56 WJEC Biology AS Level - Study And Revision Guide - Neil Roberts (1.5 Nucleic Acids And Their Functions)

Question 1

What is a code?

Answer 1

The sequence of nucleotide bases forms a code.

Question 2

What is a codon?

Answer 2

Each ‘code word’ has three letters called a triplet code or codon, which codes for a specific amino acid.

Question 3

For proline (Pro)

Answer 3

Question 4

For glycine (Gly)

Answer 4

Question 5

For tyrosine (tyr)

Answer 5

Question 6

For methionine (met)

Answer 6

Question 7

For stop codon (no amino acid)

Answer 7

Question 8

What is referred to as degeneracy or the ‘degenerate code’.

Answer 8

There are 20 amino acids that are coded by 4 to the power of 3 bases, i.e. 64 different combinations of A, G, C, T(U).

Therefore, there are ‘spare’ base codes.

This is referred to as degeneracy or the ‘degenerate code’.

Question 9

Is the code universal?

Answer 9

This code is universal, i.e. it is the same in all living things.

Question 10

What is the START codon?

Answer 10

One codon acts as a START codon.

It marks the point on the DNA where transcription begins.

This is AUG on the mRNA and codes for methionine.

Question 11

What is the one gene, one polypeptide hypothesis?

Answer 11

Each gene found on the DNA will code for a different polypeptide.

This is called the one gene, one polypeptide hypothesis.

Question 12

Where does transcription occur?

Answer 12

Transcription occurs in the nucleus.

Question 13

Where does translation occur?

Answer 13

Translation occurs at the ribosomes.

Question 14

Where does post-translational modification occur?

Answer 14

Post-translational modification occurs in the Golgi apparatus prior to packaging of the protein into vesicles.

Question 15

Describe the steps of transcription.

Answer 15

1. DNA acts as a template for the production of mRNA.
2. DNA helicase acts on a specific region of the DNA molecule called the cistron.
3. This breaks the hydrogen bonds between both DNA strands.
4. This causes the strands to separate and unwind, exposing nucleotide bases.
5. Free RNA nucleotides pair to exposed bases on the DNA template strand
6. RNA polymerase joins them by forming the phosphodiester bonds between the phosphate group on one nucleotide and the ribose sugar on the next.
7. This continues until the RNA polymerase reaches a STOP codon
8. At this point the RNA polymerase detaches and production of mRNA is complete.
9. The mRNA strand leaves the nucleus via the nuclear pores and moves to the ribosomes.

Question 16

What did scientists note regarding the number of bases in the DNA and the number of amino acids in the polypetide.

Answer 16

Scientists first noticed that there were three times the number of bases in the DNA than amino acids in the polypeptide that it coded for.

Question 17

Do you need to learn which codons code for which amino acid?

Answer 17

You do not need to learn which codons code for each amino acid, just be able to use any codon table provided.

Question 18

Answer 18

Question 19

Codon (Key-Term)

Answer 19

The triplet of bases in mRNA that codes for a particular amino acid, or a punctuation signal.

Question 20

What do you need to able to work out regarding the sequence of amino acids?

Answer 20

Make sure you are able to work out the sequence of amino acids for a DNA sequence using an mRNA codon.