3.1 Genes Flashcards
What is a gene?
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).
Alleles
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
Genome
The whole genetic information of an organism.
- An organisms complete set of DNA, including all of its genes.
Diploid cells
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
Haploid cell
Has one copy of each chromosome.
- Gametes
- Created by meiosis
Homozygous
Two copies of the same allele
Heterozygous
Two different alleles
Genotype and phenotype
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
Hierarchy of genetic information
- Genome - the whole genetic information of an organism
- Chromosome - long DNA molecules which may contain many genes
- Gene - short stretches of DNA which code for one characteristic
- Allele - a specific form of a gene
Mutations
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
Mutations can occur in 3 levels:
- Gene structure (point mutation)
- Chromosomal structure
- Chromosome number
Point mutations
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
Inherited vs non inherited mutations
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
Human Genome Project
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?
Comparison of number of genes, genome sizes and chromosome numbers
E. coli, humans, rice
- E. coli: 4000 protein encoding genes
- Humans: 20000 protein encoding genes
- Rice: 35000 protein encoding genes
Sickle cell anaemia
- 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.
Consequences of sickle cell anaemia
- 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)
How are new alleles formed?
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
Loci of genes
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
