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

Question 27

What is genetic code?

Answer 27

A set of rules by which information encoded in genetic material is translated into proteins

Question 28

What are 3 creatures of genetic code?

Answer 28

1. Triplet code— each codon corresponds to a specific amino acid
2. Degeneracy— multiple codons can code for the same amino acid
3. Universality—genetic code is nearly identical for all known prganisms

Question 29

How many amino acids are there in a human organism?

Answer 29

20

Question 30

What is the difference between free ribosomes and bound ribosomes?

Answer 30

Free: synthesize proteins that will function within the cell itself

Bound: produce proteins that will be secreted from the cell or incorporated within cellular membranes

Question 31

Protein synthesis in prokaryotes

Answer 31

Transcription and translation occur simultaneously due to lack of nuclear membrane. As mRNA is synthesised, ribosomes translate it.

Faster protein production in prokaryotes, which can be advantageous in rapidly changing environments.

Question 32

Protein synthesis in eukaryotes

Answer 32

Transcription occurs in the nucleus, and the mRNA must be processed and transported to the cytoplasm before translation can begin.

Question 33

What is a mutation?

Answer 33

A mistake when synthesising DNA or RNA despite checks and other process to find and repair errors.

Question 34

What is a point mutation?

Answer 34

A single nucleotide is changed (usually in DNA replication transcription

Can be insertion mutation (nucleotide added) or deletion mutation (deleted).

This causes a frameshift mutation where all following codons after mutation are altered.—>shift in base sequence

Question 35

Substitution mutation and its result

Answer 35

Due to degeneracy of genetic code sometimes there is no effect—> silent mutation (likely third base)

Otherwise can have a significant effect (likely change in 1st or 2nd base), changing whole protein structure.

Question 36

What is an example of a point mutation with significant effect on protein structure?

Answer 36

Sickle cell anaemia, haemoglobin mutation

Question 37

What is alternative splicing?

Answer 37

A mechanism by which different combinations of exons within a gene can be spliced together to generate multiple mRNA isoforms and increase protein diversity.

Question 38

What is an example of alternative splicing?

Answer 38

Production of troponin:

[In a developing foetus the troponin T gene in cardiac muscle cells is spliced in one way. This gives the troponin a higher sensitivity to Ca2+ and greater tolerance to acidosis (when the blood’s pH gets too low). Several weeks after the birth of the baby, its cardiac muscle cells begin to splice the troponin T gene in a different way and the troponin loses its higher sensitivity to Ca2+ and its tolerance to acidosis]

Question 39

Describe the modification of polypeptides to their functional state

Answer 39

When polypeptides are synthesised by ribosomes on the rough endoplasmic reticulum, they are packaged in vesicles which carry them to the Golgi apparatus. It is in the Golgi apparatus where many of these modifications are carried out.

Question 40

What is an example of post-translational modification?

Answer 40

Production of insulin:

Once the pre-proinsulin enters the rough endoplasmic reticulum, the signal peptide is removed. The remaining polypeptide is now called proinsulin . Disulfide bridges form between the A chain and the B chain. The proinsulin is packaged into vesicles that move to the Golgi apparatus where the C-peptide is removed and mature insulin remains.

Question 41

What is the proteome?

Answer 41

The total of all proteins made and used by the body.

Question 42

What are proteasomes?

Answer 42

All proteins in our body and in our cells that are unneeded, or damaged can be broken down and recycled for their amino acids.

This is carried out by a protein complex called a proteasome .

It hydrolyses proteins by breaking the peptide bonds between amino acids.

This allows cells to not only maintain a supply of amino acids, but also assists in maintaining correct concentrations of proteins and getting rid of any that are non-functional, keeping our proteome healthy and complete.