Causes of Genetic Diversity - Mutations and Meiosis

• Genetic diversity:

  • organisms show variation resulting from genetic diversity
  • genetic diversity is the total number of different alleles of genes in a species/population. Genetic diversity serves as a way for populations to adapt to changing environments
• Genetic diversity:

  • proteins are what make organisms so different. Sections of DNA called genes code for each proteins. Members of the same species have the same genes yet are physically different from each other (such as fur colour in mice)
  • the differences within species are therefore due to different alleles they possess. If a population has many different alleles for a particular gene, then the population is said to be genetically diverse
• Gene pool:

  • within any one species there is set number of alleles. This is known as the gene pool
  • the bigger the gene pool, the greater the variation within that species and the greater the genetic diversity
  • the greater the genetic diversity the better the survival chances of that species as they are more likely to be able to adapt to changes in the environment
• Genetic diversity:

  • genetic diversity creates variation within a population
  • specific alleles will increase in proportion overtime, and some will decrease. This is dependent on if they cause differential changes in survival and reproduction
  • genetic diversity is therefore a factor which allows natural selection to occurs
• Causes of genetic diversity:

  genetic diversity within a species (intraspecific variation) arises as a result of;

  • mutations
  • meiosis

  crossing over

  independent segregation

  • random fusion of gametes - genetic variation is further increased due to the random combination of maternal and paternal gametes during fertilisation

  Asexually reproducing organisms only show variation as a result of mutation
• Mutations:

  • a mutation is a change in the sequence of bases in the DNA of an organism
  • this can (but not always) produce a change in the characteristics (phenotype) of the organism which can be passed onto cells produced by division of the mutant cell
  • hereditary mutations are inherited from your parents and can cause genetic disorders (e.g cystic fibrosis)
  • acquired mutations occur after fertilisation and are often associated with mutagenic agents
  • mutations can affect genes or whole chromosomes
  • mutations can have positive, neutral or negative consequences
• Types of gene mutation:

  there are 6 types of gene mutation;

  • 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
• Types of gene mutation:

  • inversion of bases - a group of bases becomes separated from the DNA sequence and re-join at the same position but in the inverse order (back to front). The base sequence of this portion is therefore reversed
  • translocation of bases - a group of bases becomes separated from the DNA sequence on one chromosome and becomes inserted into the DNA sequence of the same or a different chromosome. Translocation often have significant effects on gene expression leading to an abnormal phenotype
• Silent mutations (usually substitutions):

  • the genetic code is degenerate - some amino acids are coded for by more than one DNA triplet, which means that often this type of mutation may causes no change in the polypeptide
  • in this case the mutation is silent
• Frame shift mutations (usually addition or deletion):

  • if a base is added to or removed from a gene, it changes the way in which the triplets are read following that mutation
  • this protein will likely not function normally
  • if multiples of 3 bases are added or removed, a frame shift will not occur
• Change in protein structure and function following mutation:

  • change in DNA base sequence can result in a different sequence of amino acid coded for
  • different primary structure of protein
  • different 3D tertiary structure because hydrogen/ionic/disulphide form in different positions
  • therefore, the protein function is changed
• Change in enzymes following mutation:

  • change in DNA base sequence can result in a different sequence of amino acids coded for
  • different primary structure of protein
  • different 3D tertiary structure because hydrogen/ionic/disulphide bonds form in different positions
  • active site changes shape so the substrate is no longer complementary
  • no enzyme-substrate complexes can form
• Chromosome mutations:

  this is when the structure or number of whole chromosome mutates, which occurs in two ways

  • changes in whole sets of chromosomes
  • changes in the number of individual chromosomes - this is when individual homologous pairs of chromosomes fail to separate during meiosis. This is called non-disjunction and can result in a gamete with one more or one less chromosome than they should have e.g in Down's syndrome, individuals have an extra chromosome 21
• Mutagenic agents:

  gene mutations arise randomly during DNA replication. The mutation rate can be increased by outside factors known as mutagenic agents or mutagens including;

  • high energy ionising radiation - for example X-rays and UV light. These can disrupt the structure of DNA
  • chemicals (carcinogens) - for example, nitrogen dioxide and several chemicals in cigarette tar
  • some viruses and bacteria - HPV
• Meiosis:

  • meiosis is a type of cell division that produces genetically unique gametes (sex cells) e.g. ovum, sperm, ovule and pollen grain
  • meiosis is therefore a major cause of variation within a species (intraspecific variation)
• Meiosis:

  • meiosis is part of some cell cycles and has many similarities with mitosis as well as some key differences

  the key features of meiosis are;

  • in meiosis, there are two nuclear divisions, not one as in mitosis, so 4 daughter cells are formed from each cell
  • the daughter cells have half the number of chromosomes found in the original parent cell. This is called the haploid number (n)
• Haploid number:

  • the haploid number in humans is 23. When male and female gametes join at fertilisation, the diploid number (2n) is restored. Therefore, meiosis ensures that the chromosome number is kept constant from one generation to the next
  • haploid is when the cell has only one of every pair of chromosomes. This may be before DNA replication or after DNA replication, therefore just looking at how much DNA is in a cell does not always tell you if it is diploid or haploid
• Mitosis and meiosis:

  • mitosis maintains the number of chromosomes
  • meiosis halves the number of chromosomes
• The process of meiosis:

  before meiosis starts;

  • the DNA unravels and replicates so that each chromosome consists of 2 copies of DNA. The copies are called chromatids
  • the 2 sister chromatids are identical and joined by a centromere
  • this happens during interphase (S phase) of the cell cycle
• Homologous chromosomes:

  • homologous chromosomes are matching pairs of chromosomes
  • both chromosomes are the same size and have the same genes, although they could have different alleles
  • alleles coding for the same characteristics will be found at the same fixed position (locus) on each chromosome in a homologous pair
• Meiosis:

  meiosis 1 - separation of homologous pairs, and the cells become haploid

  • prophase 1 - chromosomes condense and become visible and the nuclear membrane disintegrates, homologous chromosomes join to form a bivalent, this is where crossing over occurs
  • metaphase 1 - bivalents line up on the equator and centromeres attach to spindle fibres, this is where independent segregation occurs
  • anaphase 1 - spindle fibres contract to separate homologous chromosomes (not chromatids - centromere doesn't split)
  • telophase 1 - nuclei form, cell divides (cytokinesis)
• Meiosis:

  meiosis 2 - separation of sister chromatids (similar to mitosis)

  • prophase 2 - chromosomes condense and nuclear membrane disintegrates, centrioles move to new poles
  • metaphase 2 - chromosomes line up on the equator
  • anaphase 2 - centromeres split and chromatids separate, moving to the pole of each cell
  • telophase 2 - 4 haploid cells are produced
• Meiosis leads to variation:

  • meiosis occurs in all organisms carrying out sexual reproduction
  • meiosis provides opportunities for new combinations of alleles to occur in the gametes
  • this leads to to intraspecific variation in the offspring produced by fertilisation of the gametes

  meiosis does this in two ways;

  • crossing over
  • independent segregation
• Crossing over:

  • when the homologous pairs of chromosomes come together (bivalent) in prophase 1, the chromatids of each pair become wrapped around each other at points called chiasmata
  • this puts tension on these areas and causes sections of each chromatid to break off and re join to the chromatid of the homologous partner. This is called crossing over
  • alleles are exchanged between the maternal and paternal chromosomes - genetic recombination occurs
  • when the chromosomes move apart and separate during meiosis there may be new combinations of alleles on each chromosomes, resulting in more variation in the gametes
• Independent segregation (or random assortment) of homologous chromosomes:

  • unlike mitosis, in the first division of meiosis the pairs of homologous chromosomes line up on the equator in metaphase 1
  • when this happens, the orientation of the homologous pairs is completely random
  • subsequent separation of these pairs of chromosomes results in different combinations of maternal and paternal chromosomes in the gametes formed
  • a cell that has 'n' pairs of chromosomes has 2$^n$ different numbers of combinations
  • a cell with 3 pairs of chromosomes has 2$^3$ = 8 combinations of maternal and paternal chromosomes at the end of meiosis
• Differences between mitosis and meiosis:

  • mitosis has one nuclear division, forming two daughter cells, whereas meiosis has 2 nuclear divisions, forming 4 daughter cells
  • in mitosis the number of chromosomes remain the same (haploid to haploid or diploid to diploid), whereas in meiosis the chromosome number is halved (diploid to haploid)
  • in mitosis there is no crossing over, whereas in meiosis crossing over occurs
  • in mitosis there is no paring of homologous chromosomes and no independent assortment, whereas in meiosis homologous chromosomes pair up, allowing independent assortment
  • in mitosis daughter cells are genetically identical, whereas in meiosis daughter cells are genetically different