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Biology A-Level > Chapter 20: Gene Expression > Flashcards

Chapter 20: Gene Expression Flashcards

Learn all about what controls gene expression, how genes can be switched on or off

1
Q

Define the word mutation.

A

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

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2
Q

Name the 6 different types of mutations.

A 
Deletion
Substitution
Addition
Duplication
Inversion
Translocation
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3
Q

mutagenic agents

A

factors that can increase the rate of mutation

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4
Q

substitution

A

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

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5
Q

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

A

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
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6
Q

silent mutation

A

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

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7
Q

Give 4 examples of mutagenic agents.

A

ultraviolet radiation, ionising radiation, some chemicals and viruses

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8
Q

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

A

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

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9
Q

duplication

A

Where one or more bases are repeated

e.g. ATGCCT becomes ATCCCCT

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10
Q

addition

A

Where a bases is added to the sequence

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11
Q

inversion

A

Where a sequence of bases is flipped over or reversed

e.g. ATGCCT becomes ACCGTT

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12
Q

translocation

A

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

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13
Q

What is the epigenome?

A

The epigenome is the layer of chemicals surrounding the genome

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14
Q

What does it mean for a gene to be expressed?

A

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

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15
Q

Give 3 ways mutagenic agents can increase the rate of mutations

A
  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
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16
Q

Mutations in particular genes can lead to cancer. How?

A

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

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17
Q

Gene expression is regulated by _____ which is controlled by _____

A

i. transcription

ii. transcriptional factors

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18
Q

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

A
  • 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
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19
Q

What are transcriptional factors?

A

proteins that control the transcription of target genes

20
Q

How do transcriptional factors control transcription and therefore gene expression?

A
  • 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
21
Q

All the cells in an organism carry the same what?

A

genes

22
Q

How does RNA interference with siRNA work?

A
  • 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
23
Q

How does RNA interference with miRNA work?

A
  • miRNA (small single strandee)
  • less specific than siRNA can target more than one mRNA molecule
  • binds to target mRNA
  • prevents translation of mRNA
24
Q

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

A

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

25
Q

increased methylation

A

methylation = adding a methyl group (CH3) to a molecule

causes deacetylation

26
Q

benign tumour

A

non-cancerous tumour

27
Q

malignant tumour

A

cancerous tumour

28
Q

Give 6 characteristics of benign tumours

A
  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
29
Q

Give 8 characteristics of malignant tumors.

A
  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
30
Q

A primary tumour develops and spreads into a secondary tumour

A
  • the early tumour enlarges
31
Q

DNA probes

A
  • small section of DNA
  • radioactively or fluorescently labelled
  • sequence of nucleotides is complimentary to desired gene
32
Q

DNA hybridisation

A
  • 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
33
Q

acetylation

A

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

34
Q

deacetylation

A

deacetylation = removing adding an acetyl group from a molecule

35
Q

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

A
  • cause deacetylation of the histones
  • attracts proteins that condense the DNA-histone complex
  • DNA is inaccessible to transcriptional factors
  • prevents the transcription
36
Q

epigenetics

A

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

37
Q

epigenome

A

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.

38
Q

A less condensed DNA-histone complex means that…?

A

DNA is more accessible by transcriptional factors

39
Q

A more condensed DNA-histone complex

A

DNA is less accessible by transcriptional factors

40
Q

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

A 
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
41
Q

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

A

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

42
Q

Acetylation of histones

A

Causes less condensing

Allows transcription

43
Q

Deacetylation of histones

A

Causes more condensing

Inhibits transcription

44
Q

Increased methylation

A

Causes deacetylation
Deacetylation causes condensing
Inhibits transcription

45
Q

Why is the epigenome described as flexible?

A

It is flexible because it responds to environmental changes.

Biology A-Level (24 decks)

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