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Chromosomes and Meiosis Grade 12 Life Sciences
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Chromosomes and Meiosis:Chromosomes:
Long, thread-like structures that form part of the chromatin network in nuclei cells.
Made up of stand of DNA wound around histones (proteins)
Can only be seen as individual threads when cell is dividing.
In somatic (body) cells of diploid organisms:
No. of chromosomes in each cell is the same
Chromosomes are made up of two sets:
1. One chromosomes of each pair come from the mother (maternal
chromosome)
2. One comes from the father (paternal chromosome)
∴ called diploid cells (di-two) or 2n for short 2n = 46
For each paternal chromosome there is a matching maternal chromosome
forming a homologous pair.
>> forming a pair (chromosomes) = same size, shape, and genes in same
place, but alleles for each trait may not be the same.
DNA of each, replicates to form 2 identical threads or chromatids joined by a
centromere (takes place in interphase of cell cycle)
>> threads shorten and thicken as cell divides = visibility.
Replication of DNA is very important to ensure cell divides, each daughter cell
receives full complement of all genetic material.
What is meant by a chromosome number?
Each species has a specific number of chromosomes in its somatic cell.
(some organisms have identical chromosome nos. which are not related)
Similarities in DNA of chromosomes shows relationships, not their number.
Meiosis:
Meiosis is cell division that takes place in reproductive organs of both plants and
animal sot produce gametes (sex cells) in animals and spores in plants.
No. of chromosomes is reduced from two sets (2n) in parent cell to one set (n)
in each of daughter cells formed (halved)
Gametes or spores are formed (haploid) – only have one set of
chromosomes. i.e. one chromosome from each homologous pair.
>> In sexual reproduction, a male haploid gamete fuses with a female haploid
gamete during fertilisation to form a diploid zygote.
Where does meiosis take place?
, Animals: (testis and ovaries)
Formation of sperm cells in testis spermatogenesis
Formation of egg cells/ova in ovaries oogenesis
Plants: (formation of spores in sporangia)
Microsporangia are pollen sacs in male anthers
Megasporangia are ovules in female ovaries.
The process of meiosis:
In meiosis, replication is followed by two divisions:
>> Meiosis 1: reduction division which results in two cells being formed each with
half the no. of chromosomes of parent cell. i.e. haploid (n) number.
>> Meiosis 2: copying division which involves two haploid cells formed, each
dividing again by mitosis to form 4 haploid cells.
Meiosis 1 – a reduction division:
Early prophase 1
As in mitosis, the chromosomes become short and fat and are visible as two
chromatids joined by a centromere.
Late prophase 1
The chromosomes of the homologous pairs lie along side one another, forming a
bivalent.
The centrioles move to opposite poles.
A spindle, made up of protein threads, develops across the cell from the two
centrioles.
It is at this stage that crossing over, takes place.
Metaphase 1
The bivalents (not the chromosomes) move to the middle of the cell and line up
on the equator.
The centromeres become attached to the spindle threads.
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