Genetic Diversity

Organisms show variation due to genetic diversity - total number of different alleles of genes in a species/population - allows populations to adapt to changing environments

Organisms vary by proteins - sections of DNA called genes code for each individual protein - members of the same species have the same genes but may be physically different

Differences within species are due to different alleles - if a population contains many different alleles for a certain gene = genetically diverse

In one species there is a set number of alleles = gene pool - bigger gene pool = greater the variation within the species = greater genetic diversity

Greater genetic diversity = better chance of survival of the certain species as they are able to adapt to changes in their environment

Genetic Diversity = variation within a population

Specific Alleles increase in proportion over time and some decrease - dependent on if they cause differential changes in survival and reproduction

Causes of Genetic Diversity

Genetic Diversity in a species is resulted of :

• Mutations

• Meiosis - Crossing Over & Independent Segregation

• Random fusion of Gametes

Mutations

A mutation is a change in the sequence o bases in the DNA of an organism

Can produce a change in the characteristics of the organism which can be passed down to cells produced through division

Hereditary mutations - inherited from parents and can cause genetic disorders e.g., cystic fibrosis

Acquired Mutations - Occur after fertilisation and are often associated with mutagenic agents

Mutations can affects genes or whole chromosomes - with either a positive, neutral or negative affect

Types of Gene Mutations

• Substitution - a base is swapped with another - can result in silent mutations

• Deletion - one or several bases are removed - often results in a frame shift

• Addition - One or several bases are added - often results in a frame shift

• Duplication - One or more bases are repeated

• Inversion of bases - a group of bases become separated from DNA and re-join at the same position but in inverse order

• Translocation of bases - a group of bases become separated from DNA sequence on one chromosome and is inserted into the DNA of the same or different chromosome - Significant effects on gene expression - abnormal phenotype

Silent Mutations - The genetic code is degenerate - some amino acids are coded for by more than 1 DNA triplet - causes one change in the polypeptide chain

Frameshift - added/removed - changing the way the DNA sequence is read - Change in base sequence = different amino acids = different primary structure of proteins = Different tertiary structure due to bonds forming in different positions = proteins function changes

If the protein coded for an enzyme = Different Tertiary structure - bonds in different positions so the active site changes shape and the substrate is no longer complementary = no enzyme-substrate complexes being formed

Non-disjunction - Changes in the number of individual homologous pairs of chromosomes due to failing to separate during meiosis - Down’s Syndrome

Mutagenic Agents - Increase the rate of mutations occurring

• High Energy Ionising Radiation

• Chemicals - Carcinogens

• Some viruses/bacteria e.g. HPV

Meiosis

Cell division that produces genetically unique gametes - causes variation within a species

• Two nuclear divisions - 4 daughter cells formed

• Daughter cells are haploid cells - humans is 23(46 diploid)

Before:

• DNA unravels and replicates so each chromosome has 2 copies of the DNA

• Copies = chromatids

• 2 sister chromatids - identical and joined by a centromere

• Interphase/ S phase

Homologous Chromosomes - Matching Pairs of Chromosomes - same size and contain the same genes with different alleles - alleles with the same characteristic are found at the same fixed point(loci) at each chromosome

Meiosis :

• Meiosis 1 - Separation of Homologous Pairs and the cells become haploid

• Prophase 1 - Chromosomes are visible and join together to form a bivalent - where crossing over occurs

• Metaphase 1 - Bivalents line up on equator - independent Segregation occurs here

• Anaphase 1 - Homologous Chromosomes separate - not by centromere or into chromatids

• Telophase 1 - Nuclei forms and cell divides

• Meiosis II - Separation of sister chromatids

• Prophase II - Centrioles move to new poles

• Metaphase II - Chromosomes line up on equator

• Anaphase II - Centromeres split and chromatids separate - moving to the pole of each cell

• Telophase II - 4 haploid cells are produced

Meiosis → Variation

Creates Intraspecific Variation

Crossing Over :

• Chromosomes pairs are together as a bivalent the chromatids of each pair are wrapped around each other = chiasmata

• Tension on the areas = sections of the chromatid to break off and re-join to the chromatid of the homologous partner = crossing over

• Alleles are exchanged between maternal and paternal chromosomes - genetic recombination

• Chromosome move apart and separate during meiosis - new combinations of alleles on each chromosome = more variation in gametes

Independent Segregation :

• When Homologous Chromosomes line up on the equator during Metaphase 1

• During this the orientation of chromosome pairs are completely random

• Separation of these pairs of chromosomes result in different combinations of maternal and paternal chromosomes in the gametes formed

Differences between Mitosis & Meiosis