Topic 8- The control of gene expression

Question 1

What is a gene mutation

Answer 1

A change in the base sequence of DNA that can arise spontaneously during DNA replication

Question 2

What is a mutagenic agent

Answer 2

A factor that increaes rate of mutation

Question 3

Explain how any type of mutation can lead to the production of a non coding protein/enzyme

Answer 3

-Changes sequence of base triplets in DNA so changes sequence of codons on mRNA
-Changes sequence of amino acids in the enclosed polypeptide.
-Changes position of hydrogen/ionic/disulphide bonds
-So tertiary structure is changed in protein
-Enzymes active site no longer complementary to substrate, so E-S complex cannot form

Question 4

What is a substitution mutation?

Answer 4

A base/nucleotide is replaced by a different base/nucleotide in DNA.

Question 5

What is an addition mutation?

Answer 5

One or more bases/nucleotides are added to the DNA base sequence.

Question 6

What is a deletion mutation?

Answer 6

One or more bases/nucleotides are lost from the DNA base sequence.

Question 7

What is a duplication mutation?

Answer 7

A sequence of DNA bases/nucleotides is repeated/copied.

Question 8

What is an inversion mutation?

Answer 8

A sequence of bases/nucleotides detaches from the DNA sequence, then rejoins at the same position in the reverse order.

Question 9

What is a translocation mutation?

Answer 9

A sequence of DNA bases/nucleotides detaches and is inserted at a different location within the same or a different chromosome.

Question 10

Explain why not all gene mutations affect the order of amino acids

Answer 10

Some substitutions change only 1 triplet/codon which could still code for the same amino acid as the genetic code is degenerate so can be coded for by more than one triplet
-Some occur in introns which do not code for amino acids

Question 11

GIve examples of mutagenic agents

Answer 11

High energy ionising radiation - a&b particles as well as UV and X ray which can disrupt the structure of DNA
Chemicals- Such as nitrogen dioxide that may directly alter DNA structure or interfere with transcription. OR benzopyrene

Question 12

Explain why a change in amino acid sequence is not always harmful

Answer 12

-May not change tertiary structure of the protein (poistions of H/I/D) dont change
-May positively change the properties of the protein, giving the organsim a selective advantage.

Question 13

Explain what is meant by a frameshift

Answer 13

-When a gene mutation changes the number of nuclotides bases by any numer not divisible by 3
-This shifts the way the genetic code is read, so all DNA triplets/mRNA codons downstream from the mutation change
-The sequence of amino acids encoded changes accordingly and the effects on the encoded polypeptide are significant.

Question 14

What are stem cells

Answer 14

Undifferentiated or specialised cells capable of dividing by mitosis to replace themselves indefinently and can differentiate into other types of specialsed cells

Question 15

Explain how stem cells can be used in the treatement of human disorders giving an example of diabeties and blood caner

Answer 15

-Transplanted into patients to divide in ulimited numbers the differentiated into required healthy cells to replace faulty/damaged cells
-Potential treatement of Type 1 diabeties by creating healthy islet cells that produce insulin
-Bone marrow in patient can be destroyed so no fulty cells are produced then tranplant bone marrow stem cells rom healthy peson and they can divide and differentiate into healthy cells

Question 16

How do stem cells become specialised during development?

Answer 16

Stimuli lead to activation of some genes due to transcription factors. mRNA is transcribed from these genes and translated to form proteins, which modify cells permanently and determine cell structure/function.

Question 17

What are totipotent cells?

Answer 17

Totipotent cells occur for a limited time in early mammalian embryos. They can divide and differentiate into any type of body cell, including extra-embryonic cells like the placenta.

Question 18

What are pluripotent cells?

Answer 18

Pluripotent cells are found in mammalian embryos after the first few cell divisions. They can divide and differentiate into most cell types, but not placental cells.

Question 19

What are multipotent cells?

Answer 19

Multipotent cells are found in mature mammals and can divide AND differentiate into a limited number of cell types.

Question 20

Can you give an example of multipotent cells?

Answer 20

An example of multipotent cells is those in bone marrow, which can divide and differentiate into different types of blood cells.

Example: multipotent cells in bone marrow can divide and differentiate into different types of blood cell.

Question 21

What are unipotent cells?

Answer 21

Unipotent cells are found in mature mammals and can divide and differentiate into just one cell type.

Question 22

Can you give an example of unipotent cells?

Answer 22

An example of unipotent cells is those in the heart, which can divide and differentiate into cardiomyocytes (cardiac muscle cells).

Question 23

Describe how oestrogen stimulates transcription

Answer 23

-Oestrogen is a lipid soluble molecules so it diffuses across cell membrane
-Oestrogen binds to receptor on transcription factors forming oestrogen-receptor complex
-This changes the shape of the inactive transcription factor forming an active transcription factor
-The complex diffuses from cytoplasm into the nucleus
-This then binds to a specific DNA base sequence on the promoter region of a target gene
-This stimulates the transcription of a target gene forming mRNA by helping RNA polymerase to bind.

Question 24

Why does oestrogen only affect target cells

Answer 24

Other cells do not have oestrogen receptors

Question 25

What is epigenetics

Answer 25

Heritable changes in gene function without changes to the DNA base sequence caused by changes in the environment.

Question 26

What are transcription factors?

Answer 26

Proteins that regulate transcription of specific target cells in eukaryotes by binding to a specific DNA base sequence on a promoter region.

Question 27

How can transcription be regulated using transcription factors

Answer 27

-Transcription factors move from the cytoplasm to the nucleus - Bind to DNA at a specific DNA base sequence on a promoter region -This stimulates or inhibits transcription of target genes by helping /preventing RNA polymerase binding

Question 28

What is an epigenome?

Answer 28

All chemical modification of DNA and histone proteins, Methyl groups on DNA and acetyl groups on histones

Question 29

Describe the role of methylation in the inhibition of transcription

Answer 29

-Increase methylation means more methyl (CH3) groups added to cytosine bases in DNA. -So more nucleosomes (DNA wrapped around histone) pack more tightly together -Preventing transcription factors and RNA polymerase binding to promoter.

Question 30

Describe how acetylation can inhibit transcription

Answer 30

-Decreased acetylation of histones increases positive charge of histones -Histones bind DNA (negatively charged) more tightly -This prevents transcription factors and RNA polymerase binding to promoter

Question 31

What is RNA interference?

Answer 31

-Inhibition of translation of mRNA produced from target genes, by RNA molecules (siRNA) -This inhibits expression of a target gene

Question 32

Describe the regulation of translation by RNA interference

Answer 32

-Enzyme cuts large double stranded molecules of RNA into smaller sections (siRNA or miRNA) -This then binds with a protein/enzyme forming an RNA induced silencing complex -Single stranded miRNA/siRNA in the complex binds to the target mRNA with a complementary base sequence -This leads to the hydrolysis of MRNA into fragments which prevents ribosomes binding -This reduces/prevents translation of the target mRNA into a protein

Question 33

Describe how mutations and cancers form

Answer 33

-Mutations in the DNA/ genes controlling mitosis can lead to uncontrolled cell division -Tumour formed if this results in mass abnormal cells

Question 34

Describe the key differences and similarities and features of malignant and benign tumours

Answer 34

-They both can grow to a very large size -B grows slower M grows faster -B cells are specialised M cells are unspecialised --B cells produce adhesion molecules so they can stick together, M cells dont so they spread to other regions of the body by process called metastasis causing secondary tumours -B tumours surrounded by capsule of dense tissue so remain compact, M cells are not so can grow finger like projections -B usually removed by surgery alone, M needs chemo/radiotherapy -B rarely return after treatment, M often returns

Question 35

Explain the role of tumour suppressor genes in the development of a tumour

Answer 35

Firstly, this can be caused by a mutation in DNA base sequence which can lead to the production of non functional protein by leadijg to a change in amino acid sequence which changes tertiary structure Secondly, Decreased histone acetylation or increased DNA methylation prevents production of proteins by preventing binding of RNA polymerase to promoter region inhibiting transcription -Both of these factors lead to uncontrolled cell division and cells division cannot be slowed as the tumour suppresor gene is slienced

Question 36

What is the function of proto onco genes

Answer 36

They code for proteins that stimulate cell division

Question 37

Explain the role of oncogenes in the development of a tumour

Answer 37

An oncogene is a mutated/ abnormally expressed form of the corresponding proto oncogene It can be caused by a mutation in the DNA base sequence which leads to the overproduction of protein or permanently activated protein by leading to a change in the amino acid sequence changing tertiary structure -In addition, decreased methylation or increased histone acetylation increases the production of protein by stimulating binding RNA polymerase to promoter region, stimulating transcription. This bothleads to uncontrolled cell division as cell division is permanently stimulated.

Question 38

Suggest why tumours require mutations in both alleles of a suppressor gene but only one allele of an oncogene

Answer 38

This is because, tumour suppresor genes normally slow down cell division or trigger apoptosis and both alleles from each parent need to be mutated to cause cancer, otherwise the one non mutated one could slow down cell division. However, oncogenes normally stimulate cell division when needed, only one need to be mutated to cause cancer as the signal to divide is already being sent by that one.

Question 39

Explain the relevance of epigenetics in treating cancer

Answer 39

For mutations cause by oncogenes, treatment could involve increasing methylation or decreasing histone acetylation of oncogene which will inhibit transcription and expression -If the tumour is associated with tumour suppressor genes, Decreasing DNA methylation or increasing histone acetylation of tumour supprsesor gene will stimulate transcription which can restore normal cell behaviour

Question 40

Explain the role of increased oestrogen conc in breast cancers

Answer 40

Fat cells tend to produce more oestrogen after menopause -Some breast cancer cancers cells have oestrogen receptors, which are inactive transcription factors -If oestrogen is increased, more oestrogen binds to oestrogen receptors forming oestrogen-receptor complexes which are active trascription factors -These bind to promoter of genes that code for proteins stimulating cell division -This increases transcription/ expression of these genes, increasing the rate of cell division.

Question 41

Suggest how a drug can help treat oestrogen receptor breast cancers

Answer 41

-Drugs with a similar shape can bind to oestrogen receptors preventing binding of oestrogen -So few/no transcription factors bind to promoter region of genes that stimulate the cell cycle.