Chapter 19

Question 1

Define Mutation

Answer 1

A mutation is a change to the sequence of the bases in DNA which may affect the phenotype of the organism

Question 2

What causes mutations

Answer 2

• Mutations can happen randomly, with no external cause but the rate of mutation is increases by the presence of mutagens (EG: ionising radiations)
• Another mutagen: Free radicals can affect structures of nucleotides and also disrupt base pairing , increasing the likelihood of mutation

Question 3

What are the terms to describe the 3 types of gene mutation

Answer 3

1- Substitution
2- Deletion
3- Insertion

Question 4

What does is substitution?

Answer 4

A gene mutation where one or more bases are swapped for another

Question 5

What is Deletion?

Answer 5

A gene mutation where one or more bases are removed

Question 6

What is Insertion?

Answer 6

A gene mutation where one or more bases are added

Question 7

Why are deletion and insertion called “frameshift mutations”?

Answer 7

Insertion or deletion leads to a frameshift mutation which would change every successive
codon read from the point of mutation (since the genetic code is triplet and
non-overlapping), so the amino acids would change.

Question 8

What is a frameshift mutation?

Answer 8

A frameshift mutation in a gene refers to the insertion or deletion of nucleotide bases in numbers that are not multiples of three

Question 9

When are insertion or deletion not
frameshift mutations?

Answer 9

If the number of added or deleted nucleotides is a multiple of 3, the reading frame is not
actually changed, so the protein would be changed but would still form.

Question 10

What might the possible effects of a gene
mutation of a protein be?

Answer 10

1- Beneficial
2- Damaging
3- Neutral/silent
4- Nonsense mutations
5- Missense mutations

Question 11

Why is an effect of a gene mutation beneficial?

Answer 11

New and useful characterists develops in the phenotype; this is the basis of evolution by natural selection and is rare)

Question 12

Why is an effect of a gene mutation Damaging?

Answer 12

(proteins are no longer synthesised or are non-functional, which can interfere with
essential processes)

Question 13

Why is an effect of a gene mutation Neutral/silent?

Answer 13

(mutant triplet codes for same amino acid or mutant triplet changes amino acid but
there is no effect on the function of the protein or mutation occurs in non-coding part of DNA)

Question 14

Why is an effect of a gene mutation nonsense mutations ?

Answer 14

(codon becomes a stop codon, resulting in a shortened protein which is
likely unfunctional)

Question 15

Why is an effect of a gene mutation missense mutation?

Answer 15

(changes an amino acid in the primary structure; the new amino acid may
have similar properties to the original (conservative mutation) or vastly different properties to the
original (non-conservative mutation))

Question 16

What are the types of chromosome mutations?

Answer 16

Sections of chromosomes may :

• break off (deletion);
• be duplicated;
• break off and join another non-homologous chromosome (translocation);
• or break off, be reversed and rejoin the same chromosome (inversion)

Question 17

(b) Lac operon: Define ‘operon’ and describe
how they are expressed.

Answer 17

An operon is a sequence of DNA containing a cluster of genes under the control of a single promoter and operator.

Hence, genes within an operon will always be expressed together or not at all (their
expression patterns are linked).

Question 18

(b) Lac operon: Why are operons
advantageous in prokaryotes?

Answer 18

In general, an operon will contain genes that function in the same process. For instance, a
well-studied operon called the lac operon contains genes that encode proteins involved in
uptake and metabolism of a particular sugar, lactose.

Operons allow the cell to efficiently express sets of genes whose products are needed
at the same time.

Question 19

Lac operon: Define “promoter region”

Answer 19

Upstream sequence to which RNA polymerase binds.

Question 20

Lac operon: Define “operator region”

Answer 20

Segment of DNA to which a repressor protein binds (inhibits transcription by obstructing
RNA polymerase).

Question 21

Lac operon: Define “structural gene” and give the 3 in Lac operon

Answer 21

Genes that code for structural proteins or enzymes not involved in DNA regulation.

In lac operon:

lacZ (codes for β-galactosidase),

lacY (codes for lactose permease)

and lacA (codes for transacetylase).

Question 22

Lac operon: Give the roles of the proteins coded for lacZ , lacY and lacA

Answer 22

lacZ codes for β-galactosidase: Enzyme which catalyses hydrolysis of lactose into
glucose and galactose.

lacY codes for lactose permease: Protein which transports lactose into the cell.

lacA codes for transacetylase: Relevance in lactose metabolism is not entirely clear

Question 23

Lac operon: Define “regulatory gene” and give the 1 in Lac operon

Answer 23

A gene that codes for proteins involved with DNA regulation (activator or repressor).

In lac operon:

LacI (codes for repressor protein which prevents transcription of the structural genes in
absence of lactose).

Question 24

Is Lac operon an inducible or repressible operon?

Answer 24

inducible, meaning that it can be turned on by the presence of a particular small molecule.

Question 25

Lac operon, outline the changes in Lac operon as glucose concentration falls then rises again

Answer 25

(table)

Question 26

Lac operon: What is the role of cAMP and how does glucose affect cAMP concentration?

Answer 26

CRP (cAMP receptor protein) can only bind to RNA polymerase in the presence of
cAMP, upregulating the activity of RNA polymerase to make transcription more efficient.

The transport of glucose into an E. coli cell decreases levels of cAMP, reducing the
transcription of the genes responsible for digesting lactose.

Question 27

(b) EUKARYOTIC CONTROL: How is
gene regulation controlled at the
transcriptional level? [2]

Answer 27

Histone modification:

Negatively-charged DNA coils around positively-charged histones in
eukaryotic cells. The resulting DNA/histone complex is called a chromatin.

Genes in heterochromatin cannot be transcribed because RNA polymerase
cannot access the genes. Genes in euchromatin, however, can be freely
transcribed. This regulation ensures the proteins necessary for cell division
are synthesised in time, and that energy-consuming protein synthesis
doesn’t occur while the cells are actually dividing.

Transcription factors:

Proteins controlling rate of
transcription by promoting
(as an activator) or blocking
(as a repressor) the binding
of RNA polymerase to
specific gene to turn genes “on” and “off”

Question 28

(b) EUKARYOTIC CONTROL: How is
gene regulation controlled at the
post-transcriptional level? [2]

Answer 28

RNA-processing in the nucleus:

Pre-mRNA converted to mature mRNA:

Cap (modified nucleotide) is added to the 5’
end and tail (long chain of adenine
nucleotides) is added to the 3’ end to stabilise
the mRNA and prevent degradation in the
cytoplasm

RNA splicing by spliceosome, which removes
introns and joins exons together.

RNA-editing:

Addition, deletion or substitution of bases.

Can produce different proteins from 1 mRNA
molecule.

Question 29

(b) EUKARYOTIC CONTROL: How is
gene regulation controlled at the translational
level? [2]

Answer 29

mRNA degradation by hydrolytic enzymes in
cytoplasm:

siRNA complementary to mRNA stand bind to
the mRNA, breaking it down.

More resilient mRNA lasts longer (more
protein is produced).

Binding of inhibitory proteins:

Inhibitory proteins bind to mRNA, preventing
mRNA from binding to ribosomes.

(Not transcription factor!)

Question 30

(b) EUKARYOTIC CONTROL: How is
gene regulation controlled at the post-
translational level? [1]

Answer 30

Protein activation, e.g.:

Addition of non-protein groups (carbohydrates, lipids, phosphate groups, etc.)

Modification of amino acids and formation of disulphide bridges

Folding or shortening of proteins

Modification by cAMP (e.g. CRP binding to CRP in lac operon).

Question 31

(c) What is a homeobox gene and what is its
role?

Answer 31

A homeobox gene is a regulatory gene (codes for
transcription factor) which contains a homeobox
sequence.

The homeobox sequence is 180 base-pairs long and
codes for a 60 amino acid long homeodomain in a
homeoprotein.

The homeobox sequence is highly conserved in plants,
animals and fungi.

The homeobox gene controls the development of body
plan.

Homeobox genes regulate mitosis and apoptosis in
response to internal and external stimuli (e.g. stress).

Question 32

What are Hox genes and what is its role? What is the consequence of mutated Hox genes?

Answer 32

Group of homeobox genes only present in animals.
They are responsible for the correct positioning of body parts.
Mutated Hox genes can cause abnormalities in body plan.

Question 33

d) Why is mitosis important in controlling the development of the body form?

Answer 33

Mitosis increases the number of cells leading to growth

Question 34

d) Why is apoptosis important in controlling the development of the body form?

Answer 34

Apoptosis removing the unwanted/surplus cells allowing different body parts to be shaped e.g. allowing separation of digits of hands and feet.
Cells undergoing apoptosis may also release chemical signals which stimulate mitosis and cell proliferation, leading to the remodelling of tissues.