Lesson One: What are genes?
Objectives: understand what genes are, how to describe their location, the difference between genes and alleles, a specific example of a mutation (sickle cell anaemia), know the universal code for DNA has exceptions, and know that software can be used to compare gene sequences.
- A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic eg. eye colour
- An allele is a specific form of a gene eg. HBS is the mutant haemoglobin gene which causes sickle cell anaemia, while HBB is the normal haemoglobin gene
- A genome is the sum total of all the genes present in a cell or in an organism
- A chromosome is a tightly coiled and condensed molecule of DNA associated with histone proteins.
- A gene locus is the position of a gene on a chromosome
- Single nucleotide polymorphism: a variation of one single nucleotide in a gene
- Point mutation: a mutation that results from a change of one single nucleotide in a gene
- Introns: sections of DNA which do not code for protein production (eg. gene regulatory genes)
- Exons: sections of DNA which code for protein production
A Gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.Heritable means it can be passed on, and a characteristic is something that describes you eg. hair colour.
Genes are carried on chromosomes. Each chromosome may contain thousands of genes.
Humans have approximately 20,000 genes. A bacteria, has 1,700.
Task: Some characteristics are controlled by one gene (single gene characteristics), and others are controlled by many genes (multiple gene characteristics). Other characteristics are either partially of wholly dependent on environmental factors. Classify the following according to these rules. Discuss them with your neighbours.
- Sickle cell or normal haemoglobin production
- Anaemia in general
- Skin colouration
- Skull form and shape
- Blood antigen presence (A,B,O)
- Scarring of skin
- eye colour (iris colouration)
Introductory video: Watch and ask questions if any arise.
A gene locus is the position of a gene on a chromosome. You need to learn how to ‘read’ a gene locus.
Gene locus: a real example from the human genome
The image on the left shows the gene locus for the gene controlling haemoglobin production in red blood cells.
11p15.5 is the gene locus of this gene, called the HBB gene:
- 11 means it is on chromosome 11 out of the 23 pairs.
- p means it is on the short ‘arm’ of the chromosome
- 15.5 represents region 1, band 5, and sub-band 5 (the bands show up on microscopes if the DNA is stained)
Task: explain the gene locus of one of the most studied genes, TP53 (tumor suppressant gene).
Sickle cell anaemia. introduction video
A mutated version of the normal haemoglobin gene HBB: HBS, is responsible for sickle-celled anaemia. This occurs through a single nucleotide change (when one nucleotide changes this is called a point mutation), the change is at codon 6, from the normal codon (GAG), to a mutated codon (GTG). This causes an amino acid substitution.
Task: Using the code, state the specific point source mutation that takes place, to change the HBB gene into the mutated allele HBS.
Universal code – or is it?
the code relating base codons to specific amino acids is regarded as universal i.e. applying in all organisms, but some exceptions have been found (eg. yeast). Some species where a codon normally codes for ‘stop’, it actually codes for an additional amino acid. Even for these organisms, the majority of the code applies.
Activity: Watch this interesting video on Genes.
- Q. What is an allele?
- Q. Why do people often look like their relatives?
- Q. How many chromosomes do we have?
- Q. How much DNA do we share with a banana, and with each other?
- Q. What is a SNP?
- Q. How does a SNP form (3 ways)?
- Q. What is meant by the word ‘mutation’
- Q. What is the significance of SNPs to us?
Introducing the father of modern genetics:
Lived: 19th Century Czech Republic (formerly Austrian empire)
Occupation: Augustine Monk / Geneticist
Passion: Growing and breeding peas
Big accomplishments: Discovery of Mendel’s first and second law of genetics.
1. For every gene, there are two alleles in a body cell, and one allele in a gamete
2. Law of independent assortment. Alleles for different characteristics are inherited independently.
Mendel’s First law
The specific forms of a gene are called alleles. Gregor Mendel deduced that the inheritable differences in pea plants were caused by different alleles. He also discovered that these alleles are usually inherited in pairs (one from each parent).
image credit: www.biology.arizona.edu
You have two alleles for your blood type. The genes that you have for a specific characteristic are called your ‘genotype’. Your ‘phenotype’ is the expression of those genes, in this case, what blood antigen is present.
Task: What is your genotype for blood type? What is your phenotype for blood type. Guess which allele(s) was (were) present in your mother’s ova?
The Human Genome Project
Was an international collaboration with the aim of sequencing our entire human genome. This took 14 years (1990 – 2003).
One of the discoveries of the human genome project was that some DNA is not genes, and was called ‘junk DNA’.
It has become increasingly apparent that ‘junk DNA’ was not a good name, as these other non-coding regions of DNA are important in controlling the expression of the genes eg. switching genes on an off. They are now called introns. The DNA that codes for proteins is called exons.
Gene sequences can be compared between humans and other species, using software platforms which are available online eg. Clusterx.
Task: Is the HBB gene part of an intron or an exon? Why?
3.1 Summary powerpoint