Chromosones and meiosis
Page 1 of 10
,Official definitions
Other definitions
Centrosome - Organelle which organises/produces spindle fibres. 1 centrosome=2 centromeres.
Page 2 of 10
,Chromosones
What are chromosones?
Long , thread-like structures
Form part of the chromatin network in nuclei
A strand of DNA wound around histones (proteins)
Chromosomes can only be seen as individual threads when a cell is dividing
Relationship between chromosomes, DNA and genes
Genes are sections of DNA that contain genetic code. Chromosomes contain genes.
Terms to know
Somatic cell: Body cell in an organism. (All somatic cells are diploid in a diploid organism/2 copies)
Gamete: Sex cell. Sperm in males and an egg/ovum in females. (All gametes are haploid/1 copy)
Diploid: A diploid (2n) cell has 2 sets of chromosomes, one set from the mother (maternal) and one set from the
father (paternal) (46 chromosomes)
Haploid: A haploid (n) cell has 1 set of chromosomes, one set from the mother OR one set from the father (no pair for
chromosome, not part of homologous pair) (23 chromosomes)
Homologous: Matching chromosome pair. Chromosomes are same size, shape and have same genes in same place.
Chromosomes in somatic cells of diploid organisms
- Chromosome number in each somatic cell is the same.
- Each somatic cell is diploid (2n).
- The chromosomes form homologous pairs.
- Chromosome number is specific to a particular species, but always an even number
- DNA replication takes place at the end of interphase in preparation for cell division.
- DNA replication ensures when cell divides, each daughter cell receives full complement of all genetic material.
- After DNA replication, chromosomes consist of 2 identical DNA molecules/chromatids joined by a centromere.
A pair of homologous chromosomes after DNA replication forms a bivalent
1 bivalent = 2 homologous chromosomes= 4 chromatids
Note - chromatids are only chromatids when in a pair
Page 3 of 10
, Meiosis
What is it?
- Cell division which takes place in reproductive organs to produce sex cells (gametes in animals, spores in plants)
- Reduction of chromosomes from 2 sets in parent cell to one set in each daughter cell formed
Where is it?
Reproductive organs
Animals: testes and ovaries
- Formation of sperm cells in testis = spermatogenesis
- Formation of egg cells/ova in ovaries = oogenesis
Plants: sporangia
- Pollen sacs in male anthers = microsporangia
- Ovules in female ovaries = megasporangia
The process of meiosis
During meiosis the number of chromosomes is halved from the diploid number of chromosomes in the parent cell to
the haploid number of chromosomes in the daughter cells.
Why?
When a male gamete fuses with a female gamete during sexual reproduction (fertilisation takes place) the number of
chromosomes is doubled and a diploid zygote is formed
Meiosis therefore cancels out the doubling effect of fertilisation.
If there was no meiosis:
Meiosis consists of interphase followed by 2 divisions, called meiosis 1 and 2
Interphase: DNA of parent cells is replicated
Meiosis 1: Reduction division, 2 haploid cells formed from a diploid parent cell (chromosome number halved)
Meiosis 2: Copying devision, where 2 haploid cells from meiosis 1 are divided again to form 4 haploid cells
Page 4 of 10
Page 1 of 10
,Official definitions
Other definitions
Centrosome - Organelle which organises/produces spindle fibres. 1 centrosome=2 centromeres.
Page 2 of 10
,Chromosones
What are chromosones?
Long , thread-like structures
Form part of the chromatin network in nuclei
A strand of DNA wound around histones (proteins)
Chromosomes can only be seen as individual threads when a cell is dividing
Relationship between chromosomes, DNA and genes
Genes are sections of DNA that contain genetic code. Chromosomes contain genes.
Terms to know
Somatic cell: Body cell in an organism. (All somatic cells are diploid in a diploid organism/2 copies)
Gamete: Sex cell. Sperm in males and an egg/ovum in females. (All gametes are haploid/1 copy)
Diploid: A diploid (2n) cell has 2 sets of chromosomes, one set from the mother (maternal) and one set from the
father (paternal) (46 chromosomes)
Haploid: A haploid (n) cell has 1 set of chromosomes, one set from the mother OR one set from the father (no pair for
chromosome, not part of homologous pair) (23 chromosomes)
Homologous: Matching chromosome pair. Chromosomes are same size, shape and have same genes in same place.
Chromosomes in somatic cells of diploid organisms
- Chromosome number in each somatic cell is the same.
- Each somatic cell is diploid (2n).
- The chromosomes form homologous pairs.
- Chromosome number is specific to a particular species, but always an even number
- DNA replication takes place at the end of interphase in preparation for cell division.
- DNA replication ensures when cell divides, each daughter cell receives full complement of all genetic material.
- After DNA replication, chromosomes consist of 2 identical DNA molecules/chromatids joined by a centromere.
A pair of homologous chromosomes after DNA replication forms a bivalent
1 bivalent = 2 homologous chromosomes= 4 chromatids
Note - chromatids are only chromatids when in a pair
Page 3 of 10
, Meiosis
What is it?
- Cell division which takes place in reproductive organs to produce sex cells (gametes in animals, spores in plants)
- Reduction of chromosomes from 2 sets in parent cell to one set in each daughter cell formed
Where is it?
Reproductive organs
Animals: testes and ovaries
- Formation of sperm cells in testis = spermatogenesis
- Formation of egg cells/ova in ovaries = oogenesis
Plants: sporangia
- Pollen sacs in male anthers = microsporangia
- Ovules in female ovaries = megasporangia
The process of meiosis
During meiosis the number of chromosomes is halved from the diploid number of chromosomes in the parent cell to
the haploid number of chromosomes in the daughter cells.
Why?
When a male gamete fuses with a female gamete during sexual reproduction (fertilisation takes place) the number of
chromosomes is doubled and a diploid zygote is formed
Meiosis therefore cancels out the doubling effect of fertilisation.
If there was no meiosis:
Meiosis consists of interphase followed by 2 divisions, called meiosis 1 and 2
Interphase: DNA of parent cells is replicated
Meiosis 1: Reduction division, 2 haploid cells formed from a diploid parent cell (chromosome number halved)
Meiosis 2: Copying devision, where 2 haploid cells from meiosis 1 are divided again to form 4 haploid cells
Page 4 of 10
