Gene Expression

Question 1

What is a start codon?

Answer 1

AUG, 3 nucleotide triplet code

Question 2

What signals the end of protein synthesis?

Answer 2

Stop codon

Question 3

What is the difference between pre-messenger RNA and mRNA?

Answer 3

Pre-mRNA processed into a finished messenger RNA (mRNA), capping (5’ end: 5’ cap), tailing (3’ end: polyA tail), splicing (removal of introns, spliceosome, sequence specific regions cause splicing, mutations will cause a lack of splicing), that moves through a nuclear pore into the cytoplasm.

Question 4

What happens in the initiation stage of transcription?

Answer 4

Initiation: RNA polymerase + regulatory proteins bind specific nucleotide sequence, promoter region: TATAAA, marks beginning of gene, works in a downstream direction from +1, first base after polymerase.

Question 5

Describe transcription elongation

Answer 5

Elongation: DNA unwind + open, RNA polymerase reads the nucleotide sequence of the coding strand 5’ to 3’. As it moves along, it inserts and links together complementary RNA nucleotides = pre-mRNA. No T in RNA, so an A on the DNA template = U in the RNA transcript.

Question 6

How does transcription terminate?

Answer 6

Termination: RNA polymerase moves along, it reaches the end of the gene = nucleotides called a 3´ termination sequence, it stops adding nucleotides to the pre-mRNA, falls off DNA template strand, pre-mRNA is released.

Question 7

How does translation initiate?

Answer 7

Initiation: 5’cap end of mRNA recognised, binds to 40S ribosome subunit. Anticodon of initiator tRNA carrying the AA methionine pairs with the AUG start codon of mRNA, start of all human protein. Initiation complete when a large ribosomal subunit binds to the complex

Question 8

Explain translation elongation

Answer 8

Elongation: large subunit has E-P-A regions, charged tRNA carrying 2nd AA pairs with the 2nd mRNA codon in the A region. rRNA of the large subunit acts as an enzyme and forms a peptide bond between the two AA in A and P regions. Uncharged (no AA) tRNA exits from E region. As the ribosome moves along the mRNA, other tRNAs carrying AA pair with mRNA codons, adding AA to the growing polypeptide chain, N to C growth.

Question 9

What is translation termination?

Answer 9

Termination: ribosome reaches stop codon. UAA, UAG, and UGA do not code for AA, there are no tRNA molecules with anticodons for stop codons. Release factors bind to stop codon in A region; then the polypeptide, mRNA, and tRNA are released from the ribosome

Question 10

What is the definition of an intron?

Answer 10

DNA sequences present in some genes that are transcribed but are removed during processing and therefore are not present in mature mRNA.

Question 11

Describe an exon

Answer 11

A coding piece of DNA

Question 12

What is the difference between a Cap A and a poly-A tail?

Answer 12

Cap A –> modified base (guanine nucleotide) attached to the 5’ end of eukaryotic mRNA molecules.

Poly-A tail –> series of A nucleotides added to the 3’ end of mRNA by polyA polymerase

Question 13

What is a chain of ribosome linked by mRNA called?

Answer 13

Polyribosome

Question 14

What brings in AA?

Answer 14

tRNA

Question 15

What is the structure of tRNA?

Answer 15

brings AA in to bind to mRNA, clover model, H bonds form between complementary sequences

Question 16

in general terms where does transcription and translation take place?

Answer 16

the nucleus, the cytoplasm

Question 17

what happens during RNA processing?

Answer 17

Capping, tailing, splicing

Question 18

name the prokaryotic ribosome subunits

Answer 18

30S + 50S = 70S

Question 19

Name the eukaryotic ribosome subunits

Answer 19

40S + 60S = 80S