8. Molecular Genetics

Question 1

Outline the relationship between DNA, genes, and chromosomes

Answer 1

• DNA is the genetic material that organisms inherit from their parents. DNA is used to carry the genetic codes (genes), which is used to synthesise specific polypeptides.
• A molecule of DNA condenses during cell division to form a compact structure called a chromosome.
• Each chromosome contains one long DNA molecule (wrapped around protein), usually carrying several hundred or more genes.
• A gene is a unit of inheritance that occupies a specific locus on a DNA molecule. It is a short and specific sequence of nucleotides that code for a particular polypeptide, which fold to form a protein.
• The proteins contribute to the development of many characteristics in our bodies, like colour of the eye.

Question 2

Structure of DNA

Answer 2

One DNA molecule is a double helix of two complementary, anti-parallel polynucleotide chains.

DNA (polymer) consists of repeating nucleotides (monomer).

One nucleotide consists of:

• Phosphate group
• Deoxyribose sugar
• Nitrogenous base (adenine, thymine, guanine, cytosine)

The phosphate group of one nucleotide joins with the deoxyribose sugar of the next nucleotide to form the sugar phosphate backbone of the DNA molecule.

The nitrogenous bases pair with each other using complementary base pairing to form a double-stranded structure, comprising two polynucleotide strands joined in an anti-parallel manner.

Question 3

Complementary Base Pairing (Chargaff’s Rule)

Answer 3

Rules for complementary base pairing (also known as Chargaff’s Rule):

• Adenine (A) pairs with Thymine (T), forming double hydrogen bonds
• Guanine (G) pairs with Cytosine (C), forming triple hydrogen bonds

Question 4

State briefly the process of DNA replication

Answer 4

1. DNA replicates, following the process of semi-conservative replication.
2. Each polynucleotide strand of the original DNA acts as a template for the synthesis of a new complementary polynucleotide strand.
3. The two strands of the original double helix are first separated by enzymes. WIth the assistance of other enzymes, nucleotides are bound to the separated polynucleotide strands following the rules of complementary base pairing: adenine (A) to thymine (T) and guanine (G) to cytosine (C).
4. This results in the formation of 2 double-stranded DNA molecules — each molecule consisting of one half of the original parent strand and a newly synthesised daughter strand.

Question 5

Gene

Definition

Answer 5

A gene is a unit of inheritance that occupies a specific locus on a DNA molecule. It is a short and specific sequence of nucleotides that code for a particular polypeptide, which fold to form a protein in the rough endoplasmic reticulum or cytoplasm.

Question 6

DNA

Function

Answer 6

DNA is used to carry genetic code, which is used to synthesise polypeptides. DNA directs RNA synthesis, and through RNA, DNA controls protein synthesis.

Question 7

Describe how the information on DNA is used to synthesise polypeptides in eukaryotes

Process

Answer 7

Synthesis of polypeptides occurs through a two-step process: transcription (DNA to mRNA) and translation (mRNA to polypeptide).

1. In the nucleus, the message in the gene is copied into an mRNA (messenger RNA). This process is known as transcription.
2. The mRNA travels to the cytoplasm and attaches to a ribosome.
3. As the ribosome moves along the mRNA, it synthesises a polypeptide. The synthesis of the polypeptide chain from the mRNA is known as translation.
4. When the ribosome leaves the mRNA, the polypeptide is released.

Question 8

mRNA

Properties

Answer 8

• mRNA does not contain the base thymine (T), instead it is replaced by uracil (U).
• Three bases on the mRNA make up a codon (triplets of nucleotide bases).
• One codon codes for one amino acid.
• 20 different amino acids are coded by the codons.

Question 9

DNA vs RNA

Similarities & Differences

Answer 9

Similarities:

1. Both DNA and RNA have a sugar-phosphate backbone.
2. Both DNA and RNA contain three common nitrogenous bases: adenine, guanine and cytosine.

Differences:

1. mRNA is made up of one strand while DNA is made up of two strands.
2. mRNA is mostly a straight chain while DNA is a double helix.
3. Monomers of mRNA is ribonucleotides while monomers of DNA is deoxyribonucleotides.
4. Pentose sugar in mRNA is ribose sugar while pentose sugar in DNA is deoxyribose sugar.
5. Nitrogenous bases in mRNA are adenine, uracil, guanine and cytosine, while nitrogenous bases in DNA are adenine, thymine, guanine and cytosine.
6. Ratio of bases varies in RNA while in DNA, ratio of adenine to thymine and guanine to cytosine is one to one.
7. No hydrogen bonds if RNA is a straight chain, while hydrogen bonding is present in DNA.

Question 10

Mutation

Definition + Examples

Answer 10

Mutation is the spontaneous or induced change in the DNA sequence of genes, or a change in number or structure of chromosomes. This may result in the change in the phenotype of an organism.

Gene mutation: sickle cell anaemia
Chromosomal mutation: down syndrome

Question 11

Gene Mutation Example

Answer 11

Gene mutation example: sickle cell anaemia

• Caused by a gene mutation, which results in a substitution of a single amino acid in the haemoglobin protein.
• Sickle-cell haemoglobin molecules aggregate into long rods that deform the cells into a sickle shape.

Question 12

Chromosomal Mutation Example

Answer 12

Chromosomal Mutation Example: Down Syndrome

• Down syndrome is caused by the non-separation of chromosome 21 in meiosis during gamete formation. This results in a zygote having an extra chromosome 21.
• Each body cell ends up with 47 chromosomes instead of 46.

Question 13

Mutagens

What they are + Examples

Answer 13

Mutagens are physical or chemical agents that can cause alterations to DNA, resulting in changes in the sequence of DNA. They increase the rate of spontaneous mutation.

Examples of mutagens:

1. High energy radiation (ultraviolet light, X-ray, gamma ray)
2. Chemical mutagens (tar, formaldehyde in tobacco smoke)

Question 14

Genetic Engineering

What it is + How it is done

Answer 14

Genetic engineering is a technique used to transfer genes from one organism to another. Individual genes may be taken from cells of one organism and inserted into the cells of another organism of the same or different species, and this can be done using a vector.

Vectors carry the genes to the intended cells. Examples of vectors include plasmids or viruses. The transferred gene can express itself in the recipient organism.

Question 15

Why do we use bacteria to produce insulin?

Answer 15

1. It is relatively easy to transfer the genes into the bacterium. Bacteria contain plasmids, circular strands of DNA that replicate separately from bacterial DNA. A plasmid only has a small number of genes, which may be useful to the bacteria under certain conditions but is not necessary for its survival. The insulin gene can be inserted easily into a plasmid.
2. Bacteria take a short period to replicate. When the bacterium replicates, the plasmid is replicated along with it. This single cell reproduces through repeated cell divisions to form clones of itself. These bacteria then transcribe and translate the insulin gene in the plasmid to produce large quantities of insulin.

Question 16

Steps for inserting human insulin gene into bacteria DNA

Answer 16

1. Obtain the DNA segment in the human chromosome containing the insulin gene.
2. Cut out the gene using a restriction enzyme (restriction endonuclease). This enzyme cuts the two ends of the gene to produce ‘sticky ends’. Each ‘sticky end’ is a single strand sequence of DNA bases (not paired). These bases can pair with complementary bases to form a double strand.
3. Obtain a plasmid from a bacterium. Cut open the plasmid with the same restriction enzyme. This produces ‘sticky ends’ complementary to the ends of the insulin gene.
4. Mix the plasmid with the DNA fragment containing the insulin gene. The DNA fragment containing the human insulin gene will bind to the plasmid by complementary base pairing between their ‘sticky ends’. Add DNA ligase to seal the DNA fragment to the plasmid. This plasmid is a recombinant plasmid.
5. Mix the plasmid with E.coli bacterium. Apply temporary heat or electric shock. This opens up the pores in the cell membrane for the plasmid to enter.
6. The transgenic bacterium with insulin gene will be isolated and grown (in fermenters) on large scale under optimum conditions with the optimum oxygen level, glucose level, and temperature. The transgenic bacterium will synthesise insulin together with other proteins from the plasmid. The insulin must be extracted and purifed before use.

Question 17

Benefits of the Applications of Genetic Engineering

Answer 17

Applications & Benefits:

1. Low-cost production of medicines:

• Genetic engineering of important drugs such as human insulin has drastically reduced the cost of these medicines. With these drugs becoming more affordable, more patients can get access to them and be treated.

2. Production of crops that grow in extreme conditions (drought-resistant crops, salt tolerant crops, and crops that make more efficient use of nitrogen and other nutrients):

• This allows farmers to grow more groups even when the soil conditions are not suitable for cultivating most crops.

3. Development of crops that produces toxin to kill insect pests, and pesticide-resistant crops:

• The use of costly pesticides that may damage the environment is reduced. For example, the Bt gene from a certain bacterium can be inserted into plants to prouce a toxin that kills certain insect pests.

4. Development of food designed to meet specific nutritional goals:

• Improved nutritional qualitiy of foods. For example, two genes from daffodil and one gene from the bacterium Erwinia uredarora is inserted into rice plamts produce ‘Golden Rice’. The rice grains have high vitamin A content.

Question 18

Issues behind Genetic Engineering

Answer 18

1. Environmental Hazards
   Crop plants have been genetically engineered to produce insect toxins or to be resistant to herbicides.
   - resulting in deaths of beneficial insects that feed on GM crops, result in loss of biodiversity in longer term
   - Insects that feed on GM crops may adapt and develop resistance to the toxins in the crops. The insects might subsequently develop resistance to pesticides that act in a similar way to the GM plant toxin
   - Herbicide-resistant plants and weeds could crossbreed and create ‘superweeds’
2. Health Hazards
   - New proteins in GM food can cause allergies in humans that consume them
   - Modifying a single gene could cause alteration of some metabolic processes within the plant, resulting in production of toxins not usually found in the plant.
   - Genes that code for antibiotic resistance may be accidentally incorporated into bacteria that cause diseases, making antibiotics ineffective in treating these diseases.
3. Social and Ethical Issues
   - some biotechnology companies engineered crop plants such that the plants produce seeds that cannot germinate, so farmers have to keep buying seeds from the company and struggle to make a living.
   - Technology like gene therapy can lead to class distinctions. Only people with sufficient financial means can benefit from the technology
   - Some people feel that it is morally wrong to exploit animals for medical research