Biology 5.1.2- Meiosis and Variation

Meiosis
1. reduction division. Daughter cells have half original number of chromosomes (haploid)
2. Asexual
  1. Eukaryotes (Mitosis)
  2. Prokaryotes (Binary Fission)
3. Sexual
  1. genetically different
  2. gametes- production of special reproductive cells
  3. Zygotes- gametes (one from each parent) fuse together at fertilisation
4. Stages:
  1. 2. Meiosis (1)
    1. b Prophase (1)
      1. chromatin condenses, super coils, shortens and thickens
      2. chromosomes come together in homogolous pair from bivalent
      3. non-sister chromotids wrap around each other (chaiasmata)
      4. Crossing over, nucleolus disappears, nuclear envelope disintergrates, spindle forms
    2. c. Metaphase (1)
      1. Bivalents line up across equator of spindle attached to spindle fibres at centromere (chaiasmata present
      2. Bivalents arranged randomly with each member of homogolous pair facing opposite directions
      3. Allows chromosomes to independently segregate when they are pulled apart
    3. d. Anaphase (1)
      1. Homogolous chromosomes in each bivalent pulled to opposite poles(centromres dont divide)
      2. Chaiasmata separate
    4. e. Telophase (1)
      1. Two nuclear envelopes form
  2. 1. Pre- Meiosis (1)
    1. a. Interphase
      1. DNA replication
      2. each chromosome consists of two identical sister chromatids joined at centromere
  3. 3. Meiosis (2)
    1. f. Prophase (2)
      1. nuclear envelope breaks down again
      2. nucleolus disappears, chromosomes condense, spindle forms
    2. g, Metaphase (2)
      1. chromosomes arrange on equator of spindle
      2. chromosomes attached to spindle at centromere
      3. chromatids of each chromosome are randomly assorted
    3. h. Anaphase (2)
      1. centromeres divide and chromatids pulled to opposite poles of spindle. Randomly segregated
    4. i. Telophase (2)
      1. nuclear envelopes form around haploid daughter nuclei
      2. Animals- 2 cells now divide to give 4 haploid cells
      3. Plants- Tetrad of 4 haploid cells formed
5. Significance of Meiosis
  1. Significance
    1. sexual reproduction= more genetic variation = more evolution due to natural selection
    2. maintain original chromosome numbers
      1. gametes need to be halved (haploid)
      2. when two zygotes join- diploid
  2. How Meiosis and fertilisation lead to variation
    1. crossing over during Prophase (1)- shuffles alleles
    2. genetic reassortment- random distribution and segregation of maternal and paternal chromosomes in homogolous pair (Meiosis 1)
    3. genetic reassortment- random distribution and segregation of sister chromatids (Meiosis 2)
    4. mutations
  3. 1. Crossing over
    1. lengths of DNA swapped from one chromatid to another
    2. chromosome pairs come together to form bivalent
    3. non sister chromatids wrap around each other tightly and attach to chaiasmata
    4. may break at these points, these join on to non sister chromatids in the same bivalent
    5. produces new combinations of alleles on chromatids
  4. 2. reassortment of chromatids
    1. consequence of random distribution of maternal and paternal chromosomes
    2. each gamete aquires different mix of chromosomes
    3. individual cells produce 2 to the power n genetically different gametes, n being haploid number of chromosomes
  5. 3. Reassortment of chromatids
    1. random distribution on spindle equator of sister at metaphase 2. how they align at metaphase 2 determines segregation at anaphase 2
    2. sisters no longer identical
  6. 4. mutations during interphase
Key Terms
1. Genotype: genetic makeup of an organism. describes in terms of alleles
2. homozygous- organisms with 2 identical alleles
3. heterozygous- organisms with 2 different alleles
4. Autosomes- non sex chromosomes
  1. CF is a mutation to autosomal genes.
  2. disrupts transport of Cl- and water, changes shape of Cl- channels, cannot shift mucus
  3. sufferers= cfcf (homozygous recessive)
  4. carriers= CFcf heterozygous dominant
  5. Healthy= CFCF Homozygous dominant
5. Phenotype- characteristics expressed in the organism, observable features.
6. Dominant- Expressed in phenotype
7. recessive- expressed in phenotype if there is no presence of an identical an identical allele or abscence of dominant
8. Co- Dominant: when both alleles contribute to phenotype. Two alleles of the same gene, expressed in the phenotype of heterozygote
  1. Examples: Cattle, Blood type
9. Linkage- two or more genes located on the same chromosome. linked alleles normally inherited together and dont segregate independently unless chaiasmata forms. reduces the number of phenotypes resulting from cross
10. Sex Linkage- gene that codes for characteristics on sex chromosomes. most found on X chromosome (Haemophillia, red/green colour blindness
Genetic diagrams
1. shows parental phenotypes
2. upper case= dominant, lower case= recessive
3. when gene has more than two alleles, gene is uppercase and allele is in superscript
4. Haemophillia- recessive allele, expresses altered protein that doesnt function, therefore, increase in blood clotting time
5. Duchenne muscular dystrophy (DMD)- gene for muscle protein, muscle weakness, wheelchair bound at age 10, death by 20
6. Sickle Cell anaemia (co dominant)- Beta strand of haemoglobin differs by an amino acid at position 6, when haemoglobin is deoxygenated, not soluble, becomoes crystaline and into a linear structure. if lodged into capillaries, blood flow impedded, organs damaged, heterozygous- red blood cells made in marrow with normal and sickle. prescence of normal prevents sickling when deoxygenated
7. Roan cattle- co dominant
Interactions between gene loci
1. Epistasis- interactions of different gene loci so that one gene locus marks or suppresses the expression of another gene locus
  1. May control the phenotypic characteristics in these ways:
    1. work against each other resulting in masking antagonistically
    2. work together in a complementary factor
2. Working antagonistically
  1. homozygous presence of recessive allele may prevent expression of allele at second locus
  2. first locus = epistatic to second locus
  3. 1. Recessive epistasis
    1. e.g- flower colour of salvia
      1. 1. pure pink has genotype AAbb, crossed with pure white aaBB, therefore whole F1= AaBb (purple flower)
      2. 2. interbreeding F1 to get F2 resulting in purple, pink and white flowers in ratio of 9:3:4
      3. 3. homozygous aa is epistatic to both alleles of gene B/b neither expressed with no dominant A present
  4. 2. Dominant epistasis
    1. e.g fruit colour in summer squash
      1. 1. presence of one D allele= white fruit regaurdless of second locus D/d
      2. 2. in dd, presence of one E allele = yellow fruit. 2 ee alleles produce green fruit
      3. 3. crossing 2 white, double heterozygous (DdEe)
    2. Feather colour
      1. 1. those carrying dominant L have white feathers regaurdless of C
      2. 2. homozygous c (LLcc, Llcc, llcc) are also white
      3. 3. white leghorn- LLCC+ white wyandote Llcc= 100% white LlCc. Interbreed ratio= 13:3
3. working together/complimentary
  1. 1. Crossed 2 white flowered sweet peas ccRR x CCrr.
  2. 2. F1= White, if interbred purple:white, 9:7
  3. 3. suggests atleast one dominant for both gene loci to flower purple (C-R-
  4. 4. ccR-, C-rr produce white- homozygous recessive at either locus masks expression of dominant allele at other locus
4. Coat colour in mice
  1. Agouti (grey) A/a, black or albino
  2. Allele a is a mutation (homozygous produces black)
  3. B/b at separate locus points controls formation of pigments. B- produces pigment , genotype bb cannot, therefore albino
  4. agouti pairs crossed= AaBb. agouti:black:albino= 9:3:4
5. Combs of domestic chickens
  1. effect of P/p depends on R/r that are present
  2. true breeding pea comb (PPrr) bred with rose comb (ppRR)= 100% walnit comb (PpRr)
  3. walnut comb interbred: walnut;rose;pea;single= 9:3:3:1
Chi Squared Test
1. Used for...
  1. catagorical data
  2. strong biological theory
  3. large samples
  4. raw counts
  5. no zeros
2. calculated chi squared < critical value therefore difference is due to chance and not significant (and vice versa)
3. Equation: sum of (observed (o)- expected number (e))^2 / expected numbers
Continuous and discontinuous variation
1. continuous- guantitative differences between phenotypes. no distinct categories height, mass, yields
2. discontinuous- qualitative differences between phenotypes. clearly distinguished categories- either male or female, genders
3. Genetic basis
  1. continuous variation
    1. controlled by 2+ genes
    2. each gene provides an added component to the phenotype. different alleles at gene locus have little effect on phenotype
    3. polygenes- large number of different genes that have a combined effect on the phenotype
    4. polygenic- characteristics the polygene control. unlinked and on different chromosomes
  2. discontinuous variation
    1. different alleles at single gene locus have large effect on phenotype
    2. different gene loci have different effects on the phenotype
    3. e.g- co dominance, dominant and recessive patterns
    4. 1+ genes are involved, interact in epistatic way
    5. monogenic- discontinuous variation where there is only one gene involved
4. Genotypes and environment contribute to phenotype
  1. 1. Plants (AABBCC= 12cm)
    1. genetic potential is 12 cm but some may be shorter due to lack of water, sun or minerals which effect expression of genes
  2. 2. Animals. humans
    1. child could be genetically intelligent but in order to express genes, need to in stimulating environments, added with good nutrition for development
5. when environment changes, those who adapt well, will survive and reproduce (variation and selection)
Population genetics
1. number of alleles in group is larger than that in the individual. gives rise to pool of genetic diversity. measured by Hardy Weinberg equation. Migration, selection, genetic drift, mutations can alter genetic variation
2. Darwin deduced...
  1. Struggle for survival
  2. variation between individuals
  3. those adapted more likely to survive and breed
3. Birth of population genetics
  1. genes and alleles developed, after biologists began to understand the genetic basis of inherited variation
  2. as the study of evolution continued, they realised that populations rather than individuals are the functional units in the process
  3. Scientists needed to consider frequency of alleles and not just offspring from individual matings
  4. population genetics- biologists focus on genetic structure of populations. measure changes in alleles and genotype frequency from generation to generation
  5. population- group of individuals of the same species that can interbreed. they can expand or contract due to birth and death rates.
  6. gene pool- set of genetic info carried by a population
4. measurement of allele and genotype frequency
  1. observe phenotype to measure frequency allele. need to know: mechanisms of inheritance of traits and how many different alleles on the gene for that trait in the population
  2. Traits showing co dominance, frequency heterozygous phenotype = heterozygous genotype
    1. 1. MN blood group has 2 genes--> L^m, L^n. allele control production of specific antigens on rbc surface. Individuals may be phenotype M (genotype L^mL^M or MM), Phenotype N (genotype L^nL^n or NN) or phenotype MN (genotype L^mL^n or MN) becuase of the codominance. we can determine frequency of alleles in population
    2. if recessive, heterozygous shows some phenotype as homozygous dominant, therefore frequency of alleles not directly determined
5. Hardy Weinberg principle
  1. mathematical model to calculate allele frequencies in populations for dominant and recessive alleles
  2. makes following assumptions...
    1. population = large
    2. mating within populations is random
    3. no selective adantage
    4. no mutation, migration, genetic drift
Roles of genes and the environment in evolution
1. environmental factors can act as stabilisng or evolutionary forces of natural selection
  1. all organisms reproduce, therefore potential to increase population size
  2. may reach carrying capacity, therefore stable. Not all survive, population would continuously expand if they did
  3. environmental factors that limit population growth
    1. space
    2. light
    3. food
    4. minerals
    5. water
    6. predetation
  4. environmental factors offer environmental resistance
    1. abiotic- caused by non living components
    2. biotic- caused by living organisms
  5. populations fluctuate over time around mean, if environmental resistance is great enoug, population decrease therefore less comp and population would grow
    1. increased population, more intraspecific comp, therefore smaller pop
2. what determines which individuals will survive
  1. better adapted for surroundings and the environment
3. selection pressures- environmental factors that confers greater chances for survivalto reproductive age on some members of population
  1. greater chances for agouti rabbits due to agouti being better camoflaged
  2. natural selection- environments selects the best to survive
  3. Stabilising selection- keeping things the way they are (stable but if environment changes, selection pressure changes) e.g snow coverage, therefore white rabbits would be better
  4. Directional selection--> evolutionary changes
    1. what prevents population from freely interbreeding
      1. population split into sub groups using isolated mechanisms
        geographical and ecological barriers (rivers and mountains
        seasonal (temporal)
        reproductive mechanisms
      2. leaves 2 sub populations isolated. different alleles would be eliminated or added in each population. subs not able to breed and will be different species

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Biology 5.1.2- Meiosis and Variation

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	Created by Sarah Pirbhai about 11 years ago