Topic 4B - Diversity, classification and variation Flashcards
What are diploid and haploid numbers?
Normal body cells have the diploid number (2n) of chromosomes. Each cell contains 46 chromosomes and 23 chromosome pairs
Gametes have a haploid number (n) of chromosomes. Each cell has 23 single chromosomes
What happens in meiosis I?
Prophase I - The chromosomes arrange themselves into homologous pairs. Crossing over results in recombinant chromosomes
Metaphase I - homologous pairs arrange themselves in the middle of the cell
Anaphase I - homologous pairs separate which halves the chromosome number
Telophase I (and cytokinesis) - 2 genetically different daughter cells are produced
What happens in meiosis II?
Meiosis II the pairs of sister chromatids that make up each chromosome are separated
Prophase II - The spindle fibres form. No crossing over occurs
Metaphase II - The chromosomes line up in the middle of the cell
Anaphase II - chromatids are pulled to opposite poles of the cell by spindle fibres
Telophase II (and cytokinesis) - The 2 cells divide into 4 genetically different daughter/haploid cells (aka gametes)
What is meiosis?
A type of cell division
Meiosis forms gametes in the reproductive organs of multicellular eukaryotes
The cells that divide by meiosis are diploid and the resulting cells are haploid
How is genetic variation created in meiosis?
Crossing over of chromatids in meiosis I. The chromatids swap sections. They still contain the same genes but have a different combination of alleles
Fertilisation is random
Independent segregation of chromosomes
What is non-disjunction?
A chromosome mutation
A failure of the chromosomes to separate properly resulting in an extra copy
In humans, non-disjunction of chromosome 21 during meiosis can lead to Down’s syndrome
An extra copy of chromosome 13 leads to Patau syndrome
Different types of mutation
Substitution - one base is substituted with another
Deletion - one base is deleted
Addition - one or more bases are added to the DNA sequence
What is a mutation?
Changes to the base sequence of DNA
Why do some mutations not effect the order of amino acids?
Because the genetic code is degenerate (some amino acids are coded for by more than one DNA triplet)
Substitution mutations won’t always lead to changes in the amino acid sequence but deletion mutations will
What is a mutagenic agent? + examples
Chemical, biological or physical agents that cause changes to the base sequence of DNA
They increase the rate of mutation
E.g. Ultraviolet radiation, ionising radiation, chemicals and viruses
They increase the rate at which mutations can occur
What happens before meiosis starts?
The DNA unravels and replicates so there are two copies of each chromosome, called chromatids
The DNA then condenses to form chromosomes each made from two sister chromatids joined by a centromere
What is genetic diversity?
Genetic diversity is the number of different alleles of genes in a species or population
Genetic diversity allows natural selection to occur
How can genetic diversity within a population be increased?
Mutations in the DNA sequence can form new alleles
Different alleles can be introduced into a population due to the migration of another population. This is known as gene flow
How do genetic bottlenecks decrease diversity?
A genetic bottleneck is an event that causes a reduction in population size. This reduces the number of different alleles in the gene pool and so reduces genetic diversity
What is the gene pool?
The total number of alleles in a population
What is the Founder effect?
Describes what happens when a very small population starts a new colony from only a small number of alleles in the initial gene pool
The frequency of alleles may be very different to that in the original population e.g. an allele that was rare may now be common
The Founder effect can occur as a result of migration leading to geographical separation