all sexually reproducing organisms
start life as single cell
growth & development depends on replication of
this cell & its descendants - mitosis
mitosis cannot generate cells
capable of sexual reproduction,
eg sperm & eggs
if it did they would each have full
complement of chromosomes present in
parental cells, so embryo would have
twice as many chromosomes as their
parents cells & this number would double
at every generation
potential problem of accumulating DNA
solved by meiosis - particular type of cellular
replication found only in cells that give rise to
gametes
karyotype - complement of chromosomes
in eukaryote cell defined as number, size,
shape & arrangement of organisms
complement of chromosomes
different species, & sometimes different
populations within same species, can
have different karyotypes
set of chromosomes includes autosomes & in
some groups (incl mammals, bird, insects & some
flowering plants) sex chromosomes
autosomes usually have same complement & arrangement of
genes in both sexes, whereas sex chromosomes differ between
male & females (F - XX, M - XY)
number of chromosome sets in
individual's cell determines its
ploidy
humans have 2 sets of chromosomes,
each comprising 23 chromosomes, so
karyotype is 2n = 46
polyploid organisms have more
than 2 complete sets of
chromosomes
very common in flowering
plants & occurs in some
fungi, fish, reptiles & insects
new polyploids arise when
complete sets of chromosomes
fail to separate during meiosis
genome - entire
complement of
organisms genetic
material
sequence data of DNA from many
different organisms presents
opportunities to investigate changes to
genomes during course of evolution
considerable diversity in
genome structure - size
& organisation - among
species investigated