CHROMOSOMES AND MEIOSIS
CHROMOSOMES:
Long thread-like structures → form part of chromatin network in nuclei of
cells
● Absorb dye easily -- why called chromosomes (colored bodies)
● Visible under microscope - only seen as individual threads when cell
dividing
IN SOMATIC (BODY) CELLS:
● Number of cells is same
● Chromosomes made up of 2 sets: 1 from mother (maternal) & 1 from father
(paternal) = Diploid cells (x2) / 2n
● For each paternal chromosomes there's matching maternal chromosome
forming = Homologous Pair (same size, shape and have same genes in same
place but alleles for each trait may not be same)
● DNA of each chromosome replicates to form 2 identical threads /
chromatids joined by centromere ---> takes place during interphase
(i.e. between cell division. Threads only become visible when they
shorten and thicken as cell divides)
● Replication of DNA is NB to ensure that as cell divides each daughter
cell receives full complement of all genetic material.
WHAT IS MEANT BY CHROMOSOME NUMBER ?
● Each species has a specific number of chromosomes in its somatic cells →
its the similarities in DNA of chromosomes that their show relationships
not number.
, MEIOSIS:
WHAT IS MEIOSIS?
● Cell division that takes place in reproductive organs to produce gametes
(sex cells) in animals and plants (spores)
● In meiosis - number of chromosomes is reduced from 2 sets (2n) in parent
cell to 1 set (n) in each daughter cell formed (i.e. number of
chromosomes is halved)
● Gametes / spores formed are called haploid - only have one set of
chromosomes (i.e. 1 chromosomes 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 in ovaries → oogenesis
THE PROCESS OF MEIOSIS:
DNA of parent cells is replicated in interphase leading up to both mitosis and
meiosis
In meiosis - replication followed by 2 divisions
Meiosis 1 → reduction division
● Results in 2 cells being formed each with half number of chromosomes of
parent cell (i.e. haploid (n) number )
Meiosis 2 → copying division
● 2 haploid cells formed - each dividing again by mitosis to form 4
haploid cells
CHROMOSOMES:
Long thread-like structures → form part of chromatin network in nuclei of
cells
● Absorb dye easily -- why called chromosomes (colored bodies)
● Visible under microscope - only seen as individual threads when cell
dividing
IN SOMATIC (BODY) CELLS:
● Number of cells is same
● Chromosomes made up of 2 sets: 1 from mother (maternal) & 1 from father
(paternal) = Diploid cells (x2) / 2n
● For each paternal chromosomes there's matching maternal chromosome
forming = Homologous Pair (same size, shape and have same genes in same
place but alleles for each trait may not be same)
● DNA of each chromosome replicates to form 2 identical threads /
chromatids joined by centromere ---> takes place during interphase
(i.e. between cell division. Threads only become visible when they
shorten and thicken as cell divides)
● Replication of DNA is NB to ensure that as cell divides each daughter
cell receives full complement of all genetic material.
WHAT IS MEANT BY CHROMOSOME NUMBER ?
● Each species has a specific number of chromosomes in its somatic cells →
its the similarities in DNA of chromosomes that their show relationships
not number.
, MEIOSIS:
WHAT IS MEIOSIS?
● Cell division that takes place in reproductive organs to produce gametes
(sex cells) in animals and plants (spores)
● In meiosis - number of chromosomes is reduced from 2 sets (2n) in parent
cell to 1 set (n) in each daughter cell formed (i.e. number of
chromosomes is halved)
● Gametes / spores formed are called haploid - only have one set of
chromosomes (i.e. 1 chromosomes 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 in ovaries → oogenesis
THE PROCESS OF MEIOSIS:
DNA of parent cells is replicated in interphase leading up to both mitosis and
meiosis
In meiosis - replication followed by 2 divisions
Meiosis 1 → reduction division
● Results in 2 cells being formed each with half number of chromosomes of
parent cell (i.e. haploid (n) number )
Meiosis 2 → copying division
● 2 haploid cells formed - each dividing again by mitosis to form 4
haploid cells
