AS BIO: Meiosis and Genetic Variation & Diversity
Difference between cells at anaphase of the first meitotic division (A) would differ from cells at anaphase in the second mitotic division (B)?
2 chromatids still joined in A, the cells appear as 2 chromatids joined together, but separate in B. Diploid number of chromosomes in A, but haploid in B.
How do you calculate the possible number of different combinations of chromosomes following meiosis, without crossing over?
2ⁿ Where n = number of homologous pairs
What is produced at the end of meiosis?
4 haploid cells that are genetically different from each other
What does natural selection lead to?
A better adapted population - behavioural - physiological - anatomical
Inhibition zone:
A clear patch on the lawn of bacteria on agar jelly where the bacteria can't grow.
Homologous pair:
A pair of chromosomes that are; - the same size - have the same genes, - could have different alleles.
How many whole chromosomes should each daughter cell get in meiosis when it works properly, in humans?
All four daughter cells will end up with 20 whole chromosomes - one from each homologous pair
What is a genetic bottleneck?
An event that causes a big reduction in a population e.g. when a large number of organisms die within a population die before reproducing
Homologous pairs:
Any 2 chromosomes which determine the same characteristic, but not necessarily the same alleles. Identical in size and shape. Centromere is in the same position
Why might the founder effect occur?
As a result of migration leading to geographical separation. If a new colony is separated from the original population for another reason e.g. religion
Where do the spindles form in meiosis 2?
At right angles to the spindle in meiosis 1, and 4 daughter cells are formed
What is an example of directional selection?
Bacteria evolving antibiotic resistance
Definition of a species:
Capable of breeding together to create fertile offspring. Similar morphology, biochemistry, physiology and behaviour patterns. Similar DNA and genes
Difference between anaphase in mitosis and meiosis?
Centromeres divide only in meiosis 2, but Centromeres always divide in mitosis. Chromatids only separate in meiosis 2, but always separate in mitosis. Separating identical chromosomes in mitosis, in meiosis 1, the whole chromosomes separate, but this may not be identical due to crossing over.
How does a change in the DNA base code sequence mean a different protein is formed?
Change in amino acid sequence and so change in the primary structure; Change in hydrogen, ionic and disulfide bonds; Alters tertiary structure;
Difference between metaphase in mitosis and meiosis?
Chromatids line up on the equator of the spindle in mitosis, but pairs of chromatids line up on the equator only in meiosis 2. Centromeres line up on the same plane on the equator of the spindle, but centromeres lie equidistant above and below the equator in Meoisis 1.
What does non-disjunction of chromosome 21 mean?
Chromosome 21 fails to separate properly during meiosis, so one cell gets an extra copy of 21, and another gets none.
Describe how crossing over occurs during meiosis 1?
Chromosomes in each homologous pair twist around each other Chromatids break and rejoin to chromatid on sister chromosome
Difference between prophase in Mitosis and meiosis?
Chromosomes not visible,in mitosis but visible in meiosis. Homologous chromosomes remain separate in mitosis but pair up in meiosis. Chiasmata occurs in meiosis, but not mitosis Crossing over may occur in meiosis but not mitosis.
How are the homologous pairs separated in meiosis I?
Completely randomly The four daughter cells have completely different combinations of those maternal and paternal chromosomes.
What are the 2 main events causing genetic variation during meiosis?
Crossing over of chromatids Independent segregation of chromosomes
How does meiosis contribute to genetic variation in mature sperm cells?
Crossing over, where chromatids exchange sequences of DNA, called chiasmata. Independent segregation of chromosomes from homologous pairs - they move independently at meiosis I And meiosis II, = all cells genetically different.
What has to happen before meiosis starts?
DNA unravels and replicates = 2 copies of each chromosome, called chromatids
What is gene flow and how does it increase genetic diversity?
Different alleles are introduced into a population when individuals from another population migrate into them and reproduce
What is an allele?
Different type of a gene or Different base sequence of a gene
How does natural selection work?
Differential reproductive success in a population = organism with beneficial allele is more likely to survive, reproduce and pass on genes. = Greater prop of next generation with beneficial allele = More likely to survive, reproduce and pass on their genes = Frequency of the beneficial allele increase from generation to generation
What can non-disjunction of chromosome 21 lead to in humans?
Down's syndrome
Why do not all base substitutions cause a change to the sequence of amino acids encoded?
Due to the degenerate nature of the genetic code,
What could directional selection be in response to?
Environmental change
What is chromosome mutation caused by?
Errors during meiosis
What is non-disjunction?
Failure of the chromosomes to separate properly.
Biodiversity:
General term to describe the variety of living organisms in a particular area at a given time.
What happens when chromatids cross over?
Homologous pairs of chromosomes come together and pair up. Chromatids twist around each other and bits of chromatids swap over. Chromatids contain the same genes but now have a different combination of alleles.
What happens in Meiosis 1?
Homologous pairs separated Chromosome number halved
What does the size of the inhibition zone tell you?
How well an antibiotic works - the larger the zone, the more were inhibited from growing
What is an example of stabilising selection?
Human birth weight
How does stabilising selection work?
Humans have a range of birth weights Very small babies less likely to survive - hard to maintain body temp Giving birth to large babies = difficult, so they're less likely to survive Conditions are most favourable for medium-sized babies, so it tends to shift towards the middle of the range.
Why might a population show very low levels of genetic diversity?
Hunting - reduced population size, so much only few alleles left and they all come a few surviving population Inbreeding Population might have started with small number of individuals - by one pregnant female. Population might have been very small - genetic bottleneck
How do the chromosomes arrange themselves in meiosis 1?
In 2 planes across the equator of the spindle. The chromosomes arrange themselves randomly, the homologous pairs remain together, but the maternal and paternal chromosomes in the pair lune up independently of the other pairs.
How does the production of gametes by meiosis contribute to variation between individuals within a population?
Independent assortment and random alignment of homologous chromosomes= Different combinations of maternal and paternal chromosome Crossing over; = Different combination of alleles and an exchange of genetic material;
Differential reproductive success?
Individuals that have an allele that increases their chance are more likely to survive reproduce and pass on their genes, which contain the beneficial allele, than individuals with different alleles?
What do chromosome mutations lead to?
Inherited conditions - the errors present in the gametes - (the hereditary cells)
What does crossing over allow?
It allows exchange of genetic material between maternal and paternal chromosomes, when its the meiosis of a zygote It brings together new combinations of alleles to produce genetic variation in the gametes.
How do mutations in DNA increase genetic diversity?
It forms new alleles
Why do few gametes produce cells with cross-hatched DNA in meiosis?
It is infrequent - rare;
What happens to gametes during fertilisation?
Join together to from zygote, which divides and develops into a new organism
What is the effect of independent segregation of the choromosomes?
Leads to genetic variation in any potential offspring.
What are the benefits of a population showing greater variation?
Likely some individuals will have the alleles, and so the advantageous characteristics for survival;
Difference between mitosis and meiosis?
Me: Homologous chromosomes associate in pairs Mi: Independent - they do not pair Me: Crossing-over - chiasmata formation Mi: No crossing-over; Me: Two divisions → 4 offspring cells Mi: One division → 2 offspring cells; Me: Genetically different product Mi: Genetically identical product
When does the crossing over of chromatids occur?
Meiosis I
Difference in number of cells produced in meiosis and mitosis?
Mitosis = 2 daughter cells Meiosis = 4 daughter cells
Different outcome of the genes in meiosis and mitosis?
Mitosis = Genetically identical to each other and parent Meiosis - Genetically different from one another and the parent cell
Different outcome of the number of chromosomes in meiosis and mitosis?
Mitosis = same number of chromosomes as parent Meiosis = half the number of parents chromosomes
Why are the outcomes of meiosis and mitosis different?
Mitosis only involves 1 division - to separate the chromatids. Meiosis has 2 divisions - to separate the homologous pairs and then the sister chromatids. Mitosis = no sharing or paring of homologous pairs, no crossing over or independent segregation of chromosomes = genetically identical
What is genetic diversity within a population increased by?
Mutations in DNA Gene flow
When can mutations arise in chromosomes?
Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis.
What is down's syndrome caused by?
Non-disjunction of chromosome 21 The person has an extra copy, or part of chromosome 21
Species diversity:
Number of different species and the number of individuals of each species within any one community
What is crossing over?
Occurs at the chiasmata. Chromosomes break and the broken ends reunite with those of the non-sister chromatids. New combo of alleles are present where the crossing over has occurred.
How does natural selection lead to evolution?
Over generations the Frequency of the beneficial allele increase from generation to generation, until it becomes more common in the population.
Example of physiological adaptations
Processes inside an organism's body that increase its chance of survival. e.g. brown bears hibernate over the winter = lower rate of metabolism = energy conserved = don't have to look for food in the months where it is scarce
When do the homologous pairs pair up in meiosis?
Prophase 1
How is genetic diversity achieved?
Random fertilisation = zygotes with different combinations chromosomes to both parents. This mixing of genetic material in sexual reproduction increases genetic diversity within a species.
How does directional selection work - bacteria evolving antibiotic resistance?
Random mutation = advantageous allele that gives them antibiotic resistance Population is exposed to the antibiotic, killing bacteria without the resistant allele. Resistant survive and reproduce without competition, passing on the allele that gives antibiotic resistance to their offspring. After some time, most organisms in the population will carry the antibiotic resistance allele.
What is the effect of a genetic bottleneck?
Reduces the number of different alleles in the gene pool and so reduces genetic diversity. The survivors reproduce and a larger population is created from a few individuals
What is the effect of stabilising selection?
Reduces the range of possible characteristics
Where does meiosis take place?
Reproductive organisms
Difference between telophase in mitosis and meiosis?
Same number of chromosomes as parent cell in mitosis, but half the number present in meiosis. Both homologous pairs present in diploid cell replicating by mitosis, but only one of each homologous pair is present after meiosis.
What divides in meiosis 1?
Separates the homologous chromosomes in each pair. Separates the chromatids making up each chromosome.
What happens in mitotic division 2 that doesn't happen in division 1?
Separation of sister chromatids, or the division of centromere;
Example of anatomical adaptations
Structural features of an organisms body that increase its chance of survival e.g. whales - thick layer of blubber which keeps them warm in the cold sea.
What is separated in meiosis 2?
The chromatids making up each chromosomes are separated. Similar to mitosis but contains the haploid number of chromosomes, and the chromatids are not identical.
What is the gene pool?
The complete range of alleles in a population
Why might the founder effect lead to a higher incidence of genetic disease?
The frequency of each allele in the new colony might be very different to the frequency of those alleles in the original population An allele that was rare in the original population might be more common in the new colony
How can genetic diversity within a species or a population be compared?
The frequency of measurable or observable characteristics The base sequence of DNA The base sequence of mRNA The amino acid sequence of the proteins encoded by DNA and mRNA.
What usually happens to harmful new alleles created as a result of randomly occurring mutations?
The mutated allele dies out quickly
What is chiasmata and how are they formed?
The non-sister chromatids then form points of attachment called chiasmata.
Genetic diversity:
The number of different alleles of genes in a species or population. The variety of genes possessed by the individuals that make up any one species
Species richness:
The number of different species in a particular area at a given time.
What happens in meiosis 2?
The pairs of sister chromatids are that make up each chromosome are separated - centromere is divided
Ecosystem diversity:
The range of different habitats within a particular area.
When can mutations arise in genes?
They can arise spontaneously during DNA replication and include base deletion and base substitution.
Why are aseptic techniques used?
To prevent costamination of culture by unwanted MOs, which can affect the growth of the MOs that you're working with, and avoids contamination with disease-causing microbes that could make you ill
What is the function of meiosis?
To separate the homologous pairs of chromosomes
Describe a chromosome at the start of meiosis?
Two sister chromatids Due to DNA replication Joined by a centromere
Example of behavioural adaptations
Ways an organism acts that increases its chance of survival and reproduction e.g. possums 'play dead' if they're being threatened by a predatory to escape attack
Aseptic techniques:
Wear gloves Tie hair back to prevent it falling in anything Regularly disinfect work surfaces to minimise contamination Work near a Bunsen flame - hot rising air will draw microbes away from the culture. Sterilise utensils by passing through a flame - kill any microbes. Pass neck of glass container just after its opened and just before its closed, causing air to move out, preventing unwanted organisms from falling in. Sterilise all glassware before and after use e.g. in an autoclave
What does the founder effect describe?
When just a few organisms from a population start a new colony - only a small number of different alleles are in the initial gene pool
What is chromosome mutation?
When meiosis goes wrong and the cells produced contain variations in the numbers of whole chromosomes or parts of chromosomes e.g. 2 cells produced are normal, but 1 has two chromosomes 6's, but the other has no chromosome 6
What happens when mutations produce alleles that are beneficial to an organism?
When the allele codes for a characteristic that increases the chances of an organism surviving, its frequency within the population can increase. This is called natural selection
What is the independent segregation of chromosomes?
When the chromosomes line up in 2 planes across the equator of the spindle during meiosis 1. The chromosomes arrange themselves randomly, the homologous pairs remain together, but the maternal and paternal chromosomes in the pair line up independently of the other pairs. So as the spindle fibres contract during anaphase 1, the homologous pairs are separated
When does stabilising selection occur?
When the environment isn't changing
What happens as a result of the non-disjunction of chromosome 21?
When the gamete with the extra copy fuses to another gamete at fertilisation, the resulting zygote will have 3 copies of chromosome 21.
How does independent segregation lead to genetic vairaton'?
When the homologous pairs are split, it produces a new combination of genes as a result of independent assortment
Stabilising selection:
Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
Directional selection:
Where individuals with alleles for charactersistics of an extreme type are more likely to survive or reproduce
What is directional selection exemplified by?
exemplified by antibiotic resistance in bacteria,
What is stabilising selection exemplified by?
human birth weights.
What is the process of homologous pairs joining up called?
synapsis - chromosomes pair and coil around each other
