CHROMOSOMES AND MEIOSIS
CHROMOSOMES:
■ Long thread like structures that form part of chromatin network in nuclei of
cells
■ Made up of strand of DNA wound after proteins (histones)
In somatic [body] cells of diploid organisms:
• Number of chromosomes in each cell is the same
• Made up of two sets-
- one chromosome of each pair comes from mother (maternal)
- and one comes from father (paternal)
THEREFORE, they are called diploid cells
• For each paternal chromosome, there is a matching maternal chromosome
forming a HOMOLOGOUS PAIR
• Same size and shape and have same genes in same place, but alleles for
each trait may not be same
• DNA of each chromosome replicates to form two identical threads or
chromatids joined by a centromere
Takes place in INTERPHASE
INTERPHASE: Interphase is a time for the cell to prepare for meiosis and part of this
preparation involves doubling the number of chromosomes the cell contains.
, CHROMOSOMES AND MEIOSIS
WHAT IS MEANT BY CHROMOSOME NUMBER:
■ Each species has specific number of chromosomes in its somatic cells
■ It is the similarities in the DNA of chromosomes
MEIOSIS
MEIOSIS: is cell division that takes place in reproductive organs of both plants and animals to
produce gametes (sex cells) in animals and spores in plants.
o Number of chromosomes reduced from two sets [2n] in parent cell to one
set [n] in each of the daughter cells formed = no. chromosomes halved
o Gametes/spores formed = haploid as only have one set of chromosomes
i.e. one chromosome from each homologous pair
o In sexual reproduction, a male haploid gamete fuses with female haploid
gamete during fertilization to form diploid zygote
WHERE DOES MEIOSIS TAKE PLACE?
In ANIMALS: in reproductive organs, TESTIS and OVARIES
■ Formation of sperm cells in testis = spermatogenesis
■ Formation of egg cells or ova in ovaries = oogenesis
In PLANTS: in formation of spores in SPORANGIA
■ Microsporangia = pollen sacs in male anthers
■ Mega sporangia = ovules in female ovaries
THE PROCESS OF MEIOSIS
DNA of parent cells is replicated in interphase
❖ MEIOSIS 1: reduction division which = half number of chromosomes of
parents cell i.e. haploid [n] number
❖ MEIOSIS 2: is a 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-
■ Chromosomes become short and fat and are visible as two chromatids joined
by centromere
CHROMOSOMES:
■ Long thread like structures that form part of chromatin network in nuclei of
cells
■ Made up of strand of DNA wound after proteins (histones)
In somatic [body] cells of diploid organisms:
• Number of chromosomes in each cell is the same
• Made up of two sets-
- one chromosome of each pair comes from mother (maternal)
- and one comes from father (paternal)
THEREFORE, they are called diploid cells
• For each paternal chromosome, there is a matching maternal chromosome
forming a HOMOLOGOUS PAIR
• Same size and shape and have same genes in same place, but alleles for
each trait may not be same
• DNA of each chromosome replicates to form two identical threads or
chromatids joined by a centromere
Takes place in INTERPHASE
INTERPHASE: Interphase is a time for the cell to prepare for meiosis and part of this
preparation involves doubling the number of chromosomes the cell contains.
, CHROMOSOMES AND MEIOSIS
WHAT IS MEANT BY CHROMOSOME NUMBER:
■ Each species has specific number of chromosomes in its somatic cells
■ It is the similarities in the DNA of chromosomes
MEIOSIS
MEIOSIS: is cell division that takes place in reproductive organs of both plants and animals to
produce gametes (sex cells) in animals and spores in plants.
o Number of chromosomes reduced from two sets [2n] in parent cell to one
set [n] in each of the daughter cells formed = no. chromosomes halved
o Gametes/spores formed = haploid as only have one set of chromosomes
i.e. one chromosome from each homologous pair
o In sexual reproduction, a male haploid gamete fuses with female haploid
gamete during fertilization to form diploid zygote
WHERE DOES MEIOSIS TAKE PLACE?
In ANIMALS: in reproductive organs, TESTIS and OVARIES
■ Formation of sperm cells in testis = spermatogenesis
■ Formation of egg cells or ova in ovaries = oogenesis
In PLANTS: in formation of spores in SPORANGIA
■ Microsporangia = pollen sacs in male anthers
■ Mega sporangia = ovules in female ovaries
THE PROCESS OF MEIOSIS
DNA of parent cells is replicated in interphase
❖ MEIOSIS 1: reduction division which = half number of chromosomes of
parents cell i.e. haploid [n] number
❖ MEIOSIS 2: is a 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-
■ Chromosomes become short and fat and are visible as two chromatids joined
by centromere
