Chapter 20: Gene Expression

Question 1

Define the word mutation.

Answer 1

A mutation is a change to the sequence of nucleotide bases in DNA

Question 2

Name the 6 different types of mutations.

Answer 2

Deletion
Substitution
Addition
Duplication
Inversion
Translocation

Question 3

mutagenic agents

Answer 3

factors that can increase the rate of mutation

Question 4

substitution

Answer 4

a base is replaced by another base
this can lead to the production of a non-functional protein or to no change in the amino acid sequence of the polypeptide

Question 5

How can substitution result in the production of a non-functional protein?

Answer 5

Each of the triplets in a gene code for a particular amino acid e.g. ACT-AGG-TTC

1. the base that gets substituted could be the same as the original base so the triplets stay the same e.g. ACT-AGG-TTC
2. the base that gets substituted could be different than the original e.g. AAT-AGG-TTC
   - the new triplet AAT still codes for the same amino acid (as the genetic code is degenerate) and therefore there is no change to the sequence of amino acids in the polypeptide
   - the new triplet AAT codes for a different amino acid and therefore leads to a change in the amino acid sequence of the polypeptide which may mean the protein cannot fold into a specific shape and therefore cannot carry out its function

Question 6

silent mutation

Answer 6

A mutation that doesn’t cause a change in the amino acid order

Question 7

Give 4 examples of mutagenic agents.

Answer 7

ultraviolet radiation, ionising radiation, some chemicals and viruses

Question 8

What is a deletion mutation and what effect does it have?

Answer 8

Where a base is removed from the base sequence.

This causes a frame shift to the left, affecting all the bases afterwards

Because the genetic code is non-overlapping, the bases are still read in 3’s which means the triplets are different and so will code for different amino acids to the original.

This changes the primary structure of the polypeptide possibly leading to a non-functional protein

Question 9

duplication

Answer 9

Where one or more bases are repeated

e.g. ATGCCT becomes ATCCCCT

Question 10

addition

Answer 10

Where a bases is added to the sequence

Question 11

inversion

Answer 11

Where a sequence of bases is flipped over or reversed

e.g. ATGCCT becomes ACCGTT

Question 12

translocation

Answer 12

Where a group of bases are removed from one place and added at another location and this could be movement within the same chromosome or movement from one chromosome to another

Question 13

What is the epigenome?

Answer 13

The epigenome is the layer of chemicals surrounding the genome

Question 14

What does it mean for a gene to be expressed?

Answer 14

When a gene is said to be expressed/switched on in an organism it means the gene is producing the polypeptide it codes for

Question 15

Give 3 ways mutagenic agents can increase the rate of mutations

Answer 15

1. Some types of radiation can change the structure of DNA
2. some chemicals can alter or delete bases
3. types of chemicals called base analogs can substitute a base during replication

Question 16

Mutations in particular genes can lead to cancer. How?

Answer 16

Proto-oncogenes mutate into oncogenes which can cause them to be permanently switched on which leads to cancer
Proto-oncogenes normally stimulate a cell to divide by producing proteins. When mutated they can become overactive and
- produce excess amount of growth factors
- the receptor proteins can be permanently activated

Mutations​ in tumour suppressor genes cause them to become inactive which leads to cancer
Tumour suppressor genes are involved in slowing down cell division and in carrying out apoptosis (programmed cell death) when the cell’s DNA gets damaged.
If they are inactive it means they can’t do their job so cells divide uncontrollably

Question 17

Gene expression is regulated by _____ which is controlled by _____

Answer 17

i. transcription

ii. transcriptional factors

Question 18

Describe how a hormone such as oestrogen is able to control transcription and therefore gene expression.

Answer 18

• oestrogen (steroid hormone) diffuses into cytoplasm
• binds to an oestrogen receptor forming oestrogen-oestrogen receptor complex
• receptor is attached to a DNA binding site
  (receptor and DNA binding site together make up transcriptional factor)
• complex moves into nucleus from cytoplasm
• DNA binding site binds to specific DNA sites near start of gene
• can act as an activator or a repressor depending on the type of cell and the target gene

Question 19

What are transcriptional factors?

Answer 19

proteins that control the transcription of target genes

Question 20

How do transcriptional factors control transcription and therefore gene expression?

Answer 20

• control the rate of transcription
• by binding to specific DNA sites near the start of their target gene
• some stimulate transcription or increase its rate, called activators
• help RNA polymerase bind to start of target gene which leads to transcription
• some inhibit transcription or decrease its rate, called repressors
• prevent RNA polymerase binding to start of target gene which stops transcription

Question 21

All the cells in an organism carry the same what?

Answer 21

genes

Question 22

How does RNA interference with siRNA work?

Answer 22

• involves siRNA (small double stranded RNA)
• enzyme separates the 2 strands
• single strand of siRNA binds to target mRNA
• base sequence of siRNA complimentary to some of the bases on mRNA strand
• enzyme cuts mRNA into fragments
• mRNA cannot be translated into a protein so expression of that gene stops

Question 23

How does RNA interference with miRNA work?

Answer 23

• miRNA (small single strandee)
• less specific than siRNA can target more than one mRNA molecule
• binds to target mRNA
• prevents translation of mRNA

Question 24

How does decreased acetylation of histones allow gene expression to be controlled epigenetically?

Answer 24

increases the association between histones and DNA
so DNA-histone complex is more tightly packed
transcriptional factor cannot access DNA
cannot bind to initiate transcription
gene is switched off

Question 25

increased methylation

Answer 25

methylation = adding a methyl group (CH3) to a molecule | causes deacetylation

Question 26

benign tumour

Answer 26

non-cancerous tumour

Question 27

malignant tumour

Answer 27

cancerous tumour

Question 28

Give 6 characteristics of benign tumours

Answer 28

1. less likely to be life threatening 2. do not metastasise, adhesion molecules let them stick together = primary tissue 3. can grow to be large 4. grow more slowly 5. tend to have localised effects on the body 6. tumour surrounded by a capsule so remains compact 7. rarely reoccur after treatment

Question 29

Give 8 characteristics of malignant tumors.

Answer 29

1. more likely to be life threatening 2. tend to metastasise 3. can grow to be large 4. grow quickly 5. can affect the whole body (systemic) 6. can grow into surrounding tissue 7. tend to reoccur after treatment

Question 30

A primary tumour develops and spreads into a secondary tumour

Answer 30

- the early tumour enlarges

Question 31

DNA probes

Answer 31

- small section of DNA - radioactively or fluorescently labelled - sequence of nucleotides is complimentary to desired gene

Question 32

DNA hybridisation

Answer 32

- uses DNA probe: - small section of DNA - that is radioactively or fluorescently labelled - having a sequence of nucleotides is complimentary to the desired gene - DNA being tested has strands separated - strands mixed with the probe

Question 33

acetylation

Answer 33

acetylation = adding an acetyl group to a molecule (comes from acetylcoenzyme A)

Question 34

deacetylation

Answer 34

deacetylation = removing adding an acetyl group from a molecule

Question 35

How does decreased acetylation of histones allow gene expression to be controlled epigenetically?

Answer 35

- cause deacetylation of the histones - attracts proteins that condense the DNA-histone complex - DNA is inaccessible to transcriptional factors - prevents the transcription

Question 36

epigenetics

Answer 36

The process by which environmental factors can cause heritable changes to DNA function without changing the DNA base sequence

Question 37

epigenome

Answer 37

The layer of chemical tags surrounding the DNA-histone complex which determines the shape of the DNA-histone complex, and acts as a cellular memory as the accumulation of the signals the cell has received in its lifetime.

Question 38

A less condensed DNA-histone complex means that...?

Answer 38

DNA is more accessible by transcriptional factors

Question 39

A more condensed DNA-histone complex

Answer 39

DNA is less accessible by transcriptional factors

Question 40

What is the effect of increased methylation of DNA on gene expression?

Answer 40

``` Switches a gene off Methylation = adding a methyl group (-CH3) to a molecule Methyl added to cytosine bases of DNA Induces deacetylation Attracts proteins Makes DNA-histone complex condense DNA is inaccessible to trans. factors Inhibits transcription of DNA ```

Question 41

What is the effect of decreased acetylation of DNA on gene expression?

Answer 41

Switches a gene off Acetylation = adding acetyl group to a molecule Deacetylation = removing an acetyl group from a molecule Catalysed by histone deacetylase (HDAC) Increases attraction of histones to phosphate groups on DNA By increasing positive charges on histones Makes DNA-histone complex condense DNA not accessible to transcriptional factors Inhibits transcription

Question 42

Acetylation of histones

Answer 42

Causes less condensing | Allows transcription

Question 43

Deacetylation of histones

Answer 43

Causes more condensing | Inhibits transcription

Question 44

Increased methylation

Answer 44

Causes deacetylation Deacetylation causes condensing Inhibits transcription

Question 45

Why is the epigenome described as flexible?

Answer 45

It is flexible because it responds to environmental changes.