8. The Control Of Gene Expression

Question 1

What are the 6 types of gene mutation

Answer 1

Substitution of bases
Addition of bases
Deletion of bases
Duplication of bases
Inversion of bases
Translocation of bases

Question 2

What are the 3 possible consequences of base substitution ?

Answer 2

• Formation of one of the 3 stop codons ie. production of polypeptide would be stopped prematurely
• Formation of a codon for a different amino acid ie. shape change
• Formation of a different codon but one that produces the same amino acid as before ie. mutation will have no effect

Question 3

What is the consequence of a base deletion ?

Answer 3

Creates a frame shift to the left
Gene is read in the wrong codons and so the coded information is altered
Polypeptide will likely lead to the production of a non-functioning protein that could considerably alter the phenotype

Question 4

What is the effect of the addition of bases

Answer 4

Usually similar effect to base deletion as there is usually a frame shift (however to the right this time)
However if 3 (or any multiple of 3) bases are added there will not be a frame shift and so the resulting polypeptide will be different but to a lesser extent than if there was a frame shift

Question 5

What is the consequence of the duplication of bases

Answer 5

One of more of the bases are repeated- produces a frame shift to the right

Question 6

What is inversion of bases and what is the consequence ?

Answer 6

A group of bases becomes separated from the DNA sequence and rejoin at the same position but in the inverse order
The base sequence of this portion is therefore reversed and affects the amino acid sequence that results

Question 7

What is the translocation of bases and what is its effect?

Answer 7

A group of bases become separated from the DNA sequence on one chromosome and reinserted into the DNA sequence of a different chromosome
Leads often to abnormal phenotype, including the development of certain forms of cancer and reduced fertility

Question 8

What is included under the term mutagenic agents

Answer 8

High energy ionising radiation - can disrupt the structure of DNA

Chemicals - alters the structure of DNA or interferes with transcription

Question 9

What is cell differentiation?

Answer 9

The process by which each cell develops into a specialised structure suited to the role it will carry out

Question 10

What are stem cells

Answer 10

Undifferentiated dividing cells that occur in adult animal tissue and need to be constantly replaced. They have the ability to perform self-renewal

Question 11

What are totipotent stem cells

Answer 11

Found in the early embryo and can differentiate into any type of cell

Question 12

What are pluripotent stem cells

Answer 12

Found in embryos and can differentiate into almost any type of cell

Question 13

What are multipotent stem cells

Answer 13

Found in adults and can differentiate into a limited number of specialised cells

Question 14

What are unipotent stem cells

Answer 14

Can only differentiate into a single type of cell.
Derived from multipotent stem cells and are made in the adult tissue

Question 15

What are induced pluripotent stem cells (iPS cells) ?

Answer 15

A type of pluripotent cell that is produced from unipotent stem cells.
These cells are genetically altered in a lab to make them acquire the characteristics of embryonic stem cells (involves inducing genes and transcriptional factors within the cells to express themselves)

Question 16

What are transcription factors?

Answer 16

Proteins that control the rate of protein synthesis by switching some genes on and some off

Question 17

What is a promoter region

Answer 17

The short sequence of DNA at the start of a gene (where dna/rna polymerase attaches)

Question 18

Give the steps in controlling gene expression by controlling transcription

Answer 18

Transcriptional factors move from the cytoplasm into the nucleus
Each transcriptional factor has a promoter region
When it binds, it causes this region to begin the process of transcription
mRNA is produced and the info it carries is then translated into a polypeptide

Question 19

What is the effect of hormones (such as oestrogen) on gene transcription

Answer 19

• oestrogen is lipid soluble & therefore diffuses easily across the phospholipid bilayer
• oestrogen then binds with a site on the receptor molecule of the transcription factor by complementary pairing
• the binding of the oestrogen changes the shape of the DNA binding site of the transcription factor which can now bind to the DNA
• the transcription factor enters the nucleus through the nuclear pores and binds to the specific base sequence on DNA (promoter region)
• transcription starts

Question 20

What is epigenetics

Answer 20

Provides explanations as to how environmental influences such as diet, stress, toxins etc can alter genetic inheritance of an organisms offspring

Question 21

What is the epigenome

Answer 21

Both DNA and histones are covered in chemicals sometimes called tags.
These tags form the epigenome

Question 22

What is the role of the epigenome

Answer 22

Determines the shape of the DNA-histone complex
Keeps genes that are inactive in a tightly packed arrangement, ensuring they cannot be read (switches the gene off)
Unwraps active genes so the DNA is exposed and can easily be transcribed (switches the gene on)

Question 23

What is epigenetic silencing referring to

Answer 23

The action of switching genes off

Question 24

What is the epigenome referred to as ‘flexible’

Answer 24

Because the chemicals tags respond to environmental changes
Factors such as diet and stress.can cause the chemical tags to adjust the wrapping and unwrapping of DNA

Question 25

What are the 2 processes that inhibit transcription

Answer 25

Decreased acetylation of histones
Methylation of DNA

Question 26

When the association of histones with DNA is weak…

Answer 26

DNA-histone complex is less condensed
Therefore accessible to transcription factors which can initiate production of mRNA (switch the gene on)

Question 27

When the association of DNA with histones is strong…

Answer 27

DNA-histone complex is more condensed
Therefore DNA is not accessible to transcription factors which therefore cannot initiate mRNA production (gene is switched off)

Question 28

How does decreased acetylation inhibit transcription

Answer 28

Decreased acetylation increases the positive charges on histones and therefore increases their attraction to the phosphate groups of DNA
Association between DNA and histones is stronger so DNA is not accessible to transcription factors = mRNA production is not initiated
Gene is switched off

Question 29

How does methylation of DNA inhibit transcription ?

Answer 29

2 ways:
- prevents the binding of transcriptional factors to DNA
- attracts proteins that condense the DNA-histone complex making the DNA inaccessible to transcription factors

Question 30

Give the step by step process of siRNA operation

Answer 30

• An enzyme cuts large double stranded molecules of RNA (dsRNA) into smaller sections (siRNA)
• One of the 2 siRNA strands combines with an enzyme to form a siRNA-protein complex
• The siRNA molecule guides the enzyme to a mRNA by pairing its bases with the complementary ones on the mRNA molecule
• The enzyme then cuts the mRNA into smaller sections
• The mRNA is no longer capable of being translated into a polypeptide
• Gene has been blocked

Question 31

What are cancerous tumours referred to as ?

Answer 31

Malignant
(non-cancerous tumours are referred to as Benign)