Gene Expression - Transcription & Translation

Question 1

What is the central dogma?

Answer 1

DNA –> RNA –> Protein

The process of transcription followed by translation.

Question 2

What are the differences between DNA and RNA?

Answer 2

1. RNA is single stranded.
2. RNA’s sugar molecule is ribose, rather than deoxyribose.
3. RNA has the base uracil instead of thymine.

Question 3

What are the 3 stages of transcription?

Answer 3

1. Initiation.
2. Elongation.
3. Termination.

Question 4

What happens in the initiation phase of transcription?

Answer 4

RNA polymerase recognises and binds to a promoter sequence (initiation site) on DNA.

Question 5

What happens in the elongation stage of transcription?

Answer 5

RNA polymerase elongates (lengthens) the nascent (growing) RNA molecule in a 5’ to 3’ direction, antiparallel to the template DNA strand. Nucleotides are added by complementary base pairing with the template strand and the substrates (ribonucleoside triphosphates) are hydrolysed as added, releasing energy for the process.

Question 6

What happens in the termination stage of transcription?

Answer 6

Particular DNA sequences and sometimes proteins terminate transcription. The transcript is released from the DNA and is called a pre-mRNA which is processed to generate the mature mRNA.

Question 7

What is transcription and translation?

Answer 7

Transcription - The process of synthesising RNA from DNA.

Translation - the process of synthesising proteins from decoding mRNA.

Question 8

What is a codon and how many possible ones are there?

Answer 8

A codon is a triplet of nucleotides. There are 64 possible codons:

• 1 start codon (encodes methionine)
• 3 termination codons (stops translation).
• 60 codons for the 20 amino acids.

Question 9

What is the difference in translation between prokaryotes and eukaryotes?

Answer 9

Prokaryotes - Translation begins before mRNA synthesis/transcription is completed.
Eukaryotes - Transcription occurs in the nucleus and translation occurs in the cytoplasm.

Question 10

What are the 4 components that translation requires?

Answer 10

1. Ribosomes
2. tRNA’s
3. Activating enzymes
4. mRNA (template).

Question 11

What is tRNA (transfer RNA) role in translation?

Answer 11

It acts as the adapter, brining the amino acids anticodon corresponding to the mRNA’s codon.

Question 12

What is the role of aminoacyl-tRNA syntheses (family of activating enzymes) in translation?

Answer 12

They attach specific amino aids to their appropriate tRNA’s, forming charged tRNA’s

Question 13

What do the ribosomes do/what are their subunits/sites?

Answer 13

The mRNA and charged tRNA’s meet at the ribosome. The ribosome has 2 subunits: small and large, and 4 sites:E, P, A and T.

Question 14

What is the initiation process of translation?

Answer 14

An initiation complex forms, consisting of the initiator tRNA charged with methionine, and the small ribosomal subunit bound to mRNA triggers the start of translation.

Question 15

What is the process of elongation in translation?

Answer 15

The ribosome moves along the mRNA one codon at a time in a 5’ to 3’ direction. Polypeptides grow from the N terminus towards the C terminus. Charged tRNA’s bring amino acids to the ribosome sequentially..

Question 16

What is specificity provided by in the elongation stage of translation?

Answer 16

• The anticodon to codon interaction.

- The accuracy of aminoacyl-tRNA synthesis.

Question 17

What is the process of termination in translation?

Answer 17

The presence of a stop codon in the A site of the ribosome causes translation to terminate. The completed protein is then released.

Question 18

What do polysomes allow in translation and how?

Answer 18

Polysomes allow the synthesis of multiple copies of the same protein from a single mRNA. In a polysome, more than one ribosome moves along the mRNA at one time.