GENETIC DIVERSITY Flashcards
whats a gene muatation
-change in the base sequence of DNA on chromosome
-can arise spontaneously during DNA replication (interphase)
-involves base deletion/substitution
changes to one or more bases in DNA triplet could result what
change in amino acid sequence of the polypeptide
Why might a mutation not lead to a change in the amino acid sequence
-Genetic code is degenerate, so may code for the same amino acid
-may change an intron which is not expressed
When does gene mutation by deletion of bases occur
when a nucleotide is lost from the normal DNA sequence
what does 1 deleted nucleotide cause
-causes all triplets in a sequence to be read differently
-because each has been shifted to the left by one base
two forms chromosome mutation can take
-changes in whole set of chromosome
and
- changes in the number of individual chromosomes
describe changes in whole set of chromosomes
-occurs when organisms have 3 or more sets of chromosomes rather than usual 2
-called polyploidy + occurs mostly in plants
describe changes in the number of individual chromosomes
-sometimes individual homologous pairs of chromosomes fail to separate during meiosis (non-disjunction)
-usually results in gamete having either 1 more or 1 fewer chromosome
-eg down syndrome
What is a mutagenic agent?
a factor that increases the risk of mutations
What is a frame shift mutation?
-the mutation results in the rest of the base sequence being impacted,
-therefore causing a greater impact of the codons
-therefore the amino acid sequence that is coded for.
mutation can lead to production of non-functional protein/enzyme
-changes in base/ triplet sequence of DNA
-changes sequence of codons on mRNA
-changes sequence of a.a in primary structure of polypeptide
-changes position of H/ ionic /S-S bonds in tertiary structure of protein
-changes tertiary structure and thus shape of protein (and active site if enzyme)
-(if enzyme substrate can’t bind to active site and form enzyme substrate complex)
Base deletion process
-one nucleotide / base removed from DNA sequence
-changes triplet/codon from point of mutation (frameshift)
-changes sequence of codons on mRNA after point of mutation
-changes sequence of a.a in primary structure of polypeptide
-changes position of H/ionic/S-S bonds in tertiary structure of protein
-changes tertiary structure, thus shape of protein
Base substitution process
-nucleotide /base in DNA replaced with another nucleotide/base
-change in one base = changes one triplet
-changes one mRNA codon and one a.a > sequence of a.a in primary structure of polypeptide changes etc
-OR due to degenerate nature of genetic code , the new triplet may still code for same a.a so sequence of a.a in primary structure of polypeptide remains unchanged
products of meiosis
-4 daughter cells
-each with half the number of chromosomes as the parent cell
Process of meiosis
-before meiosis starts. DNA replicates so there’s 2 copies of each chromosome (sister chromatids joined by centromere)
-homologous pairs line up randomly and one from each pair move either side of the cell
-independent segregation (increases genetic variation in gametes)
-crossing over - recombination occurs where chromosomes swap genetic material
-homologous chromosomes separate
-4 new daughter cells are formed
How does meiosis create genetic variation
1)crossing over between homologous chromosomes
- alleles exchanged between chromosomes
-creates new combination of maternal and paternal alleles on chromosome
2) independent segregation of homologous chromosomes
- random alignment of homologous pairs at equator > random which chromosomes from each pair goes to each daughter cell
-creates different combinations of maternal and paternal chromosomes and alleles in daughter cells
3) random fertilisation when 2 gametes fuse to form zygote
Importance of meiosis
-2 divisions > creates haploid gametes (half number of chromosomes)
-diploid number restored at fertilisation
-maintains chromosome number from one generation to the next
-independent segregation and crossing over creates genetic variation
Why is meiosis important?
-so that gametes formed are haploid
-therefore fuse to make a cell with a diploid number of chromosomes.
-Without meiosis the number of chromosomes would increase with each generation
-therefore it is needed to keep the number constant/stable
What is 2 differences between meiosis and mitosis?
-mitosis; genetically identical daughter cells
-meiosis; genetic variation in daughter cells
-mitosis; 1 nuclear division
-meiosis; 2 nuclear divisions
-mitosis; diploid
-meiosis; haploid
genetic recombination by crossing over
-chromatids of each pair become twisted around one another
-during this twisting process, tensions created and portions of chromatids break off
-these broken portions might then rejoin with the chromatids its homologous partner
usually it is the equivalent portions of homologous chromosomes that are exchanged
-in this way new genetic combination of maternal and paternal alleles are produced
What is exchanged in crossing over?
equivalent sections of DNA/homologous chromosomes
What does crossing over produce?
a new combination of alleles
How can you calculate the possible number of chromosome combinations following meiosis?
2^n where n = number of pairs of homologous chromosomes
Genetic diversity
the total number of different alleles in a population
Define population
group of interbreeding individuals of the same species
advantage of a greater number of different alleles that all members of a species possess
the greater the genetic diversity of that species
when is genetic diversity reduced
when a species has fewer different alleles
What advantage does high genetic diversity provide?
the stability to adapt to a change in the environment, allows natural selection to occur
Why does greater genetic diversity mean more likely that some individuals in population survive and environmental change
-because of a wider range of alleles
-therefore a wider range of characteristics
-giving a greater probability that some individuals will possess a characteristic that suits the new environmental conditions
-genetic diversity is a factor that enables natural selection to occur
Why are not all alleles of a population are equally likely to be passed to the next generation
-because only certain individuals are reproductively successful and so pass on their alleles
reproductive success and allele frequency
-within population of a species there’ll be a gene pool containing a wide variety of alleles
-random mutation of alleles within this gene pool> new allele of a gene that could be harmful
-but in certain environments, new allele of gene might give its possessor an advantage over other individuals in population due to being better adapted > survive in their competition with others
-more likely to obtain available resources > grow more rapidly and live longer > have better chance of breeding successfully and producing more offspring that contain the ‘new allele’
-so they will be more likely to survive and reproduce successfully
-over many generations , the number of individuals with ‘new advantageous allele’ will increase at the expense of individuals with ‘less advantageous’ alleles
Explain how natural selection results in the development of new characteristics
-mutations -> new alleles
-some alleles provide a selective advantage, meaning they are more likely to survive and reproduce
-the offspring receive the allele and pass it to next generation, allele frequency increases
