Genetics

Question 1

Describe the structure of RNA

Answer 1

Single stranded. Made up of ribose nucleotides. U replaces T

Question 2

How is the DNA sequence converted into proteins?

Answer 2

Transcription: reads DNA sequence and produces mRNA
Translation: reads mRNA and produces proteins

Question 3

Outline the process of transcription.

Answer 3

• DNA uncoils into 2 strands with exposed bases, one is used as a template
• Free nucleotides line up next to their complementary base pairs, joined together by RNA polymerase
• mRNA is synthesised from the antisense strand
• U pairs with A

Question 4

What happens to mRNA after transcription?

Answer 4

It moves out of the nucleus into the cytoplasm and attaches to a ribosome, ready for translation

Question 5

Outline the process of translation.

Answer 5

• mRNA attaches to a ribosome
• Each tRNA carries a specific amino acid
• The anti-codon of tRNA attaches to complementary bases on the mRNA
• Amino acids bonded to tRNA forms peptide bonds, continuing to form a polypeptide chain until a stop codon is reached

Question 6

Identify features of the genetic code.

Answer 6

• A triplet of bases code for a particular amino acid (e.g, 12 bases codes for 4 amino acids)
• Non overlapping = each triplet is only read once
  (e.g, AAT + ACC + AGT gives 4 distinct amino acids)
• Degenerate = more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids)

Question 7

What is a gene?

Answer 7

A sequence of bases on a DNA molecule that codes for a specific sequence of amino acids in a polypeptide chain

Question 8

What is substitution?

Answer 8

When one base is substituted for another

Question 9

What is deletion?

Answer 9

When a base is deleted

Question 10

What is insertion?

Answer 10

When an extra base is added

Question 11

What is a silent mutation?

Answer 11

When substitution of a base still codes for the same amino acid, this is due to its degenerate nature

Question 12

What is a nonsense mutation?

Answer 12

When a substitution of a base occurs leading to a premature ‘stop codon’ being coded for

Question 13

What is a missense mutation?

Answer 13

When a change in base (substitution) leads to a different amino acid being coded for

Question 14

What is a frameshift mutation?

Answer 14

An insertion or a deletion of a base and a nucleotide is lost from the DNA sequence

Question 15

Define the term heterzygous

Answer 15

2 different alleles

Question 16

Define the term homozygous

Answer 16

2 identical alleles

Question 17

What is meant by incomplete dominance?

Answer 17

An allele whose characteristics is not completely expressed, the phenotype is a combination of 2 alleles

Question 18

What is meant by genetic screening?

Answer 18

Determining if an individuals DNA contains a certain allele, usually one that results in a genetic disorder

Question 19

Describe the process of chorionic villus sampling

Answer 19

• A sample of embryonic tissue is taken from the placenta at around 11-14 weeks of pregnancy and is screened for various disorders

Question 20

Describe the process of amniocentesis

Answer 20

A sample of amniotic fluid is taken at around 15-20 weeks of pregnancy, collect foetal cells that have fallen off the placenta and sample the cells for diseases

Question 21

Define the process of PIGD

Answer 21

Checking genes of embryos created through IVF for genetic diseases

Question 22

What are some social and ethical issues surrounding prenatal genetic screening?

Answer 22

• procedures carry risk of harming foetus
• abortion
• high cost of bringing a child up with a condition
• emotional and mental stress on parents

Question 23

What is a stem cell?

Answer 23

Undifferentiated cells, that can divide indefinitely and can turn into other specific cell types

Question 24

What is the meaning of totipotent and pluripotent stem cells?

Answer 24

Totipotent = can develop into any cell type including the placenta and embryonic cells
Pluripotent = can develop into any cell type excluding placental cells

Question 25

Give some uses of stem cells.

Answer 25

- Treat diseases - Replace damaged tissue

Question 26

Why is the use of some stem cells controversial?

Answer 26

- Involves killing embryos - Risk of infection transplant

Question 27

How do cells become specialised (gene expression)?

Answer 27

1. Under the right conditions (stimulus), some genes are activated and others are inactivated 2. mRNA is only transcribed from the active genes it is then translated into proteins 3. These proteins modify the cell, they determine cell structure and control cell processes 4. Changes to these cells causes differentiation

Question 28

What is meant by epigenetics?

Answer 28

A heritable change in base sequence without change to the base sequence of DNA

Question 29

How does DNA methylation affect gene transcription?

Answer 29

1. Involves the addition of a -CH3 group at a CpG site (c and g base next to eachother) 2. It changes DNA structure so that the proteins and enzymes needed for transcription can't bind to the gene and transcribe it - the gene is repressed/inactivated

Question 30

What are histones?

Answer 30

- Histones are proteins that DNA wraps around to form chromatin - Chromatin can be highly condensed or less condensed - how condensed it is affects accessibility of the DNA and how easily it can be transcribed

Question 31

How can histone modification affect gene expression?

Answer 31

- When histones are acetylated (-COCH3) the chromatin is less condensed, this means proteins in transcription can bind to the DNA - When acetyl groups are removed, the chromatin becomes highly condensed and genes in the DNA cannot be transcribed

Question 32

How are epigenetics passed on?

Answer 32

All cells that arise from the altered cells will possess the modifications

Question 33

What are transcription factors?

Answer 33

Proteins that bind to DNA and activate or deactivate genes by increasing or decreasing rate of transcription

Question 34

What are activators and repressors?

Answer 34

Activators = help RNA polymerase bind to the DNA and begin transcription Repressors = prevent RNA polymerase binding and so stops transcription

Question 35

What are control elements?

Answer 35

Promoter and operator

Question 36

What is an operon?

Answer 36

A section of DNA that contains a cluster of structural genes (useful proteins) that are transcribed together

Question 37

What is lac operon?

Answer 37

When glucose isn't available for E.coli to respire, it can use lactose, the genes that produces these enzymes needed to respire lactose are found on an operon called lac operon

Question 38

What 3 structural genes does lac operon have?

Answer 38

lacZ, lacY and lacA

Question 39

What happens when lactose isn't present in E.coli?

Answer 39

1. The regulatory gene lacI produces the lac repressor, which binds to the operator site when there is no lactose present 2. This blocks transcription so lacZ, Y and A aren't transcribed

Question 40

What happens when lactose is present in E.coli?

Answer 40

1. When lactose is present, it binds to the repressor changing its shape 2. RNA polymerase can now begin transcription of the structural genes - lacZ, Y and A are transcribed and produces proteins that help the digestion of lactose

Question 41

What proteins help the bacteria digest lactose?

Answer 41

B-galactosidase and lactose permease

Question 42

Scientists have suggested that epigenetic events are involved in regulating collagen synthesis in rats given alcohol. i) Explain how epigenetic events might be involved in regulating collagen synthesis in rats given alcohol.

Answer 42

- Exposure to alcohol may lead to histone modification such as adding acetyl groups - Collagen gene more likely to be expressed

Question 43

Describe how you could use a plant tissue culture technique to show totipotency in cotton plant seedlings.

Answer 43

- Aseptic technique carried out - Agar used - Using a specific part of the seed - Good supply of light