Topic 4B- Diversity, Classification and Variation

Meiosis and Genetic Variation: DNA from one generation is passed to the next by gametes.

• Gametes are the sperm cells in males and egg cells in females. They join together at fertilisation to form a zygote, which divides and develops into a new organism.

• Normal body cells have the diploid number (2n) of chromosomes- each cell contains two of each chromosome, one from the mum and one from the dad.

• Gametes have a haploid (n) number of chromosomes- there’s only one copy of each chromosome.

• At fertilisation, a haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes.

• During sexual reproduction, any sperm can fertilise any egg- fertilisation is random.

• Random fertilisation produces zygotes with different combinations of chromosomes to both parents.

• Mixing of genetic material increases genetic diversity within a species.

Gametes are formed by Meiosis:

• Meiosis is a type of cell division which takes place in the reproductive organs.

• Cells that divide by meiosis are diploid to start with, but the cells that are formed are haploid- the chromosome number halves.

  1- DNA unravels and replicates so there are two copies of each chromosome, called chromatids.

  2- The DNA condenses to form double-armed chromosomes, each made from two sister chromatids joined in the middle by a centromere.

  3- Meiosis 1- chromosomes arrange themselves in homologous pairs. These pairs are separated, halving the chromosome number.

  4- Meiosis 2- sister chromatids that make up each chromosome are separated (centromere is divided).

  5- Four haploid cells that are genetically different from each other are produced.

Chromatids cross over in Meiosis 1

Homologous pairs of chromosomes come together and pair up- the chromatids swap over, still contain the same genes but now have a different combination of alleles.

Meiosis produces cells that are genetically different

Genetic variation:

1- Crossing Over of Chromatids

The crossing over of chromatids in meiosis 1 means that each of the four daughter cells contain chromatids with different alleles.

2- Independent segregation of chromosomes

Each homologous pair of chromosomes is made up of one maternal chromosome and one paternal chromosome. When separated in meiosis 1, it is random which chromosome from each pair ends up in which daughter cell. Four daughter cells have different combinations of maternal and paternal chromosomes- called independent segregation. This ‘shuffling’ creates genetic variation in potential offspring.

Meiosis has a different outcome to mitosis

Mitosis- produces cells with the same number of chromosomes as parent cell- daughter cells are all genetically identical to each other - two daughter cells.

Meiosis- cells have half number of chromosomes as parent cell- daughter cells are genetically different from one another and the parent cell- produces four daughter cells.

This is because mitosis has one division whereas meiosis has two divisions. There’s no pairing or separating of homologous chromosomes in mitosis and so no crossing over or independent segregatation of chromosomes. This produces genetically identical daughter cells- unlike meiosis.

Chromosome Mutations- caused by errors in cell division

• Sometimes meiosis goes wrong and cells produced contain variations in the numbers of chromosomes or parts of chromosomes.

• Chromosome mutation is caused by errors during meiosis.

• Chromosome mutations lead to inherited conditions because the errors are present in the gametes.

• One type of chromosome mutation is called non-disjunction- it is a failure of the chromosomes to separate properly. This can lead to Down’s Syndrome- extra copy of chromosome 21- fails to separate properly during meiosis, one cell gets an extra copy and another gets none.

Mutations

• A Mutation is a random change to the base sequence of DNA.

• As the order of the DNA bases in a gene determines the order of amino acids in a protein a mutation in a gene could change the primary structure of a protein.

• If a base is changed in a codon, it could then code for a different amino acid, the sequence of amino acids would be altered and the protein made could be different or damaged.

Types of Mutations:

• Addition- one base is added

• Deletion- one base is deleted

• Substitution- one base is substituted with another

Not all Mutations affect the order of amino acids

Degenerate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet. Not all substitution mutations will result in a change to the amino acid sequence- some substitutions will still code for the same amino acid.

Deletions will always lead to changes in the amino acid sequence- it changes the number of bases present, which will cause a frame shift in all the base triplets after it.

Mutagenic Agents Increase Mutation

Mutations occur spontaneously- some things can increase the rate of mutations- mutagenic agents. Ultraviolet Radiation, ionising radiation, some chemicals and viruses

Lots of Different Alleles- High genetic diversity

Alleles- different versions of a single gene.

Genetic diversity- number of different alleles of genes in a species or population.

A population is a group of organisms of one species living in a particular habitat.

Genetic diversity within a population is increased by:

• Mutations in DNA- forming new alleles

• Different alleles being introduced into a population when individuals migrate into them- gene flow

Genetic diversity allows natural selection to occur.

Natural Selection

• Natural Selection increases advantageous alleles in a population

• Mutations sometimes result in a new allele being formed. Some mutations can produce alleles that are beneficial to an organism helping the organism to survive.

• Increases the chances of an organism surviving, its frequency within the population can increase- Natural Selection.

  1- There is variation within a population.

  2- A random mutation can result in a new allele being formed.

  3- The individuals with the advantageous allele will be more adapted to the environment.

  4- Therefore, they are more likely to survive, reproduce and pass on these advantageous alleles to their offspring.

  5- The frequency of the advantageous alleles will increase in the population over generations/time.