Chromosomes & Meiosis
Chromosomes
Long thread-like structures that form the chromatin network in the
nuclei
- Strand of DNA wound around protein histones
- Absorb dye easily hence the name chromosomes (coloured bodies)
In somatic cells of diploid organisms
- Number of chromosomes in each cells is the same
- Two sets: mother (maternal) & father (paternal) therefore diploid cells (2n)
- Matching maternal & paternal chromosomes form a homologous pair (identical but
alleles may differ)
- DNA of each chromosome replicated to form 2 chromatids joined by a centromere
(interphase, only visible when short and thick)
- Replication is important so that each daughter cell receives a full complement of
genetic material
Meiosis
Cell division that takes place in the reproductive organs to produce gametes (animals) & spores (plants)
- The number of chromosomes is reduced from diploid (2 sets/2n) to haploid (1 set/n) in each daughter
cell
- In sexual reproduction a male haploid gamete fuses with a female haploid gamete during fertilisation
to form a zygote
Location:
Animals (reproductive organs: testis & ovaries)
- Formation of sperm cells: spermatogenesis
- Formation of egg cells: oogenesis
Plants (Sporangia)
- Microsporangia are the pollen sacs in male anthers
- Megasporangia are the ovules in female ovaries
, Process
Dna replicated in interphase (before mitosis & meiosis)
In meiosis replication is followed by 2 divisions
- Meiosis 1: reduction division resulting in 2 cells being formed each with half the number of
chromosomes of the parent cell (2n ->n+n)
- Meiosis 2: Copying division where the 2 haploid cells divide by mitosis to form 4 haploid cells
(n+n -> n+n+n+n)
Meiosis 1 - reduction division
Early prophase 1
- (like mitosis) chromosomes become shorter and fat & are visible as 2 chromatids joined by a centromere
Late prophase 1
- Chromosomes of homologous pairs lie alongside one another, forming a
bivalent
- Centrioles move to opposite poles
- Spindle made up of protein threads, develops across the cells from the
two centrioles
THIS IS WHEN CROSSING OVER HAPPENS
Metaphase 1
- Bivalents (not chromosomes) move to middle of cell and line on the
equator
- Centromeres attach to spindle threads
Anaphase 1
- Centromeres do not split, bivalents separate and the chromosomes
(not chromatids) are pulled away from each other via contracting
spindles
- Chromosomes move to opposite poles
Telophase 1
- Cytoplasm divides (cytokinesis) to form 2 haploid cells
Chromosomes
Long thread-like structures that form the chromatin network in the
nuclei
- Strand of DNA wound around protein histones
- Absorb dye easily hence the name chromosomes (coloured bodies)
In somatic cells of diploid organisms
- Number of chromosomes in each cells is the same
- Two sets: mother (maternal) & father (paternal) therefore diploid cells (2n)
- Matching maternal & paternal chromosomes form a homologous pair (identical but
alleles may differ)
- DNA of each chromosome replicated to form 2 chromatids joined by a centromere
(interphase, only visible when short and thick)
- Replication is important so that each daughter cell receives a full complement of
genetic material
Meiosis
Cell division that takes place in the reproductive organs to produce gametes (animals) & spores (plants)
- The number of chromosomes is reduced from diploid (2 sets/2n) to haploid (1 set/n) in each daughter
cell
- In sexual reproduction a male haploid gamete fuses with a female haploid gamete during fertilisation
to form a zygote
Location:
Animals (reproductive organs: testis & ovaries)
- Formation of sperm cells: spermatogenesis
- Formation of egg cells: oogenesis
Plants (Sporangia)
- Microsporangia are the pollen sacs in male anthers
- Megasporangia are the ovules in female ovaries
, Process
Dna replicated in interphase (before mitosis & meiosis)
In meiosis replication is followed by 2 divisions
- Meiosis 1: reduction division resulting in 2 cells being formed each with half the number of
chromosomes of the parent cell (2n ->n+n)
- Meiosis 2: Copying division where the 2 haploid cells divide by mitosis to form 4 haploid cells
(n+n -> n+n+n+n)
Meiosis 1 - reduction division
Early prophase 1
- (like mitosis) chromosomes become shorter and fat & are visible as 2 chromatids joined by a centromere
Late prophase 1
- Chromosomes of homologous pairs lie alongside one another, forming a
bivalent
- Centrioles move to opposite poles
- Spindle made up of protein threads, develops across the cells from the
two centrioles
THIS IS WHEN CROSSING OVER HAPPENS
Metaphase 1
- Bivalents (not chromosomes) move to middle of cell and line on the
equator
- Centromeres attach to spindle threads
Anaphase 1
- Centromeres do not split, bivalents separate and the chromosomes
(not chromatids) are pulled away from each other via contracting
spindles
- Chromosomes move to opposite poles
Telophase 1
- Cytoplasm divides (cytokinesis) to form 2 haploid cells
