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