D1.2 Protein Synthesis

Question 1

Define transcription

Answer 1

The process by which genetic information encoded in DNA is copied into mRNA

Question 2

What is the enzyme responsible for transcription?

Answer 2

RNA polymerase

Question 3

What is the role of RNA polymerases in the initiation phase of transcription?

Answer 3

1. In the initiation phase the enzyme binds to the promoter sequence on the DNA at the start of the gene

Question 4

What is the role of RNA polymerases in the elongation phase of transcription?

Answer 4

2. It moves along the gene, separating the double strand into two, single strands
3. It starts encoding for themolecule of mRNA on the anti-sense (template/og) DNA strand 5’—>3’ by moving along the strand reading it one base at a time, adding free RNA nucleotides to the growing mRNA molecule

[however, it reads the DNA strand 3’—>5’]

Question 5

What is the role of RNA polymerases in the termination phase of transcription?

Answer 5

4. When a terminator sequence in the DNA is reached and the pre-mRNA is released, the RNA polymerase detaches from the DNA strand, allowing the 2 strands to come back together
5. Splicing occurs to convert the pre-mRNA into mature mRNA before it moves out of the nucleus for translation

Question 6

How does the complementary base pairing rule work in transcription?

Answer 6

After RNA polymerase breaks H-bonds between the bases to unzip the DNA, it adds free RNA nucleotides along the template strand based on the rule: A-U, C-G and temporarily add H-bonds between them.

1. Holds structure
2. Ensures genetic code in DNA is accurately transcribed

Question 7

How many H-bonds form between A-T/U and C-G?

Answer 7

A-T/U—> 2 H-bonds

C-G—> 3 H-bonds

Question 8

What is the antisense strand?

Answer 8

The template/original strand (DNA)

Question 9

Why is it important that the DNA strand are stable and remain unchanged by the transcription process?

Answer 9

The DNA within a cell is often transcribed many times and, for somatic cells (that do not go through regular cell divisions), the DNA needs to remain intact throughout the life of the cell so it maintains and conserves the original code.

If the DNA was to be degraded by the transcription process it would not be able to continue producing functioning proteins, which could stop the cell from functioning and even lead to the cell dying.

Question 10

What happens for gene expression in transcription?

Answer 10

Not all genes in a cell are expressed at any given time, transcription, being the first stage of gene expression, is a key stage at which expression of a gene can be switched on and off.

Question 11

What is a somatic cell?

Answer 11

A cell that doesn’t divide to replace itself, meaning that the genetic code must stay unchanged throughout its lifetime.

Question 12

How is stability ensured?

Answer 12

By the sugar-phosphate backbone and H-bonds between nucleotides.

Question 13

What are factors that can compromise DNA stability?

Answer 13

Free radicals
Chemicals
Cigarette smoke
UV radiation
Nuclear radiation

Question 14

Define translation

Answer 14

The process by which ribosomes use the genetic information carried by mRNA to synthesise proteins.

Question 15

Where do transcription and translation take place respectively?

Answer 15

Nucleus (in eukaryotic cells), the cytoplasm

Question 16

What are the three essential components for translation to be carried out?

Answer 16

mRNA
Ribosomes
tRNAs

Question 17

What is the role of ribosomes in translation?

Answer 17

Act as the binding site for mRNA and tRNA.

They catalyse the peptide bonds between the amino acids to form the polypeptide.

Question 18

What is the role of mRNA in translation?

Answer 18

Binds to the small subunit of ribosomes, it has the sequence of codons that specifies the amino acid sequence of the polypeptide.

Question 19

What are ribosomes?

Answer 19

Are composed of 2 subunits: one large and one small.
They have 3 binding sites; 2 for tRNA molecules, one for the mRNA.

Question 20

What is the role of tRNA in translation?

Answer 20

2 tRNA molecules attach to the large ribosomal subunit.
Each tRNA has a specific corresponding amino-acid attached.

Question 21

Describe the structure of ribosomes, tRNA, and mRNA

Question 22

Outline the process of splicing

Answer 22

The 3’ end is capped with Poly-A as well as a 5’ cap for the 5’ end—> this is to protect the mRNA from degrading

RNA splicing removes the introns (non-coding regions of pre-mRNA) and keeps the exons to form a mature mRNA

Question 23

What happens in the initiation phase of translation?

Answer 23

The tRNA molecule (met) enters the ribosome at the P-site

Question 24

What happens in the elongation phase of translation?

Answer 24

1. Another tRNA molecule enters the at the A-site
2. In the meantime, a covalent bond is formed between the two amino acids of the two tRNAs
3. As the process continues, the tRNA in the initial P-site will move to the E-site and loose its amino acid

[this is repeated until a polypeptide chain grows to form a protein]

Question 25

What happens in the termination phase of translation?

Answer 25

When stop codons are detected at the A-site they cause a release factor to enter the A-site which causes the ribosomal subunits to disassemble and the protein to be released.

Translation has ended, afterwards, the protein leaves the ribosome to the Golgi body where the proteins undergo folding to form a specific shape for a specific function.

Question 26

What are the stop codons?

Answer 26

“Nonsense” codons that do not code for any genetic information:

UAA
AUG
UGA