In depth summarised notes on the entirety of chromosomes and meiosis with all diagrams necessary done according to the IEB syllabus for matric (Grade 12) students.
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CHROMOSOMES AND MEIOSIS visible when they shorten
- chromosomes = long, thread- + thicken as cell divides
like structures that form part replication of DNA is very
of chromatin network in nuclei NB to ensure that as a cell
of cells divides each daughter cell
- are made up of a strand of receives full complement
DNA wound around proteins of all genetic material
called histones
- were discovered by chance in
1888
- absorb dye very easily – why What is meant by
they were called chromosome number?
chromosomes - each species has specific
- makes them visible under a number of chromosomes in
microscope but can only be somatic cells
seen as individual threads - some organisms have
when cell is dividing identical chromosome
- in somatic (body) cells of numbers but not necessarily
diploid organisms: related, is similarities in DNA
number of chromosomes of chromosomes that shows
in each cell = same relationships, not number
chromosomes made up of
two sets; one chromosome What is meiosis?
of each pair comes from - meiosis = cell division that
mother (maternal takes place in reproductive
chromosome) and one organs of plants + animals to
from father (paternal produce gametes (sex cells) in
chromosome) – therefore animals and spores in plants
called diploid cells (2n) - in meiosis number of
for each paternal chromosomes reduced from
chromosome, is a two sets (2n) in parent cell to
matching maternal one set (n) in each daughter
chromosome forming a cell formed, i.e. the number of
homologous pair – chromosomes is halved
chromosomes forming a - gametes/spores formed called
pair = same size + shape haploid as only have one set
and have same genes in of chromosomes, i.e. one
same place but alleles for chromosome from each
each trait may not be homologous pair
same - in sexual reproduction, male
DNA of each chromosome haploid gamete fuses w/ a
replicates to form two female haploid gamete during
identical threads or fertilisation to form diploid
chromatids joined by zygote
centromere, takes place in Where does meiosis take
interphase of a cell cycle, place?
i.e. between cell divisions – - in animals meiosis takes place
threads only become in reproductive organs, the
testes and ovaries
1
, - formation of sperm cells in the - centrioles move to opposite
testes = spermatogenesis poles
- formation of egg cells/ova in - spindle, made up of protein
ovaries = called oogenesis threads, develops across cell
- in plants meiosis takes place from the two centrioles
in formation of spores in - crossing over takes place
sporangia
- in seed-bearing plants:
microsporangia = pollen
sacs in male anthers
megasporangia = ovules in
female ovaries
The process of meiosis
- DNA of parent cells is
replicated in interphase
preceding both mitosis and
meiosis Metaphase 1
- in meiosis replication followed - bivalents (NOT chromosomes)
by two divisions move to the middle of the cell
- meiosis 1 = reduction division and line up on equator
which results in two cells - centromeres become attached
being formed each w/ half the to spindle threads
number chromosomes of
parent cell, i.e. the haploid (n)
number
- meiosis 2 = copying division
which involves two haploid
cells formed, each dividing
again by mitosis to form 4
haploid cells
Anaphase 1
Meiosis 1
- centromeres do not split
- interphase occurs first
Early prophase 1 - bivalents separate and
chromosomes (NOT
- as in mitosis, chromosomes
chromatids) are pulled away
become short + fat and are
from each other by
visible as two chromatids
contracting spindle threads
joined by a centromere
- chromosomes move to
- from here on, behaviour of the
opposite poles of cell
chromosomes in meiosis 1
and mitosis differs
Late prophase 1
- chromosomes of homologous
pairs lie alongside one
another, forming a bivalent
2
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