Structure of a Gene / System for Transcription

Question 1

Structure of DNA

Answer 1

At each base of a DNA strand (A-T, C-G) there is a carbon sugar. The end where there is a 5C sugar= 5 prime. 3C sugar= 3 prime. A strand of DNA is put together by 5 prime 3 prime linkage with a 5’ free on one end and 3’ free on the other. On the opposite strand, the 5’/3’ is reversed. The linkage between two strands is formed by the nuclear paired bases. A DNA sequence is read from 5’ to 3’ end.

Question 2

What provides the copying mechanism for gene synthesis?

Answer 2

The double helix of DNA.

Question 3

Antisense strand

Answer 3

This is complimentary to the sense strand and is how transcription occurs.

Question 4

What is a codon?

Answer 4

3 base pair sequence which codes for an amino acid

Question 5

What is the start codon?

Answer 5

ATG, this tells the translation system where its going to start making the protein

Question 6

What are the stop codons?

Answer 6

TAA, TAG, TGA

Question 7

Gene Structure

Answer 7

From 5’ to 3’=
Enhancer sequence, Promoter, TATA/INR, TSS of 5’ UTR, ATG, coding sequence (incl. introns and exons), Stop codon, 3’ UTR.

Question 8

What is the enhancer sequence on a gene?

Answer 8

It regulates gene expression. It’s part of the transcription initiation complex as regulatory enhancers/silencers bind to it.

Question 9

What is the promoter on a gene?

Answer 9

Part of the gene necessary to initiate gene expression

Question 10

What is UTR?

Answer 10

There is a region either side of the coding sequence which = untranslated region. At the 5’ end of the 5’ UTR there is the TSS

Question 11

What is TSS (found at the 5’ end of the 5’ UTR)

Answer 11

Transcription start sequence

Question 12

What is the TATA box/INR?

Answer 12

Initiation sequence. This is where the proteins initiate the process of transcription as transcription factors bind to the TATA box. This pulls in RNA polymerase which takes the DNA sequence and produces mRNA.

Question 13

Which strand is used for transcription?

Answer 13

Antisense strand is the template strand so the mRNA produced will be complimentary to it and therefore the same as the sense strand. In RNA there is no T base only U.

Question 14

What are the post-transcriptional mRNA processes?

Answer 14

5’ capping, 3’ polyadenylation. exon splicing and translation

Question 15

What is 5’ capping?

Answer 15

Guanine is added to the 5’ end of the 5’ UTR via 5’-5’ link. Guanine is then methylated.

Question 16

what is the purpose of 5’ capping?

Answer 16

mRNA is very unstable and open to enzyme degradation so this prevents degradation of mRNA by 5’-3’ nucleases and promotes nuclear export.

Question 17

What is 3’ polyadenylation?

Answer 17

At the 3’ end there is a run of adenines = Poly A tail. This can be hundreds of bases.

Question 18

What’s the purpose of 3’ polyadenylation?

Answer 18

Enhances stability and protects the mRNA from 5’-3’ nucleases and help with nuclear export of mRNA from nucleus to cytoplasm.

Question 19

What is exon splicing?

Answer 19

Introns are spliced out of the coding sequence, the exons are then spliced together which creates the mature mRNA ready for translation.