20. Gene Expression

Question 1

What is a gene mutation?

Answer 1

A change to one or more nucleotide bases, or any rearrangement of the bases

Question 2

What are the 6 types of gene mutation?

Answer 2

• Substitution
• Deletion
• Addition
• Duplication
• Inversion
• Translocation

Question 3

Explain the substitution gene mutation

Answer 3

A nucleotide in a section of a DNA molecule is replaced by another nucleotide that has a different base

Question 4

What are the 3 consequences that can arise from a substitution gene mutation?

Answer 4

• Formation of a stop codon that would mean the polypeptide being coded would be stopped prematurely
• Formation of a codon for a different amino acid, changing the tertiary structure of the protein, possibly making it non-functional
• The formation of a different codon that still produces the same amino acid, therefore having no effect

Question 5

Explain the deletion gene mutation

Answer 5

The removal of a nucleotide from a DNA sequence, resulting in a frame shift to the left, which changes all the triplets past where the deletion occurred. This makes most triplets produce different amino acids, resulting in a different, non-coding polypeptide being made

Question 6

Explain the addition gene mutation

Answer 6

An extra base is inserted into the sequence, resulting in a frame shift to the right. Multiple bases can be added at once and if a multiple of 3 is added, no frame shift will occur. Results in either a completely different polypeptide or just an extra codon, which will still affect the polypeptide

Question 7

Explain the duplication gene mutation

Answer 7

One or more bases are repeated, producing a frame shift to the right

Question 8

Explain the inversion gene mutation

Answer 8

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

Question 9

Explain the translocation gene mutation

Answer 9

A group of bases become separated from the sequence on one chromosome and become inserted into the DNA sequence of a different chromosome. Translocations often have significant effects on gene expression leading to abnormal phenotype. These effects include the development of certain forms of cancer and also reduced fertility

Question 10

What are outside factors that can influence the basic mutation rate called?

Answer 10

Mutagenic agents

Question 11

What are 2 examples of mutagenic agents?

Answer 11

• High energy ionising radiation, e.g. α or ß particles
• Chemicals, e.g. nitrogen dioxide

Question 12

How are the proteins a cell produces coded for?

Answer 12

By the genes that are expressed

Question 14

What cells can mature into any body cell?

Answer 14

Totipotent cells

Question 15

What are stem cells?

Answer 15

Undifferentiated dividing cells that occur in adult animal tissues and need to be constantly replaced

Question 16

What sources do stem cells originate from in mammals?

Answer 16

• Embryonic stem cells
• Umbilical cord blood stem cells
• Placental stem cells
• Adult stem cells

Question 17

What are the 4 types of stem cell?

Answer 17

• Totipotent stem cells
• Pluripotent stem cells
• Multipotent stem cells
• Unipotent stem cells

Question 18

What are totipotent stem cells?
Where are they found?

Answer 18

Found in the early embryo
Can differentiate into any type of cell

Question 19

What are pluripotent stem cells?
Where are they found?

Answer 19

Found in embryos
Can differentiate into almost any type of cell

Question 20

What are multipotent stem cells?
Where are they found?

Answer 20

Found in adults
Can differentiate into a limited number of specialised cells

Question 21

What are unipotent stem cells?
Where are they found?

Answer 21

Found and made in adult tissue
Can only differentiate into a single type of cell

Question 22

What are induced pluripotent stem cells (iPS cells)? How are they made?

Answer 22

A type of pluripotent cell that is produced from unipotent stem cells

Made by genetically altering body cells to make them acquire the characteristics of embryonic stem cells, which are pluripotent

Question 23

How can iPS cells be beneficial to humans?

Answer 23

They can self-renew indefinitely, so can provide a limitless supply of embryonic stem cells, which can be used in treatment without using embryos

Question 24

What are 3 ways that pluripotent stem cells can be used to treat humans?

Answer 24

Nerve cells - to treat neurodegenerative disease such as Parkinson’s
Skin cells - to treat burns and wounds
Heart muscle cells - to treat heart damage

Question 25

What is the general process of controlling the expression of a gene by controlling transcription? (6)

Answer 25

- For transcription to begin the gene is switched on by specific molecules that move from the cytoplasm into the nucleus. These molecules are called **transcriptional factors** - Each transcriptional factor has a site that binds to a specific base sequence of the DNA in the nucleus - When it binds, it causes this region of DNA to begin the process of transcription - mRNA is produced and the information it carries is then translated into a polypeptide - When a gene is not being expressed (switched off) the site on the transcriptional factor that binds to the DNA is not active - As the site on the transcriptional factor binding to the DNA is inactive it cannot cause transcription and polypeptide synthesis

Question 26

What is the process of oestrogen switching on a gene to start transcription?

Answer 26

- Oestrogen diffuses through the phospholipid bilayer as it is lipid-soluble - Once in the cytoplasm, it binds with a site on a receptor molecule of the transcriptional factor. The shape of this site and the shape of the oestrogen molecule complement one another - By binding with the site, the oestrogen changes the shape of the DNA binding site on the transcriptional factor, which is now activated - The transcriptional factor can now enter the nucleus through a nuclear pore and bind to specific base sequences on DNA - The combination of the transcriptional factor with DNA stimulates transcription of the gene that makes up the portion of DNA

Question 27

What is the epigenome?

Answer 27

DNA and histones are covered in chemicals, known as tags, forming a second layer, which is the epigenome

Question 28

What does the epigenome determine?

Answer 28

The shape of the DNA-histone complex. Genes that are inactive are kept in a tightly packed arrangement and therefore ensures that they cannot be read, known as **epigenetic silencing**

Question 29

What is the epigenome that DNA code is not? Why?

Answer 29

DNA code is fixed whereas the epigenome is flexible. This is because its chemical tags respond to environmental changes. Factors like diet and stress can cause the chemical tags to adjust the wrapping and unwrapping of the DNA, switching genes on and off