Thursday, November 27, 2008

15.4: Errors and Exceptions in Chromosomal Inheritance

When the members of a pair of homologous chromosomes do not separate properly during meiosis I, or sister chromatids don't separate properly during meiosis II, nondisjunction occurs.

As a result of nondisjunction, one gamete receives two copies of the gene while the other gamete receives none. In the next step, the faulty gametes engage in fertilization, the offspring will have an incorrect chromosome number. This is called aneuploidy.

Fertilized eggs that have received three copies of the chromosome in question are said to be trisomic; those that have received just one copy of a chromosome are said to be monosomic for the chromosome.

If nondisjunction occurs during mitosis (and early in embryonic development) it will be passed on to a large number of the organism's cells and have a significant effect on the organism.

Trisomy and monosomy often occur on the 21st and 18th chromosome pairs. When a pair of 21st chromosomes in one of the gametes fails to separate, the zygote ends up with three chromosomes, resulting in Trisomy 21 (aka Down Syndrome). Down syndrome is the most common chromosomal error. It occurs in 1 of every 733 births. Ninety percent of Down syndrome babies are miscarried. This is why Mrs. Lyon says that people with Down syndrome are a gift to us; they have overcome amazing odds to be alive.

*Above is a karyotype if a person who has Down syndrome.
People with Down syndrome are more likely to have congenital heart defects, respiratory and hearing problems, Alzheimer's disease, childhood leukemia, and thyroid conditions.
They also often have low muscle tone, small stature, an upward slant to the eyes, and a single deep crease across the center of the palm. The severity of each case varies so some people with Down syndrome may demonstrate all of the above traits while others may show very few.
People with Down syndrome all have cognitive delays, but some are much worse than others in the same way that physical problems vary.

Polyploidy is the condition of having more than two complete sets of chromosomes. This condition is somewhat common in plants. Terms such as triploidy (3n) and tetraploidy (4n) indicate that there are three or four chromosomal sets. Bananas are triploid and wheat is hexaploid (6n-meaning it has 6 chromosomal sets).
The following conditions are just based on chromosomes. You should know how to recognize them but you do not need to know what they do:

Deletion refers to a chromosome segment that has no centromere. It is broken off and lost during segregation The cell that receives the partial chromosome will be missing all of the genes located on the chromosome fragment.
-How bad the situation is depends on what has been deleted.
-If the centromere is deleted, the entire chromosome will be lost.

If the chromosome fragment that broke off (causing the deletion) becomes attached to its sister chromatid, a duplication occurs. In this case, the zygote will get a double dose of the genes located on that chromosome.
-Good Example: If the extra DNA codes for the production of a protein that is safe and helpful in excess.
-Bad Example: If the extra DNA codes for a disease, the person is more likely to get the disease.
An inversion refers to a chromosome fragment breaking off and then reattaching to its original position, but backwards so that the part of the fragment that was originally at the attachment point is now at the end of the chromosome.

-Example: abcdefghi becomes abcfedghi (the portion def in the pattern was inverted)
A translocation occurs when the chromosome fragment joins a nonhomologous chromosome. This moves a segment of one chromosome to a nonhomologious chromsome. A translocation can be reciprocal, meaning that the nonhomologous chromosomes can exchange segments.

Quiz:
1. Which of the following is an inversion of ABCDEFGHI?
a) ABFGHI
b) ABCDEHGFI
c) ABDCFGHIE
d) ABCABCDEFGHI
2. Which of the following might cause nondisjunction?
a) the breakage of chromosomes
b) centromere duplication
c) problems in Meiosis II
d) failure of synapsis in Meiosis I
3. A somatic cell that does not have an exact multiple of the haploid chromosome number is called:
a) aneuploid
b) polyploid
c) diploid
d) non-disjunctioned
Answers: 1-b, 2-d, 3-a

**If anyone wants to practice linkage map problems, this is a great website: http://library.thinkquest.org/20465/g_linkagemap.html.
I hope everyone had a wonderful, relaxing Thanksgiving Break!
Here are some reminders for this week:
Monday: 15.1 Quiz
Tuesday: Chapter 9 Vocab Check
Wednesday: Chapter 9 Vocab Quiz and Unit III Test
Friday: Lab 3 Mitosis

Monday, November 24, 2008

I thought everyone would enjoy this...

15.2 - 15.3 Notes

Hey guys! So today in class we took the chapter 15 vocab quiz, had some interesting discussions, and took notes on sections 15.2 and 15.3. Here are some notes:

As you know, humans have two types of sex chromosomes, X and Y. Females have two X chromosomes and males have an X and Y chromosome. Because there are two sex chromosomes, there is a 50/50 chance that the offspring will be male or female.

15.2 Linked Genes tend to be inherited together because they are located near each other on the same chromosome


Genes that tend to be inherited together because they are on the same chromosome are referred to as linked genes. sex-linked genes carry many genes that are not related to sex. An example of this is color blindness.


Sex linked chromosomes are also called X-lined chromosomes because they reside on the X chromosome. Therefore, fathers can only pass their sex-linked genes on to their daughters, not their sons. Females will express a sex-linked trait only if they are homozygous for it. Males are more likely to express a sex-linked trait because they only have one X and have no other allele to mask the effects.


Genetic Recombination and Linkage

Genetic Recombination is the production of offspring with combinations of traits differing from those found in either parent.

Mendel learned during his experiments that some offspring have combinations of traits. For example, a cross between a pea plant with yellow-round seeds that is heterozygous for both seed color and shape (YyRr) crossed with a plant with green-wrinkled seeds (homozygous, yyrr) can produce offspring that are genetically different from the parents. The offspring will have genotypes of YyRr, yyrr, (Yyrr, yyRr these two are recombiant offspring). In fact, 50% of all offspring are recombinants as in this example.


Recombination of Linked Genes: Crossing Over

crossing over ( down) causes recombination. one maternal and one paternal chromatid break at corresponding points then are rejoined to each other. (they trade places)

Linkage Maping Using Recombination Data: Scientific Inquiry


A genetic map is an ordered list of the genetic loci along a particular chromosome. Alfred H. Sturtevant discovered that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the highter the recombination frequency. This is true. A genetic map that is based on the recombination frequencies is specifically called a linkage map (left). The distance is measured in map units which is equivalent to a 1% recombination frequency. Today, they are often called centimorgans in honor of Morgan. The frequency of crossing over has a maximum value of 50%. Cytogenetic maps locate genes with respect to chromosomal features.
15.3- Sex-linked genes exhibit unique patterns of inheritance


The Chromosomal Basis of Sex


As stated above, humans have two types of sex chromosomes- X and Y. Females have two X's and males have one X and one Y. In both testis and ovaries, the two sex chromosomes segretate like any other pair of chromosomes pair during meiosis. Females can only give an X chromosome, whereas, males can give either an X or a Y. Half the sperm a male produces will contain a Y chromosome and the other half will contain an X. During the second month of development the gene SRY of a Y chromosome will turn on and is required for the development of testes in males. SRY also codes for other genes on the Y chromosome. If these genes are absent the XY individual is male but cannot produce normal sperm.


Inheritance of Sex-Linked Genes


Sex chromosomes have genes that are unrelated to sex. Any gene that is located on either sex chromosome is reffered to as a sex-linked gene. As stated above, females will only express a recessive sex-linked trait if she is homozygous. Men only need to inherit the recessive allele from their mother to express the trait. There are many sex-linked disorders. Examples inlcude Duchenne muscular distrophy which is a degenerative muscle disorder caused by the absense of a muscle protein called dystraphin and hemophilia. Hemophilia is a disorder that affects the blood's ability to clot. (examples of sex-linked traits below)

X-Inactivation in Female Mammals

During embryonic development, one X chromosome in each cell become "inactivated". Because of this, both females and males have the same amount of genes with loci on the X chromosome. The X chromosome that has been inactivated condenses into a compact object called a barr body. The genes on the inactivated X chromosome are not expressed in the ovaries. The are however, reactivated so that every female gamete has an active X chromosome.

The selection of which chromosome is inactivated is random. Because of this, females have a mosaic of two types of cells- those with the active X from the father and ones with the active X from their mother. Descendents of a cell have the same X chromosome that is active and the same one that is inactive. In humans, this mosaicism can be ovserved in a recessive X-linked mutation that prevents for the development of sweat glands. If a woman is heterozygous for this trait she will have patches of skin that don't have sweat glands mixed with patches of skin that do.

The inactivation of an X chromosome involves modification of DNA and scientists have discovered the gene XIST that is active only on the barr-body chromsome. The many copies of the RNA produced from this gene attach to the X chromosome on which they are made which seems to initiate X inactivation.
QUIZ:
1) All are examples of sex-linked disorders:
a. color-blindness b. Duchenne muscular distrophy c. Fragile X-syndrome d. Huntington's Disease
2) True or false: females must be homozygous for a sex-linked trait in order to express it
3) True or false: crossing over occurs more commonly on genes that are closer together
answers....1) D 2) true 3) false

Don't forget- if you are absent tomorrow (Tuesday) you are still requried to take the chapter 15 quiz on Monday. We have a unit exam next Tuesday and a lab on Monday, so its going to be a busy week. Happy Thanksgiving! :)
























Sunday, November 23, 2008

More on Mendel's Laws

Mendel’s First Law
The Law of Segregation
This law states that the alleles in a pair segregate into different gametes during gamete formation. Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete. In terms of chromosomes, this segregation corresponds to the distribution of the two members of a homologous pair of chromosomes to different gametes during Meiosis.



Mendel’s Second Law
The Law of Independent Assortment
This law states that each pair of alleles segregates independently of each other pair of alleles during gamete formation. This law applies only to allele pairs located on different chromosomes (chromosomes that are not homologous).

Mendel performed dihybrid crosses (mating of parent plants that differ in two traits) in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive. The resulting offspring or F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).





Mendel then allowed all of the F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 9:3:3:1 ratio. About 9 of the F2 plants had green pods and yellow seeds, 3 had green pods and green seeds, 3 had yellow pods and yellow seeds and 1 had a yellow pod and green seeds.


Mendel performed similar experiments focusing on several other traits like seed color and seed shape, pod color and pod shape, and flower position and stem length. He noticed the same ratios in each case. From these experiments Mendel formulated what is now known as Mendel's law of independent assortment. This law states that allele pairs separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another.
(http://biology.about.com/library/weekly/aa110603a.htm)

Chapter 15
The Chromosomal Basis of Inheritance
In the early 1900’s, the chromosome theory of heredity was formed. It stated that genes have specific locations (called loci) on chromosomes, and that it is chromosomes that segregate and assort independently.

After the chromosome theory of heredity was formed, Thomas Hunt Morgan discovered a sex-linked gene. A sex-linked gene is one located on a sex chromosome (x or y in humans). Non sex-linked genes found on non-sex chromosomes are called autosomes.

Review Questions

1.) The fact that all seven of the garden pea traits studied by Mendel obeyed the law of Independent Assortment means that the
a.) Haploid number of garden peas is 7.
b.) Diploid number of garden peas is 7.
c.) Seven pairs of alleles determining these traits are on the same pair of homologous chromosomes.
d.) Formation of gametes in plants is by mitosis only.
e.) Seven pairs of alleles determining these traits behave as if they are on different chromosomes.
2.) In dihybrid crosses, the phenotypic ratio is always…
a.) 1:2:2:1
b.) 3:1
c.) 9:3:3:1
d.) 3:9:9:1
3.) The Law of Segregation corresponds to which of the following?
a.) Meiosis
b.) Mitosis
c.) Cytokinesis
d.) DNA Replication

Answers: e,c,a

Sorry for the delay. Hope you all had a good weekend. Remember we have chapter 15 vocabulary quiz tomorrow and your edited essay is due.

Wednesday, November 19, 2008

Chapter 14: Section 3 & 4

Today in class we took the first chapter 14 quiz; I hope you all did well. :) Then we took notes on the 2nd half of chapter 14. 

14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics. 

1. Complete Dominance: dominance in which heterozygote and homozygote for the dominant allele are indistinguishable 

2. Incomplete Dominance: type of dominance in which F1 hybrids have an appearance that is in-between that of the two parents.


3. Codominance: two alleles both affect the phenotype in separate, distinguishable ways
-ex: the human MN blood group

4. Relationship between Dominance and Phenotype: the dominant allele does not subdue the recessive allele; they don't physically interact at all.
-ex: In Mendel's study of pea plants, some were round (dominant allele: RR) and some peas were wrinkled (recessive allele: rr). The dominant allele codes of an enzyme converting unbranched starch in the seed to branched starch. The recessive allele codes for a defective form of this same enzyme resulting in an accumulation of unbranched starch. This leads to excess water entering the seed through osmosis; as the seed later dries, it wrinkles. Heterozygote (Rr) peas were round as well because the one dominant allele generates enough branched starch that no excess water enters the seed.

5. Multiple Alleles: when two or more different alleles exist for the same trait
-ex: ABO blood groups in humans, for instance, are determined by 3 alleles of a single gene: IA, IB, and i.

6. Pleiotropy: ability of a single gene to affect many different traits in an organism
-ex: in peas, the same gene codes for flower color (purple or white) and outer seed color (gray or white)

7. Polygenic Inheritance: two or more genes have an additive effect on a single character in the phenotype
-basically opposite of pleiotropy
-ex: at least three different separately inherited genes code for human skin color

8. Nature and Nurture: Environmental Impact on Phenotype: phenotype sometimes depends on environment as well as genotype
-ex: a tree's leaves vary in shape, size, and greenness depending on wind and sun exposure

14.4: Many human traits follow Mendelian patterns of inheritance

1. Family pedigree: information collected about a family's history of a particular trait assembled into a family tree
-helps calculate probability that a child will have a particular genotype or phenotype
-used especially when alleles in question lead to deadly or disabling diseases (cystic fibrosis)
This family pedigree is analyzing Dystonia, a neurological movement disorder.

2. Behavior of recessive alleles: when functioning for a genetic disorder, the allele either codes for a malfunctioning protein or no protein at all
-recessive disorder only shows up in recessive homozygous individuals
-heterozygote individuals considered a carrier because they carry one recessive allele and therefore have the ability to pass it along to their offspring

3. Dominantly Inherited Disorders: some human disorders are caused from dominant alleles but it is much less common and they are rarely lethal
-ex: some types of dwarfism (anchondroplasia
-if dominant allele is lethal before maturation of the offspring, it will not be passed on to future generations because the offspring never had time to reproduce
-exception: Huntington's disease, which is a lethal degenerative disease of the nervous system, is caused by a dominant allele. However, phenotypic symptoms do not show up until age 35 to 45. Any children born to the affected parent has a 50% chance of inheriting the allele/disorder

4. Genetic testing can often be a highly controversial issue. 
-parents can be tested to find out whether they are carriers of a disorder
-the amniotic fluid of a 14-16 week old uterus can be tested for disorders as well
-disease are now screened for routinely at birth
-ex: phenylketonuria, a mental retardation, can be screen for at birth and actually prevented by a special diet (most disorders can't be treated as an infant)

Quiz...
1. One gene coding for different traits in an organism is an example of:
a) codominance
b) pleiotropy
c) multiple alleles
d) polygenic inheritance

2. A family pedigree can help answer which of the following questions?
a) What is the likelihood that my child will have a widows pea?
b) Is there a potential my child will have cystic fibrosis?
c) Will my child have friends?
d) Both a & b  

3. A carrier of a disorder passed on through a recessive allele that is not affected physically but has the ability to pass it on must be:
a) homozygous dominant
b) heterozygous 
c) homozygous recessive
d) trizygous recessive

Answers...
1. b
2. d
3. b

Good luck on the quiz tomorrow!

Monday, November 17, 2008

Chapter 13 Continued

Meiosis Reduces Chromosome Number From Diploid to Haploid
Meiosis and mitosis look similar-both are preceded by the replication of cell's DNA, for instance, but in meiosis this replication is followed by two stages of cell division, meiosis I, and meiosis II.

The result of meiosis is four daughter cells, each of which has half as many chromosomes as the parent cell.

The Stages of Meiosis are as follows

Interphase
The chromosomes replicate resulting in two sister chromatids attached at the centromere and the centrosomes are replicated.
Prophase I
The chromosomes begin to condense, and homologs loosely pair along their lengths, aligned gene by gene. Crossing over (the exchange of corresponding segments of DNA molecules by nonsister chromatids) is completed while homologs are in synapsis, (joining of two homologous chromosomes along their length) held tightly together by proteins along their lenghts. The new structure is a tetrad. Synapsis ends in mid-prophase, and the chromosomes in each pair move apart slightly. Each homologous pair has one or more chiasmata, points where crossing over has occurred and the homologs are still associated due to cohesion between sister chromatids. Centrosome movement, spindle formation, and nuclear envelope breakdown also all occurs in Prophase I. In late Prophase I, microtubules from one pole or the other attach to the two kinetochores. The homologous pairs then move toward the metaphase plate.
Metaphase I
Homologous pairs of chromosomes are lined up at the metaphase plate and microtubules from each pole attach to each of the members of homolog
ous pairs in representation for pulling them to opposite ends of the cell.

Anaphase I
The spindle apparatus helps to move the chromosomes toward the opposite ends of the cell; sister chromatids stay connected and move together toward the poles. The breakdown of proteins responsible for sister chromatid cohesion
along chromatid arms allow homologs to separate.Telophase I and Cytokinesis
At the beginning of telophase I, each half of the cell has a complete haploid set of replicated chromosomes. Each chromosome is composed of two sister chromatids; one or both chromatids include regions of nonsister chromatid
DNA. Homologous chromosomes move until they reach opposite poles, so that each pole contains a haploid set of chromosomes with each chromosome still made up of two sister chromatids. Cytokinesis occurs at the same time as telophasae-a cleavage furrow occurs in animal cells and cell plates occur in plant cells. Both result in the formation of two daughter cells. No replication occurs between meiosis I and meiosis II.
Prophase II
A spindle apparatus forms and sister chromatids move towards the metaphase plate.Metaphase II
The chromosomes are lined up on the meta
phase plate, and the kinetochores of each sister chromatid prepare to move to opposite poles of the cell. Because of crossing over in meiosis I, the two sister chromatids of each centromere are not genetically identical.

Anaphase II
The centromeres of the sister chromatids separate, and individual chromosomes move to opposite ends of the cell.Telophase II and Cystokinesis
The chromatids have moved all the way to opposite ends of the cell; nuclei reappear, and cytokinesis occurs. Each daughter cell (there are a total of 4) has the haploid number of chromosomes. Each of the four daughter cells is genetically distinct from the other daughter cells and from the parent cells.
Origins of Genetic Variation
Here are some of the processes that contribute to variation in offspring of sexully reproducing organisms:
1. Independent Assortment
2. Crossing Over
3. Random Fertilization

1. Independent Assortment of C
hromosomes
In metaphase I, when the homologous chromosomes are lined up on the metaphase plate, they can pair up in any combination, with any of t
he two homologous pairs facing either pole. This means that there is a 50-50 chance that a particular daughter cell will get a maternal chromosome or a paternal chromosome from the homologous pair.2. Crossing Over
After prophase I, homologous chromosomes synapse nd the homolous chromosomes exchange homologous parts of two on-sister chromatids. T
hen, during metaphase II, chromosomes that now have recombinant chromatids can be facing either of the two poles with respect to each other, which further increases variation in reproduction.3. Random Fertilization
This refers to the fact that fertilization (in which an egg meets with a sperm) is random. Since each egg and sperm is different, as a result of independent assortment and crossing over, each combination of egg and sperm is unique.


Chapter 14 Vocabulary
norm of reaction
The range of phenotypes produced by a single genotype, due to enviornmental influences.

genotype
The genetic makeup, or sets of alleles of an organism.

Law of Segregation
Mendel's first law statting that the two alleles in a pair segregate (separate) into different gametes during gamete formation.

polygenic inheritance
An additive effect of two or more genes on a single phenotypic character.

dihybrid
An organism that is heterozygous with respect to two genes of interest. All offspring form a cross between parents doubly homozygous for different alleles are dihybrids.

recessive allele
An allele whose phenotypic effect is not observed in a heterozygote.

alleles
Any of the alternative versions of a gene that produce distinguishable phenotypic affects.

phenotype
The physical and physiological traits of an organism, which are determined by its genetic makeup.

quantitative characters
A heritable feature that varies continuously over a range rather than in an either-or.

test cross
Breeeding an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype. The ratio in the offspring reveals the unknown genotype.

Dominant Allele
An allele that is fully expressed in the phenotype of a heterozygous.

Monohybrid
An organism that is heterozygous with respect to a single geneof interest. All the offspring from a cross between parents homozygous for different alleles are monohybrids.

codominance
The situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways.

Complete Dominance
The situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.

incomplete dominance
The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.

plietropy
The ability of a single gene to have multiple effects.

epistasis
A type of gene interaction in which one gene alters the phenotypic effects of anothr gene that is independently inherited.

hybridization
In genetics, the mating or crossing, of two true-breeding varieties.

true-breeding
Referring to plants that produce offspring of the same variety when they self-pollinate.

law of independent assortment
Mendel's second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes.

Roots
pleio-more
epi-besides
pedi-a child
-centesis-a puncture
geno-offspring

Questions

1. All occurs in Telophase I and Cytokinesis except
a. Homologous chromsomes move until they reach opposite poles
b. Each pole contains a haploid set of chromosomes
c. Spindle apparatus helps move chromosomes towards opposite ends of the cell
d. The formation of two dadughter cells

2. True/False Meiosis II separates homologous chromosomes.

3. ________ occurs after prophase I when homologous chromosomes synapse and the homologous chromosomes exchange homologous parts two non-sister chromatids.

4. Meiosis and mitosis both are preceded by
a. Condensing of chromosomes
b. Replication of cell's DNA
c. The pairing of homologs
d. Chromosomes moving towards the metaphase plate

5. What are the origins of genetic variation
a. Random Fertilization
b. Independent Assortment
c. Crossing Over
d. all of the above


answers: 1)c 2) false 3) Crossing Over 4) b 5) d



Friday, November 14, 2008

Section 13.2

  • Here we introduce Gametes is a cell that fuses with another gamete during fertilization in organisms that reproduce sexually. Gametes are also haploid cells and examples of gametes are sperm and egg (ovum). Haploid cells contain half of chromosomes of somatic cells. They contain 22 autosomes with a single sex chromosome being either or X or Y, so including the X and Y making it a haploid(n) number of 23. (n being the symbol for haploid number of chromosomes).
  • n=3
Fertilization
The union of gametes (sperm and egg). So the combination of the haploid gametes from the parents fuse, and the fertilized egg that results is called a zygote. Because of the union of gametes or of two haploids it turns into a diploid which has two sets of chromosomes. The diploid number of chromosomes 2n because of the two haploids (n) making it 2n.

  • 2n=46

Meiosis
Through the production of gametes the number of chromosomes is half so that the haploid gametes are formed. Mitosis conserves chromosome number, meiosis reduces the chromosome number by half. Human sperm and ova each have a haploid set of 23 different chromosomes, one from each homologous pair. After fertilization restores the diploid condition by combining two haploid sets of chromosomes, the human life cycle is repeated, from generation to generation.

KEEP IN MIND THE THREE TYPES OF LIFE CYCLES


  • humans and animals
  • fungi and pictists
  • plant and some algae
1. Humans and animal life cycles
Meiosis only occurs during the productgion of gametes, which undergo further cell division before fertilization. The diploid zygote divides by mitosis making it a multicellular organism that is a diploid









2. Fungi and protists life cycles
After gametes begin to form the diploid zygote, meiosis produces haploid cells. After they then divide by mitosis to give a haploid multicellular organism.











3. Plants and some algae life cycles
Alternation of generations occurs. The multicellular stage is called sporophyte. Meiosis here in the sporophyte produces haploid cells called spores that divide to produce a gametophyte which produces haploid gametes (mitosis) and fertilization happens then producing a diploid zygote.




Questions:

1. Multicellular haploid organisms:


a. are typically called sporophytes.

b. produce new cells for growth by meiosis.

c. produce gametes by mitosis.

d. are found only in aquatic environments.

e. are the direct result of syngamy.



2.The immidiate product of meiosis in a plant is a


a. spore.

b. gamete.

c. zygote.

d. sporophyte.

e. gametophyte.



3. Homologous chromosomes move to opposite poles of a dividing

a. mitosis.

b. meiosis I

c. meiosis II

d. fertilization

e. binary fission.



4.True or False


Meiosis is the process by which, the gamete production, chromosome number is halved so that the haploid gametes are formed.

Answers: 1. b , 2. a, 3. b, 4. True

Book about Stroke

Some of you were interested about the book of the brain scientist that had a stroke.

Here is the title:

My Stroke of Insight: A Brain Scientist's Personal Journey

Here is the video about meiosis:


Thursday, November 13, 2008

Work Day

Hey guys, so because today was a work day I'm just going to post a couple of things for you to look at, starting with vocab for tomorrows quiz.

Chapter 13 Vocab

Heredity- The transmission of traits from one generation to the next.


Variation- Differences between members of the same species.


Synapsis- The pairing and physical connection of replicated homologous chromosomes during prophase I of meiosis.



Tetrad- (1) Four homologous chromatids in a bundle in the first meiotic prophase and metaphase. The meiotic configuration of four chromatids first seen in pachytene. There is one tetrad per pair (bivalent) of homologous chromosomes. (2) The four haploid product cells from a single meiosis.


Gene- A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).


Meiosis- A modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell.


Chiasma- The X-shaped, microscopically visible region where homologous nonsister chromatids have exchanged genetic material through crossing over during meiosis, the two homologs remaining associated due to sister chromatid cohesion.


Crossing over- The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis.

Homologous structure- A pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism’s father, the other from the mother. Also called homologs, or a homologous pair.


Locus- A specific place along the length of a chromosome where a given gene is located.

Karyotype- A display of the chromosome pairs of a cell arranged by size and shape.


Clone- (1) A lineage of genetically identical individuals or cells. (2) In popular usage, a single individual organism that is genetically identical to another individual. (3) As a verb, to make one or more genetic replicas of an individual or cell. See also gene cloning.


Syngamy- The fusion of two gametes in fertilization.


And the following words can be found in the last image, the key shows the colors for haploid and diploid structures, with blue as haploid and green as diploid.


Diploid cell- A cell containing two sets of chromosomes (2n), one set inherited from each parent.


Haploid cell- A cell containing only one set of chromosomes (n).


Spore- In the life cycle of a plant or alga undergoing alternation of generations, a haploid cell produced in the sporophyte by meiosis. A spore can divide by mitosis to develop into a multicellular haploid individual, the gametophyte, without fusing with another cell. (2) In fungi, a haploid cell, produced either sexually or asexually, that produces a mycelium after germination.


Sporophyte- In organisms (plants and some algae) that have alternation of generations, the multicellular diploid form that results from the union of gametes. The sporophyte produces haploid spores by meiosis that develop into gametophytes.


Gametophyte- In organisms (plants and some algae) that have alternation of generations, the multicellular haploid form that produces haploid gametes by mitosis. The haploid gametes unite and develop into sporophytes.




Roots:

-apsis-juncture

tetra- four
chiasm- marked crosswise

syn-together
fertil-fruitful

homo-like
haplo-single

sporo-a seed



And now the meiotic division of an animal cell...

Meiosis is divided into two parts, the first part is called Meiosis I: Separates Homologous Chromosomes

Prophase I:
  • Chromosomes begin to condense, and homologs loosely pair along their lengths lined up gene by gene
  • Crossing over is completed while homologs are in synapsis and held tightly together by proteins along their lenghts
  • Synapsis ends in mid-prophase and the chromosomes in each pair move apart slightly
  • Each homologous pair has one or more chiasmata, points where crossing over has occured and the homologs are still associated due to cohesion between sister chromatids
  • Centrosome movement, spindle formation, and nuclear envelope breakdown occur
  • In late propahse I, micortubules from one pole or another attach to the two kinetechores, protein structures at the centromeres of the two homologs
  • The two homologous pairs then move toward then move towards the metaphase plate
Metaphase I
  • Pairs of homologous chromosomes are arranged at the metaphase plate with one chromosome in each pair facing each pole
  • Both chromatids of one homolog are attached to kinetechore microtubules from one pole, the others are attached to the opposite pole
Anapahse I
  • Breakdown of proteins responsible for sister chromatid cohesion along chromatid arms allowds homologs to seperate
  • The homologs move toward opposite poles, guided by the spindle apparatus
  • Sister chromatid cohesion persists at the centromere causing chromatids to move as a unit toward the same pole
Telophase I and Cytokinesis
  • At the beginning of telophase I each half of the cell has a complete haploid set of replicated chromosomes, each chromosome is composed of two sister chromatids. One or both chromatids include regions of nonsister chromatid DNA
  • Cytokinesis usually occurs at the same time as telophase I, forming two haploid daughter cells
  • In animal cells, a cleavage furrow forms, in plant cells, a cell plate forms
  • No replication occurs between Meiosis I and II
Meiosis II: Seperates sister chromatids

Propahse II
  • A spindle apparatus forms
  • In late propahse II chromosomes move towards the metaphase plate
Metaphase II
  • The chromosomes are posistioned on the metaphase plate
  • Because of crossing over in meiosis I the two sister chromatids of each chromosome are not genetically identical
  • The kinetechores of sister chromatids are attached to microtubules extending from opposite poles
Anaphase II
  • Breakdown of proteins holding the sister chromatids together at the centromere allows the chromatids to seperate
  • The chromatids move towards opposite poles
Telophase II and cytokineses
  • Nuclei form, the chromosomes begin decondensing, and cytokineses occurs
  • The meiotic division of one parent cell produces four daughter cells , each with a haploid set of unreplicated chromosomes
  • Each of the four daughter cells is genetically distinct from the other daughter cells and from the parent cell


Questions:

1. Meiosis has______round(s) of DNA replication
  1. one
  2. two
  3. four
  4. six
2. Crossing over occurs in______
  1. Prophase II
  2. Metaphase I
  3. Prophase I
  4. Metaphase II
3. True or false? A Haploid cell has 46 chromosomes.

4. Humans have what kinds of chromosomes?

Answers:

1. 1 2. 3 3. False 4. X, Y and autosomes