Genomics II Flashcards
Define gene.
Gene is a unit of inheritance made of a specific sequence of nucleotides that codes for a specific sequence of amino acids in a polypeptide chain or RNA molecule
Define genetic code.
It is a set of rules specifying the relationship between triplet nucleotides in DNA , RNA or amino acids of a protein.
What are the features of the genetic code?
a. Triplet code > The genetic code is a triplet code with codons on the mRNA that are complimentary to the triplets of bases of the DNA template.
b.Degenerate > More than one codon can code for the same amino acid but no codons can code for more than one amino acids.
c. Non-overlapping > The codons are non-overlapping and are read in order without skipping any nucleotides.
d. Punctuated > There are start codons for initiation and termination of translation. Start codon is AUG and it signals for the initiation of translation of an mRNA into a polypeptide. The stop codons are UGA , UAG and UAA that signal the termination of translation.
e. Universal > The same triplet of bases codes for the same amino acid in all organisms.
Define transcription.
The process where a sequence of bases on the DNA template of a gene is copied onto a complementary sequence of bases on a primary RNA transcript.
What happens during initiation?
Transcription begins in the promoter. General transcription binding proteins will bind onto the TATA box and recruit other proteins like RNA polymerase. RNA polymerase will bind to the promoter and cause the unwinding of DNA double helix. The template strand is used as a template strand duh
What happens during Elongation?
RNA polymerase catalyzes the formation of phosphodiester bonds within adjacent ribonucleotides , adding ribonucleotides to the 3’OH end. The new RNA strand is synthesized in the 5’-3’ direction. As RNA polymerase moves along the DNA template strand in the 3’-5’ direction , it continues to unwind the double helix. Free ribonucleotides complementary to the bases on the template strand are taken up and matched via complementary base pairing.
What happens during termination?
Elongation occurs until stop codon is reaches where it signals for the termination of transcription. The resulting RNA is known as primary RNA transcript. The DNA rewinds.
What are the post-transcription modifications?
A. 5’capping where 7-methylguanosine is added to 5’ end after transcription , catalyzed by capping enzymes through a 5’-5’ triphosphate bridge.
B. At the 3’end where the polyadenylation signal sequence is found , poly(A) polymerase is adds more Adenine to form a poly(A) tail.
C. RNA splicing where introns are removed from the primary transcript RNA while simultaneously the exons are spliced to form mature mRNA. This is carries out by spliceosome.
Functions of A&B:
-to facilitate transport of mature mRNA out of nucleus
-help ribosome attach to the 5’ end of mRNA once it reaches the nucleus
-prevents degradation by hydrolytic enzymes.
Function of C:
Ensure a continuous coding sequence as the non-coding sequence is removed.
Define translation.
Translation is the formation of a specific sequence of amino acids coded by a sequence of codons on mRNA.
How does the structure of ribosome adapts to its function in protein synthesis?
rRNA associates with ribosomal protein to form the small and large ribosomal subunits. This gives rise to the a site , p site and e site. - A site holds the tRNA that carries the next amino acid to be added to the chain. P site holds the tRNA that carries the growing polypeptide chain. E site is the site where discharged tRNAs leave the ribosome.
rRNA facilitates the binding of codons on the mRNA to the anti-codons of the tRNA. This ensures that the correct amino acids is brought by the tRNA according to the sequence on the mRNA.
rRNA in large ribosomal subunit has catalytic activity and ribozyme. This provide peptidyl transferase activity for the formation of peptide bonds during elongation.
rRNA has base sequence which is complementary to the 5’ of UTRs in mRNA. Facilitates binding of small ribosomal subunit to the 5’ end of UTR of the mRNA strand during initiation.
What happens during initiation of translation?
A small ribosomal subunit will bind with mRNA and initiator tRNA with amino acid methionine. rRNA has base sequence complementary to the ones found in 5’UTR in mRNA. Small ribosomal subunit will move along the mRNA in 5’-3’ direction until it reaches the start codon of mRNA. Initiation factors will bring in the large ribosomal subunit and initiation` translation is complete. This process requires energy from the hydrolysis of GTP.
What happens during elongation of translation?
CODON RECOGNITION
Anti-codon of second aminoacyl-tRNA , held at the A site of the ribosome , is complementary to the codons on the mRNA. Hydrolysis of GTP increase accuracy and efficiency.
PEPTIDE BOND FORMATION
The first amino acid , methionine , on the P site is transferred to the second aminoacyl-tRNA on the A site to form peptidyl-tRNA. The two amino acids are joined by peptide bonds which is catalyzed by petidyl transferase activity found in the large ribosomal subunit. rRNA is responsible for the catalytic activity found in the large ribosomal subunit. Peptide group is formed between amino group and carboxyl group on P site.
Translocation
Several conformational changes in the ribosome shifts the two tRNAs. Uncharged tRNA moves from P site to E site , thus exiting the complex. Peptidyl tRNA moves from a site to p site. Ribosome shifts one codon down in the mRNA. The empty a site receives a aminoacyl tRNA that has an anti-codon complimentary to the third codon on the mRNA. GTP hydrolysis is required.
What happens during Termination of translation.
It happens when the stop codon of a mRNA reaches the A site of the ribosome. Instead of tRNA , A site will accept release factor. A molecule of water is added to hydrolyze the polypeptide chain from the peptidyl-tRNA on the P site. Translation initiation complex comes apart , ribosome release the mRNA and dissociates into its two subunits.
What happens to the polypeptide chain when its released?
-undergoes spontaneous folding and coiling into protein
-undergoes post-translational modification in the sER and Gogli apparatus to become a functional protein
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What is a polyribosome and what are its function?
Polyribosome is a single mRNA strand attached to many ribosomes.
this allows simultaneous translation and to make many copies of a polypeptide at a faster.
