PROTEIN SYNTHESIS
Protein synthesis is the process that describes how enzymes and other proteins are made from DNA.
The three steps in protein synthesis are transcription, RNA processing, and translation.
Transcription:
This is the process by which DNA makes RNA. The three types of RNA are mRNA, tRNA, and rRNA. Messenger RNA (mRNA) carries messages directly from DNA to the cytoplasm and varies in length, depending on the length of the message. Transfer RNA (tRNA) is shaped like a cloverleaf and carries amino acids to the mRNA at the ribosome. Ribosomal RNA (rRNA) is strcutural, this makes up the ribosome, which is formed in the nucleolus.
Transcription consists of three stages: initiaition, elongation, and termination.
At the start of initiation an enzyme, RNA polymerase, recognizes and binds to DNA at the promoter region. This begins to unzip the DNA into two strands. A promoter region for mRNA transcriptions often contains the sequence T-A-T-A (the TATA box). The completed assembly of transcription factors and RNA polymerase bound to the promoter is called the transcription initiation complex. Once RNA polmerase is attached to promoter, DNA transcription of the DNA template begins.
Elongation occurs as the RNA polymerase unzips the DNA and assembles RNA nucleotides using one strand of the DNA as a template. As in DNA replication , elongation of the RNA molecule occurs in the 5' --> 3' direction. In contrast to DNA replication, new nucleotides (rather than DNA nucleotides), and only on DNA strand in transcribed.
Termination is the final stage. It continues for a short distance after the RNA polymerase transcribes the termination sequence. At this point, mRNA is cut free from the DNA template. In eukaryotes, the termination region often ocntains the DNA sequence AAAAAAA.
RNA Processing Before the newly formed RNA strand is shipped out to the ribosome in the cytoplasm, it is altered or processed by a series of enzymse.
A 5' cap consisting of a modified guanine nucleotide is added to the 5' end. This cap helps protect the RNA strand from degradation by hydrolytic enzymes and also helps the RNA strands bind to the ribosome in the cytolpasm.
A poly-A tail is attahced to the 3' end of the mRNA. The tail consists of 30 to 200 adenine nucleotides. The tail protects the RNA strand from degradation by hydrolytic enzymes, helps the ribosome attach to the RNA, and facilitates the release of the RNA into the cytplasm.
Noncoding regions of the mRNA called introns or intervening sequences are removed by the snRNPs, small nuclear ribonucleoproteins, and splicesomes.This removal allows only exons, which are expressed sequences, to leaves the nucleus. As a result of this processing, the mRNA that leaves the nucleus is a great deal shorter than the original transcription unit.
Translation
After transcription, the mRNA, tRNA, and ribosomal subunits are transported across the nuclear envelope and into the cytoplasm. In the cytoplasm, amino acids attach to the 3' end of the tRNA's, forming an aminoacyl-tRNA. The reaction requires an enzyme specific to each tRNA and the energy from one ATP. The amino acid-tRNA bond that results is a high-energy bond, creating a activated amino acid.
Like transcription, translation is categorized into three steps: intiation. elongation, and termination. Energy for translation is provided by several GTP molecules. GTP acts as an energ supplier in the same manner as ATP.
Intiation begins with when mRNA becomes attached to a subunit of the ribosome. The first codon is always AUG. It codes for methione and must be positioned correctly in order for transcription of an amino acid sequence to begin. At this point, the ribosomal subunit is a complex of mRNA, AUG, the tRNA anticodon, UAC, the amino acid methione.
Elongation continues as tRNA brings amino acids to the ribosome and a polypeptide chain is formed.
Termination of an mRNA strand is complete when a ribosome reaches one of three termination or stop codons. A release factor hydrolyzes the bond between ther tRNA and the last amino acid of the polypeptide chain. The polypeptide is freed from the ribosome, and the mRNA is broken down.
Quiz:1) If AUU is the codon, what is the anticodon?
a. AUU
b. TAA
c. UUA
d. UAA
2) What are the regions of DNA called that code for proteins?
a. introns
b. codons
c. anticodons
d. exons
3) The end products of translation are
a. polypeptides
b. amino acids
c. RNA
d. DNA
4) Which if the following contains a code for a protein?
a. mRNA
b. RNA polymerase
c. rRNA
d. tRNA
5) Proteins synthesis consists of all the following steps except:
a. translation
b. transcription
c. replication
d. elongation
ANSWERS: 1)D 2)D 3)A 4)A 5)C
Just keep reviewing the rest of the week!!!! :-)
No comments:
Post a Comment