Topic 4: DNA, protein synthesis and mutations

Question 1

The genetic code is said to be degenerate, universal and non-overlapping. Explain what each term means.

Answer 1

Degenerate
More than one codon/ triplet of bases codes for the same amino acid.

Universal
The same triplet codes for the same amino acid in all organisms.

Non-overlapping.
Each base is part of only one triplet.

Question 2

What is the advantage of the genetic code being: Degenerate

Answer 2

Even if there is a gene mutation, the triplet of bases may still code for the same amino acid.

Question 3

What is the advantage of the genetic code being: non-overlapping

Answer 3

If there is a mutation, it will only affect 1 codon, and therefore only 1 amino acid. This reduces the potential change in tertiary structure of the protein.

Question 4

What is a codon? (2 marks)

Answer 4

A triplet of bases on mRNA that codes for an amino acid.

Question 5

What is the difference between an exon and an intron?

Answer 5

Introns are the non-coding sections of DNA (do not code for a protein). Exons are the coding sections of DNA (do code for proteins).

Question 6

Name the two types of RNA involved in protein synthesis?

Answer 6

mRNA and rRNA

Question 7

What is mRNA and why is it necessary?

Answer 7

mRNA is a copy of a gene from DNA. mRNA is created in the
nucleus and then enters the cytoplasm where it attaches to a
ribosome.

DNA is too large to leave the nucleus and would be at risk of being damaged by enzymes. mRNA is much shorter because it is only the length of one gene, and can therefore leave the nucleus as it is small enough to fit through the nuclear pores.

mRNA is single-stranded and every three bases in the sequence code for a specific amino acid, these three bases are called codons.

Question 8

Structure and function of tRNA?

Answer 8

tRNA is found in the cytoplasm. It is single-stranded but folded to create a cloverleaf shape, held in place by hydrogen bonds. The function of tRNA is to attach to one of the 20 amino acids and transfer this amino acid to the mRNA to create the polypeptide chain.
Specific amino acids attach to specific tRNA molecules and this is determined by 3 bases found on the tRNA which are complementary to the 3 bases on mRNA. These are called the anticodon because they are complementary to the codon on mRNA.

Question 9

Protein synthesis is split into 2 stages. Name the stages, the location they occur & the products:

Answer 9

Stage 1 : Transcription

Location: Nucleus
Product: mRNA (or pre-mRNA)

Stage 2: Translation

Location: The cytoplasm attached to a ribosome Product: polypeptide chain

Question 11

Name the two enzymes involved in transcription & describe their function:

Answer 11

Enzyme 1 -
DNA helicase - breaks the hydrogen bonds between DNA strands.

Enzyme 2 -
RNA polymerase - joins adjacent RNA nucleotides.

Question 12

In eukaryotic organisms, pre-mRNA is modified before leaving the nucleus.

What modification is made?

Answer 12

The introns are removed (spliced) by a (splicesome.)

Question 13

Why does this modification stage (splicing) not occur in prokaryotic organisms?

Answer 13

Prokaryotes do not contain introns in their DNA.

Question 14

Use your knowledge & the diagram to outline the process of transcription:

Answer 14

The DNA helix unwinds to expose the bases to act as a template.

Only one chain of the DNA acts as a template.

The unwinding is catalysed by DNA helicase. DNA helicase breaks the hydrogen bonds between bases.
Free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases.
The enzyme RNA polymerase bonds together the RNA nucleotides to create a new RNA polymer chain. One entire gene is copied.

Question 15

Use your knowledge & the diagram to outline the process of translation:

Answer 15

mRNA attaches to a ribosome in the cytoplasm. The ribosome attaches at the start codon.

The tRNA molecule with the complementary anticodon to the start codon aligns opposite the mRNA, which is held in place by the ribosome.

The ribosome will move along the mRNA molecule to enable another complementary tRNA to attach to the next codon on the mRNA.

The two amino acids that have been delivered by the tRNA molecule are joined by a peptide bond. This is catalysed by an enzyme and requires ATP.

This continues until the ribosome reaches the stop codon at the end of the mRNA molecule. The stop codon does not code for an amino acid and therefore the ribosome detaches and translation ends.

Question 16

What is a gene mutation?

Answer 16

A change in the base sequence of the DNA

Question 17

When in the cell cycle do gene mutations occur?

Answer 17

During DNA replication

Question 18

What is a mutagenic agent?

Answer 18

radiation and chemicals increase risk of mutation

Question 19

Give two reasons why gene mutations may not cause a change to the primary structure of a protein?

Answer 19

The genetic code is degenerate, so the new triplet of bases may still code for the same amino acid.

If the mutation occurs in an intron.

Question 20

A gene mutation could result in either a base being deleted or substituted for a different one e.g.
TAC CCA AGT GGC - original DNA sequence
TAC ACA AGT GGC - mutation 1
TAC CAA GTG GC - mutation 2

Which type of gene mutation is:
mutation 1

Answer 20

substitution

Question 21

A gene mutation could result in either a base being deleted or substituted for a different one e.g.
TAC CCA AGT GGC - original DNA sequence
TAC ACA AGT GGC - mutation 1
TAC CAA GTG GC - mutation 2

Which type of gene mutation is:
mutation 2

Answer 21

deletion

Question 22

What is non-disjunction?

Answer 22

non-disjunction is when the chromosomes or chromatids do not split equally during anaphase