Genetic Diversity - A10 Flashcards
What does it mean if body cells have the diploid no. of chromosomes?
They contain two of each chromosome (a pair) -> 1 from the mother and 1 from the father
True or false: each pair of chromosomes are the same size and contain the same genes
True -> could have different versions pf the same alleles
What are pairs of matching chromosomes called?
homolagous pairs
How many homologous pairs do humans have?
-23 homologous pairs
-46 is the diploid number in humans
What are the gametes and males and females respectively?
males - sperm
females - egg
Are gametes haploid or diploid?
haploid - contain one copy of each chromosome in a homologous pair
What is the human haploid number?
23
What happens to the gametes during fertilisation?
The two gametes’ nuclei fuse during fertilisation, forming a zygote
What happens to the zygote during fertilisation?
it divides by mitosis to make an embryo
Is the zygote haploid or diploid?
diploid zygote made from haploid gametes
How is fertilisation random?
Any sperm can fertilise any egg - fertilisation is random/random fusion of gametes -> genetic diversity increases by producing new allele combinations
produces zygotes with different combinations of chromosomes to both parents
What are the stages of meiosis 1(the first division)
prophase 1 :
-DNA condenses and becomes visible
-(DNA replication occurs in interphase) so chromosomes consist of two sister chromatids joined by a centromere
-crossing over occurs
-spindle forms
-nuclear envelope breaks down
metaphase 1:
-homologous chromosome pairs/bivalents line up along the equator
anaphase 1 :
-homologous chromosome pairs/bivalents are pulled apart to opposite sides of the cell by spindle fibres
telophase 1:
-nuclear envelopes form around the two groups of chromosomes
cytokinesis:
-division of cytoplasm
-produces 2 haploid cells
What happens to the chromosome in meiosis 1?
The chromosome number is halved
What are the stages of meiosis 2?
No interphase
prophase 2:
-nuclear envelope breaks down
-chromosomes condense and become visible
metaphase 2:
-chromosomes line up in single file along the equator
anaphase 2:
-sister chromatids are separated by spindle fibres to opposite poles
telophase 2:
-nuclear membranes form around each group of chromosomes
cytokinesis:
-cytoplasm divides creating 4 haploid gametes
What are the condensed mark points for meiosis?
-produces 4 genetically different haploid gametes
-DNA replication occurs
-homologous chromosomes pair up and are separated
-1st division occurs
-sister chromatids are separated
-2nd division occurs
What is meiosis used for?
cell division to produce gametes in the reproductive organs of multicellular, eukaryotic organisms
What are the 4 ways of creating genetic variation in gametes, and which occur in meiosis?
1)mutation
2)random fertilisation
in meiosis:
3)crossing over of chromatids
4)independent segregation pf chromosomes
How does crossing over of chromatids create genetic variation?
-during meiosis 1(between prophase 1 and metaphase 1)
-homologous pairs of chromosomes associate/formed a bivalent
-non-sister chromatids twist together forming chiasmata
-equal lengths of non-sister chromatids and therefore alleles are exchanged
-producing new combinations of alleles
Define homologous pairs.
two chromosomes that carry the same gene
How does independent segregation cause genetic variation?
-in metaphase 1
-each homologous pair of chromosomes is made of one maternal chromosome and one paternal chromosome
-when homologous pairs are separated in meiosis 1, it’s random which chromosomes from each pair ends up in which daughter cell
-the 4 cells produced by meiosis end up with different combinations of those maternal and paternal chromosomes -> maternal and paternal are reshuffled in different combinations
What do the number changes mean in life cycles?
-need to be able to spot where meiosis is occurring in a life cycle you haven’t seen before
-stays the same :mitosis
-halves: meiosis
-doubles: fertilisation
What is a mutation?
change in the base sequence of DNA
What can cause mutations?
errors during DNA replication
What can increase the rate of mutations?
mutagenic agents
What are the 6 types of mutation?
substitution, addition, deletion, duplication, inversion, translocation
What is a duplication mutation?
one or more bases are repeated
What is an inversion mutation?
the sequence of bases is reversed
e.g. GCC becomes CCG
What is a translocation mutation?
A sequence of bases is moved from one location in the genome to another. This could be movement within the same chromosome or movement to a different chromosome
What is the effect of mutations?
Change in DNA base sequence causes a change in the amino acid sequence of a polypeptide may change the final tertiary structure of the protein by changing the hydrogen/ionic/disulphide bonds formed, altering it’s function
Why do not all mutations effect the order of amino acids in the protein?
The degenerate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet
What happens if the number of bases in the DNA code has changed?
-additions, duplications, and deletions within a gene will almost always change the amino acid sequence of a polypeptide
-these mutations all change the no. of bases in the DNA code
-causes a frameshift in the base triplets that are downstream
What are mutagenic agents?
-mutations occur spontaneously(DNA replication)
-mutagenic agents increase the rate of a mutation
-e.g. UV radiation, ionisation radiation, some chemicals, some viruses
What happens when meiosis goes wrong?
(chromosome mutations)
-meiosis can go wrong - cells produced have variations in the numbers of hole chromosomes or part chromosomes
-e.g. one cell might end up with two chromosome 6s and the having none
What can chromosome mutations caused by errors in meiosis lead to?
- lead to inherited conditions because the errors are present in the gametes
-example -> chromosome non-disjunction - a failure of the chromosomes to separate properly
-another example is Down’s Syndrome
What is natural selection?
Mutation
Variation
Adventageous allele ->characteristic
Out compete(differential reproductive success)
Survive
Reproduce
Inherit advantageous allele
Many generations
Allele frequency changes (increase/decrease)
What happens if the advantageous allele is useful?
if allele is useful the allele will code for a protein and therefore a characteristic which is advantageous for survival
What are the different types of adaptation?
-Behavioural -> acts in a certain way to increase the chance of survival
-physiological -> process inside the organism’s body that increase it’s chances of survival
-anatomical -> structural features of an organism’s body which increase it’s chances of survival
What is stabilising selection?
-individuals with alleles for characteristics towards like middle of the range are more likely to survive and reproduce(passing on favourable alleles)
-selection pressures at both ends of the distribution
-ultimately reduces the range of possible phenotypes by eliminating the extreme alleles and therefore extreme phenotypes
-occurs when the environment isn’t changing
What is directional selection?
-individuals with alleles for a single extreme phenotype are more likely to survive and reproduce(passing on favourable allele)
-could be in response to an environmental change
-so the frequency of that allele will increase in the population
Define genetic diversity.
The number of different alleles of genes in a species or population
If there is a large number of alleles, is there a small or large genetic diversity?
large genetic diversity
Whys is genetic diversity important?
low genetic diversity = population at risk of being wiped out by a change in environment/new selection pressures(e.g. disease)
How is genetic diversity increased?
-mutations in DNA forms new alleles - some can be advantageous and some can be harmful
-different alleles being introduced into a population through migration - gene flow
-genetic diversity allows natural selection to occur because some characteristics are more advantageous then others
What is a genetic bottleneck?
-an event causes a bug reduction in a population
-reduces the number of different alleles in the gene pool and reduces allele frequency and therefore genetic diversity
-survivors reproduce and a larger population is created from a few individuals ->meaning less similar alleles->less able to adapt to changes in the environment in the future
What is the founder effect?
-a few organisms from a population start a new colony(often due to migration, causing a geographical isolation) and there are only a small number of different alleles in the initial gene pool
-the allele frequency in the new population may be very different to allele frequency in the original population e.g. an allele that was quite rare in the original population may be more common in the new colony
-sometimes lead to higher incidence of genetic disease
