B9 Flashcards
mutation
change in the quantity or base sequence of the DNA of an organism
when do mutations occur
during the formation of gametes
substitution of bases
a type of gene mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide with a different base
–> if this changes the amino acid coded for, the polypeptide will be different
the significance of this will depend on the role of the original amino acid
— if it is important in forming bonds that determine the tertiary structure of the final protein, the new aa may not form the same bonds
e.g. enzyme may have diff active site, no longer complementary to substrate, no enzyme-substrate complexes formed, so cannot catalyse reaction
deletion of bases
arises when a nucleotide is lost from the normal DNA sequence
–> causes all triplets in a sequence to be read differently because each has been shifted to the left by one base
chromosome mutations
changes in the structure/ number of whole chromosomes
chromosome mutations can arise spontaneously and take two forms:
changes in whole sets of chromosomes
- when organisms have three or more sets of chromosomes rather than usual 2
–> POLIPLOIDY + occurs mostly in plants
changes in the number of individual chromosomes
- sometimes individual homologous pairs of chromosomes fail to separate during meiosis
- usually results in a gamete having either one more or one fewer chromosome
–> NON DISJUNCTION
- on fertilisation with a gamete that has the normal amount of chromosomes, the resultant offspring will have diff no. chromosomes in ALL their body cells
hybridisation and polyploidy
hybridisation: combining the genes of different varieties/ species of organism to produce a hybrid
–> sometimes this is followed by organisms that have additional complete sets of chromosomes- polyploidy
polyploidy can arise in many ways:
- chromosomes do not separate into two distinct sets during meiosis*
- gametes could then be produced that have both sets i.e. they are diploid rather than haploid
- if these fused with each other, the offspring could have four sets of chromosomes- tetraploid
- or, if a diploid fuses with a haploid gamete, the offspring would have 3 sets of chromosomes- triploid
Hybrids can be formed by combining sets of chromosomes from two different species
e.g. by cross-pollination between two closely related plants, leading to successful fertilisation
these hybrids are usually sterile -
if, however, the hybrid has a chromosome number that is a multiple of the original chromosome number, a new fertile species can arise because chromosomes have homologous partners and so meiosis is possible.
what does mitosis produce
2 daughter cells with the same number of chromosomes as the parent cell and each other
what does meiosis produce
4 daughter cells with half the number of chromosomes as the parent cell
importance of meiosis
in sexual reproduction, 2 gametes fuse
- to maintain constant number of chromosomes in adults of a species, chromosome number must be halved at some stage in the life cycle
—> this halving occurs as a result of mitosis
- every diploid cell of an organism has 2 complete sets of chromosomes: one set provided by each parent
- during meiosis, homologous pairs separate , so that only one chromosome from each pair enters a daughter cell
—-> this is known as the haploid number of chromosomes, which, in humans, is 23 - when 2 haploid gametes fuse at fertilisation, the diploid number is restored
the process of meiosis
meiosis involves two nuclear divisions that normally occur immediately one after the other
- meiosis 1
- in the first division, homologous chromosomes pair up and their chromatids wrap around each other
equivalent portions of these chromatids may be exchanged - CROSSING OVER
- by the end of this division, the homologous pairs have separated, with one chromosome from each pair going to one of the 2 daughter cells - in the second division, the chromatids move apart
- at the end of meiosis 2, four cells have usually been formed
- in humans, each of these cells contain 23 chromosomes
how does meiosis bring about genetic variation
- independent segregation of homologous chromosomes
- new combinations of maternal and paternal alleles by crossing over
gene
a length of DNA that codes for a polypeptide and functional RNA
locus
the position of a gene on a chromosome/ DNA molecule
allele
one of the different forms of a particular gene
homologous chromosomes
a pair of chromosomes, one maternal/paternal, that have the same gene loci
independent segregation of homologous chromosomes
- during meiosis 1, each chromosome lines up alongside its homologous pair
- in humans –> 23 homologous pairs of chromosomes lying side by side
- when these chromosomes line up, they do so AT RANDOM
- one of each pair will pass to each daughter cell, and with one of the other pairs, depending on how they line up
- since the pairs line up at random, the combination of chromosomes of maternal and paternal origin that go to the daughter cells in meiosis 1 = down to chance
crossing over –> recombination
- if there is no recombination by crossing over in meiosis 1, only 2 different types of cell = produced
- if recombination does occur, 4 different types of cells = produced
- crossing over therefore further increases genetic variety
possible chromosome combinations following meiosis
homologous pairs of chromosomes line up at the equator of a cell during meiosis 1
either one of a pair can pass into each daughter cell- INDEPENDENT SEGREGATION
—> so there is a large number of possible combinations of chromosomes in any daughter cell
- number of possible combinations of chromosomes in each daughter cell = 2^n, where n = no. pairs of homologous chromosomes
- variety = further increased by random pairing of male and female gametes
–> where the gametes come from different parents, two different genetic complements with different alleles are combined, providing yet more variety
(2^n)^2, where n = no. pairs of homologous chromosomes
these calculations = based on chromosomes staying intact throughout meiosis
- CROSSING OVER between chromatids during meiosis 1 exchange sections of chromosomes between homologous pairs in recombination
—as recombination occurs every time gametes are made, it will greatly increase the number of possible chromosome combinations in the gametes
