3.1 Genes

Question 1

What is a gene?

Answer 1

A heritable factor that consists of a length of DNA and influences a specific characteristic.
- It occupies a specific position on a chromosome.
- A single chromosome may have lots of genes, each in a specific locus (plural: loci).

Question 2

Alleles

Answer 2

Various forms of a gene with slight differences in their base sequences (alleles only differ from each other by one or a few bases).
- At the same loci

Question 3

Genome

Answer 3

The whole genetic information of an organism.
- An organisms complete set of DNA, including all of its genes.

Question 4

Diploid cells

Answer 4

Has a pair of each kind of chromosome.
- Somatic cells
- Humans are diploid and inherit one copy of each gene maternally and the other paternally –> humans have two copies of every gene

Question 5

Haploid cell

Answer 5

Has one copy of each chromosome.
- Gametes
- Created by meiosis

Question 6

Homozygous

Answer 6

Two copies of the same allele

Question 7

Heterozygous

Answer 7

Two different alleles

Question 8

Genotype and phenotype

Answer 8

Genotype: the alleles you have for a gene
Phenotype: the trait you have as a result from your alleles
Example: if a plant inherits a red petal colour allele from both parents, its genotype would be homozygous for red, and its phenotype would be flowers with red petals

Question 9

Hierarchy of genetic information

Answer 9

1. Genome - the whole genetic information of an organism
2. Chromosome - long DNA molecules which may contain many genes
3. Gene - short stretches of DNA which code for one characteristic
4. Allele - a specific form of a gene

Question 10

Mutations

Answer 10

A permanent change in the sequence of nucleotides (bases) in DNA.
- Caused by mutagens (chemicals and radiation)
- New alleles are formed, they are usually harmful or have no effect but in some cases they can be beneficial

Question 11

Mutations can occur in 3 levels:

Answer 11

• Gene structure (point mutation)
• Chromosomal structure
• Chromosome number

Question 12

Point mutations

Answer 12

When a single nucleotide or triplet of nucleotides is altered which will usually only affect one gene.
Caused by:
1. substitutions - one base in the DNA is substituted for another base
2. insertions of deletions of one or more bases

Question 13

Inherited vs non inherited mutations

Answer 13

Inherited: occurs in gametes
Non inherited: occurs in body cells, may trigger the onset of cancer through the disruption of normal controls regulating cell division

Question 14

Human Genome Project

Answer 14

The entire base sequence of human genes was sequenced in the Human Genome Project.

Ethical issues:
- Genetic manipulation
- How do we collect the information?
- Who has access to the information?

Question 15

Comparison of number of genes, genome sizes and chromosome numbers

E. coli, humans, rice

Answer 15

• E. coli: 4000 protein encoding genes
• Humans: 20000 protein encoding genes
• Rice: 35000 protein encoding genes

Question 16

Sickle cell anaemia

Answer 16

• Caused by a point mutation (single base substitution) within the haemoglobin gene.
• Mutation in the DNA sequence that substitutes a thymine instead of an adenine. From GAG to GTG (the not transcribed strand)
• On the transcribed strand it changes from CTC to CAC.
• Therefore, when the DNA is transcribed, it changes the mRNA code at that location from GAG to GUG which changes the amino at that location from glutamic acid to valine.
• As a result, haemoglobin molecules don’t form properly, causing red blood cells to be rigid and have a concave shape. This makes it difficult for cells to carry oxygen and move around the bloodstream as they can often get stuck and further block bloodflow.

Question 17

Consequences of sickle cell anaemia

Answer 17

• The amino acid change from Glu to Val alters the structure of haemoglobin causing it to form insoluble fibrous strands
• The insoluble haemoglobin cannot carry oxygen as effectively, causing the individual to feel constantly tired
• The shape of the sickled red blood vessels can cause them to get stuck and clog vessels anywhere in the body
• The sickle cells are destroyed more rapidly than normal cells, leading to a low red blood cell count (anaemia)

Question 18

How are new alleles formed?

Answer 18

By mutations.

Point mutations:
1. substitutions - one base in the DNA is substituted for another base
2. insertions of deletions of one or more bases

Question 19

Loci of genes

Answer 19

It’s important to remember that the same gene for each person has the same loci.
- The one thing that may differ is the allele