Protein Synthesis

Question 1

Why does protein synthesis occur

Answer 1

Chromosomal DNA is too large to leave the nucleus to supply the coding information needed to determine the protein’s sequence of amino acids (primary structure).

Question 2

What are the two main stages in protein synthesis and where does each one take place

Answer 2

The first stage is transcription which takes place in the nucleus
The second stage is translation which takes place on a ribosome

Question 3

What is the first stage of transcription

Answer 3

In the first stage of transcription, DNA helicase breaks the hydrogen bonds between the two polynucleotide strands.

Question 4

What happens after the first stage

Answer 4

Complementary RNA nucleotides then move into place and form hydrogen bonds on the exposed nucleotides on one of the DNA strands.

Question 5

What is the second stage/ second enzyme involved in transcription

Answer 5

The enzyme RNA polymerase joins the RNA nucleotides by forming phosphodiester bonds between them.

Question 6

What does RNA polymerase continue until

Answer 6

It continues along the DNA until they reach the end of the gene.

Question 7

After these stages what occurs

Answer 7

A strand of mRNA has now been formed

Question 8

What is the mRNA complementary to

Answer 8

It is complementary to the antisense or template strand which runs from 3’ to 5’.

Question 9

What is the strand that the mRNA is not complementary to called and in which direction does it run

Answer 9

The other strand is called the sense strand and it runs from 5’ to 3’.

Question 10

Once the mRNA has been synthesised what happens

Answer 10

The RNA polymerase detaches from the DNA and DNA returns to its double helix structure. The mRNA then moves out of the nucleus through a nuclear pore.

Question 11

How long are mRNA molecules

Answer 11

They are hundreds of nucleotides long

Question 12

What happens once the mRNA are in the cytoplasm

Answer 12

They can take part in translation

Question 13

How are the mRNA nucleotides read and what are they called

Answer 13

mRNA nucleotides are read as a series of triplets, and the triplets are called codons.

Question 14

What is the mRNA sequence used to determine in translation

Answer 14

It is used to determine the amino acid sequence of a polypeptide.

Question 15

What does the start codon determine

Answer 15

Where to start translating the mRNA molecule.

Question 16

What is required for the mRNA to be read

Answer 16

Another type of RNA which is tRNA is involved which is needed for the mRNA to be read.

Question 17

What is the structure of tRNA

Answer 17

At the top of tRNA is a binding site for an amino acid.
At the bottom there is a triplet of bases called the anticodon.
The tRNA anticodon is complementary to the mRNA codon for that amino acid.

Question 18

What happens once the mRNA moves from the nucleus to the cytoplasm

Answer 18

The small subunit of a ribosome binds with the mRNA at the start codon.

Question 19

What do ribosomes contain

Answer 19

They contain a type of RNA called ribosomal or rRNA.

Question 20

What is the purpose of ribosomal RNA or rRNA

Answer 20

It is important in maintaining the structural stability of the protein synthesis sequence and plays a biochemical role in catalysing the reaction.

Question 21

After it binds to the small subunit of the ribosome what happens

Answer 21

A tRNA molecule with an anticodon complimentary to the codon attaches.

Question 22

What are the codon and anticodon held together by

Answer 22

They are held in place by hydrogen bonds between the complementary bases on the mRNA and tRNA.

Question 23

What does the second tRNA molecule then do

Answer 23

It then moves into place, and has an anticodon which is complementary to the second codon on the mRNA.

Question 24

What forms between the amino acids on the tRNA

Answer 24

A peptide bond then forms between the two amino acids which is catalysed by peptidyl transferase, which is part of the ribosomal RNA molecule.

Question 25

What does the formation of this peptide bond require

Answer 25

It requires energy which is provided by ATP.

Question 26

What does the ribosome then do

Answer 26

It moves to the next codon and forms a peptide bond to the next amino acid and the first tRNA molecule is released.

Question 27

What do the released tRNA molecules do

Answer 27

They are then attached to their amino acids by enzymes in the cytoplasm.

Question 28

What does the ribosome continue to do

Answer 28

Moving down the mRNA forming a polypeptide/protein.

Question 29

What happens when the ribosome gets to the stop codon

Answer 29

It detaches and the polypeptide chain is released.

Question 30

How can multiple ribosomes translate the same mRNA

Answer 30

Once the first ribosome has started moving along another ribosome can attach to the start codon and start translating.

Question 31

What does multiple ribosomes translating the mRNA mean

Answer 31

That a large number of polypeptides can be produced rapidly.

Question 32

Once the protein has folded what can it do

Answer 32

It can then carry out its functions in the cell.

Question 33

Where may the protein be further modified

Answer 33

It may undergo further changes in the Golgi apparatus.