4B: Diversity and Selection

?

Meiosis and Genetic Variation

Diploid number (2n): Normal body cells, 2 of each chromosome, 1 mum, 1 dad. Same size, same genes, but different versions of genes (alleles). Pairs of matching chromosomes = homologous pairs (HP). Humans = 23 homologous pairs, 46 chromosomes in total. Diploid no. = 46.

Gametes: Sperm cells, egg cells, haploid no. of chromosomes, 1 copy of each chromosome in HP. Human = 23 haploid. 2 gametes join at fertilisation = zygote; divides, develops into new organism.

Fertilisation: Haploid sperm fuses with haploid egg, making cell with normal diploid no. of chromosomes. 1/2 father, 1/2 mother. Random fertilisation produces zygotes with different combinations of chromosomes to both parents. Mixing of genetic material in sexual reproduction increases genetic diversity within a species.

Meiosis: In reproductive organs of multicellular, eukaryotic organisms. Diploid>haploid. Humans&mammals produce gametes directly, plant&insect produces haploid then later divided by mitosis to gametes. Without meiosis, x2 chromosomes when gametes fuse.

1 of 7

Meiosis (2)

1. Before meiosis starts, DNA unravels&replicates so 2 copies of each chromosome, chromatids. 2. DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids, joined in middle by centromere. 3. Meiosis I (1st division): Chromosomes arrange themselves into HPs. 4. HPs then separated, 1/2ing chromosome no. 5. Meiosis II (2nd division): Pairs of sister chromatids that make up each chromosome are separated (centromere divided). 6. 4 haploid cells, genetically different from each other produced.

Genetic Variation (Crossing over of chromatids): During meiosis I, homologous chromosomes pair up. Chromatids twist around each other and bits of chromatids swap over. Still contain same genes but now different combination of alleles. Means each of 4 daughter cells formed from meiosis II contain chromatids with different alleles.

Genetic Variation (Independent segregation of chromosomes): Each HP made from 1 mum chromosome, 1 dad chromosome. When HPs separated in meiosis I, random which chromosome from each pair ends up in daughter cell. 4 daughter cells produced by meiosis have different combinations of maternal&paternal chromosomes. "Shuffling" of chromosomes leads to genetic variation in offspring.

2 of 7

Meiosis vs Mitosis

Mitosis: Produces cells with same no. of chromosomes as parent cell VS Meiosis: Produces cells with 1/2 no. of chromosomes as parent cell.

Mitosis: Daughter cells genetically identical to each other and parent cell VS Meiosis: Daughter cells genetically different from one another and parent cell

Mitosis: Produces 2 daughter cells VS Meiosis: Produces 4 daughter cells

Mitosis: 1 division to separate sister chromatids VS Meiosis: 2 divisions to separate HPs then sister chromatids.

Mitosis: No pairing/separating of homologous chromosomes so no crossing over/independent segregation of chromosomes.

3 of 7

Mutations

Involves a change in the DNA base sequence of chromosomes. Substitution: 1 base substituted with another. Addition: 1 base added. Deletion: 1 base is deleted. Order of DNA bases in gene determines order of amino acids in particular protein. If mutation occurs in gene, sequence of amino acids it codes for could be altered.

Effects: Degenerate nature of genetic code means some amino acids coded for by more than 1 DNA triplet. Not all substitution mutations result int change to amino acid sequence of protein. Some substitutions still code for same amino acid. Deletion of a base will change no. of bases present, causes shift in all base triplets after it.

Mutagenic agents (MA): Mutations spontaneous; DNA misread during replication. MAs increase rate of mutation and probability of mutation occurring. UV radiation, ionising radiation, chemicals, viruses.

Chromosome mutations: Meiosis goes wrong and cells produced contain variations in no. of whole chromosomes or parts of chromosomes. Leads to inherited conditions because errors present in gametes. 1 type is chromosome non-disjunction; failure of chromosomes to separate properly.

4 of 7

Genetic Diversity

Genetic diversity (GD): No. of different alleles of genes in a species or population. If population has low GD, might not be able to adapt to change in environment and whole population could be wiped out by single event. GD within population increased by: Mutations in DNA forming new alleles, different alleles being introduced into population when individuals from another population migrate into it and reproduce - gene flow. GD allows natural selection to occur.

Genetic bottlenecks: Event causes big reduction in population. Reduces no. of different alleles in gene pool so reduces GD. Survivors reproduce and larger population is created from a few individuals.

Founder effect: When a few organisms from population start new colony, only small no. of different alleles in initial gene pool. Frequency of each allele in new colony might be different to frequency of those alleles in original population. E.g. rare allele in original population might be more common in new colony. May lead to higher incidence of genetic disease. Can occur as a result of migration leading to geographical separation or if new colony separated due to another reason e.g. religion.

5 of 7

Natural Selection

Randomly occurring mutations sometimes result in new allele formed. Harmful/Beneficial. When allele codes for characteristic that increases chances of organism surviving, frequency within population can increase = natural selection.

1. Not all individuals as likely to reproduce as each other; differential reproductive success; individual with allele that increases chance of survival more likely to survive, reproduce. pass on genes than individuals with less advantageous alleles. 2. Means greater proportion of next generation inherits beneficial allele. 3. They in turn more likely to survive, reproduce, pass on genes. 4. Frequency of beneficial allele in population increases from generation to generation. 5. Over generations leads to evolution as advantageous alleles become more common in population.

Behavioural adaptations: Ways organism acts to increase chance of survival. Possums play dead.

Physiological adaptations: Processes inside organisms body increase chance of survival. Hibernation, bacteria produce antibiotics.

Anatomical adaptations: Structural features of organisms body increase chance of survival. Otters have streamlined shape, whales have thick layer of blubber. 

6 of 7

The Effects of Selection

Directional selection: Where individuals with alleles for characteristics of extreme type more likely to survive and reproduce. May be in response to environmental change. Antibiotic resistance.

Stabilising selection: Individuals with alleles for characteristics towards middle of range more likely to survive and reproduce. When environment isnt changing, reduces range of possible characteristics. Human birth rate (3kg).

7 of 7

Comments

No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Ecology and biodiversity resources »