genetic diversity (section 4) Flashcards
what is a diploid
a cell with 46 chromosomes or 23 pairs of chromosomes
what is a haploid
a cell with 23 chromosomes(no pairs yet)
what is chiasmata
the points where the chromosomes break and rejoin
what does it mean to condense
chromosomes become visible
what is a homologous chromosome
two chromosomes in a pair - normally one inherited from the mother and one from the father
what is cytokinesis
a process of cell division , which divides the cytoplasm of a parental cell into two daughter cells forming a new cell membrane
what is a centromere
links a pair of sister chromosomes toghether during cell division . this is usually in the middle of the homologous pairs
what is a chromatid
one of the two identical halves of a chromosome that has been replicated in preparation for cell division
what is chromatin
a mixture of loosely packed DNA and proteins that form the chromosomes
what is meisosis
meiosis is a type of cell division that produces 4 gametes (haploid cells) that are used in sexual reproduction
where does meiosis occur
meiosis occurs in the ovaries and testes of animals and the ovaries and anthers of flowering plants (so animals and plants)
how many chromosomes do the gametes produce have
23 chromosomes
how is meiosis different from mitosis
meiosis has 2 divisions , meiosis 1 and meiosis 11
what stages are in both meiosis 1 and 11
prophase , metaphase , anaphase and telophase
what are the two important roles of meiosis
-results in haploid cells
-creates genetic variation
how does it produce variation
meiosis produces daughter cells that are genetically different from eachother and the parent cell , this is due to two processes that happen
-independent segregation
-crossing over
what happens at independent segregation
this is when homologous chromosomes line up in random orientations at the middle of the cell during metaphase. when pulled to either side of the cell this will result in different combinations of alleles in each gamete
what happens at crossing over
the process whereby a chromatid breaks during meiosis and joins to the chromatid on its homologous chromosome so that its alleles are exchanged
what is crossing over
chromosomes in each homologous pair twist around each other , chromatids break and rejoin to the chromatid on sister chromosome
what are sister chromatids
exactly the same ‘replicated’
what is meant by bivalent
a pair of homologous chromosomes ( 4 chromatids )
describe the process of meiosis
interphase - DNA replication occurs which doubles the amount of chromosomes
prophase 1 - DNA condenses and becomes visible as chromosomes , nuclear envelope breaks down, the chromosomes are arranged side by side in homologous pairs ( bivalent ) , crossing over can occur exchanging alleles , centroles migrate to oppositte poles and the spindle is formed
metaphase 1 - the pairs of homologous chromosomes (bivalents) are aligned at the centre of the cell . independent segregation occurs where they can align in different orientations
anaphase 1 - spindle fibres pull homologous chromosomes towards oppositte poles
telophase 1 and cytokinesis - nuclear envelope forms around the two groups of chromosomes and nuclei reform . cytokinesis is when the division of the cytoplasm occurs
prophase 2 - chromosomes condense and become visible again . nuclear membrane breaks down and spindle fibres develop
metaphase 2 - the chromosomes are aligned at the centre of the cell in single file this time
anaphase 2 - centromere of each chromosome divide and the chromatids are pulled to opposite poles of the cell by spindle fibres
telophase 2 and cytokinesis - the chromatids are now at either side of the cell and each cell undegoes cytokinesis to produce 4 haploid cells
What is a gene definition
Short sequence of dna bases that code for a protein
What is an allele definition
A version of a gene
What is the definition for genetic diversity
Number of different alleles or genes in a species or population
Why is genetic diversity important
-species less likely to adapt to change if didn’t have it
-more likely to be a wipeout by an event ( if we didn’t have it )
How can we increase genetic diversity
-mutations - new alleles forms eg from migrating population , this is gene flow
What is a genetic bottle neck
Event causes huge reduction in a population which reduces different alleles in a gene pool . Then there’s less genetic diversity
What is the founder effect
A few organisms from a population start a new colony , the new colony starts from a smaller number of different alleles which causes low genetic diversity
What are the steps for natural selection
1) genetic variation in a population due to mutations
2) organism with advantageous allele is more likely to survive and reproduce
3) advantageous allele is passed onto offspring
4) frequency of allele increases over each generation
5) over many generations this leads to evolution
What are the 3 types of advantageous alleles
- behavioural adaptations
-physiological adaptations
-anatomical adaptations
What are behavioural adaptations
Ways they act , eg migration
What are physiological adaptations
Processes inside the body eg slower metabolism for hibernation
What are anatomical adaptations
Structural features of the body eg spikes on hedgehogs to deter predators
What are the 3 types of selection
-directional
-stabilising
-disruptive
What is directional selection
One sided . Selection favours one extreme phenotype therefore normal distribution curve shifts in the direction of this phenotype
What is stabilising selection
Mean survive . The average phenotype has the selective advantage and therefore variation of traits decreases over generations
What is disruptive selection
Extremes survive , mean doesn’t
What is a mutation
A change in the sequence of bases as a new strand of DNA is being built . This usually occurs during DNA replication
What’s the result of a mutation
The wrong amino acid will be coded for which means the full protein structure will not be folded properly and therefore is dysfunctional
What are the causes of mutations
-Can occur by mistake during DNA replication , these can spontaneously occur
-mutations can also occur when DNA polymerase makes a mistake , this happens once every 100,000,000 bases
What is a mutagenic agent
-these can increase the frequency of mutations above the rate of naturally occurring ones
-it is a chemical or physical agent that changes the genetic material of an organism
-they include high energy radiation chemicals that alter DNA structure or interfere with transcription
What are the types of base mutations
Substitution- a single nucleotide is replaced by another. This could alter the triplet which means it could code for the wrong amino acid
Insertion - an extra base is inserted into the sequence
Deletion - when a nucleotide is lost from the DNA sequence . This means almost the whole sequence will be read differently because each has been shifted to the left by one base
Reverse order - triplets are organised in the opposite order
How would a substitution not cause a change
Sometimes there is no change to the function or tertiary structure of the protein because of the degenerate nature of the genetic code as it could still code for the same amino acid
What is a chromosome mutation
Changes in the structure or number of chromosomes . This normally happens spontaneously
What are the changes of chromosomes
-changes in whole sets of chromosomes
Polyploid plants are plants that have more than two sets of chromosomes in their cells
This is polyploidy , you could have 3 or more sets and this mostly occurs in plants
-changes in the number of individual chromosomes
Homologous pairs of chromosomes fail to separate during meiosis . This is known as non - disjunction and results in a gamete having either one or more or fewer chromosomes
What are the advantages of polyploidy
Bigger plants , increased size and can be more stress resistant to disease and cold
What are the disadvantages
Difficulty in reproducing them with other plants , meiosis is more difficult resulting in more mutations at base level
What is an example of non - disjunction
Down’s syndrome where individuals have an additional chromosome 21
How does meiosis provide genetic diversity other than co and independent s
Variation due to random fertilisation of gametetes , or populations have further mutations
How can a gene mutation have
-a positive effect on an individual
-no effect on an individual
A mutation is a change in the DNA base sequence therefore results in the formation of a new allele
- this may have no effect on an individual as genetic code is degenerate and therefore the amino acid sequence may not change , additionally if the mutation is in an intron then amino acid sequence may not change. Or there may be a change in the amino acid but not a change in the tertiary structure or if the new Allele is recessive so does not influence phenotype
A mutation can have a positive effect if
It results in a change in the polypeptide that positively changes the properties eg an advantageous allele which could result in increased reproductive success or survival
Describe the role of the centromere in mitosis
The centromere holds together the chromatids which have been replicated in dna replication . The centromere attaches the chromatids to the spindle allowing them to be separated and pulled to opposite poles
What should you never say in meiosis
‘Split’ - always say separated
explain how the chromosome number is halved during meiosis
- homologous chromosomes pair
- one of each pair goes to each daughter cell ( they do divide again but technically number is halved in second division)
what is the equation for the possible number of different combinations with n pairs of homolgous chromosomes after random fertilisation of gametes
2n)^2
