CHROMOSOMES AND MEIOSIS
CHROMOSOMES
^ chromosomes are long, thread-like structures that form part of the chromatin network in th
they are made up of a strand of DNA wound around proteins called histones
chromosomes were discovered by chance in 1888
they absorb dye which is why they are called chromosomes -- coloured bodies
this makes them visible under a microscope
they can only be seen and individual threads
- In somatic (body) cells of diploid organisms:
the number of chromosomes in each cell is the same
chromosomes are made up of two sets: one chromosome of each pair comes from the:
mother (maternal chromosome)
father (paternal chromosome)
for each paternal chromosome there is a matching maternal chromosome
homologous pair
they are the same size, shape and have the same genes in the same place
but the alleles for each trait may not be the same
the DNA of each chromosomes replicates to form two identical threads or chromatids joine
this takes place in the interphase of a cell cycle
these threads only become visible when they shorten and thicken as the cell divides
replication of DNA is important to ensure that each daughter cell receives a full complement o
, the DNA of the parent cells is replicated in interphase preceding both mitosis and meiosis
in meiosis, replication is followed by two divisions:
1.) meiosis 1 --> reduction division
results in two cells being formed each with half the number of chromosomes from
2.) meiosis 2 --> copying division
two haploid cells each dividing again by mitosis to form 4 haploid cells
________________________________________________________________
MEIOSIS 1 --> REDUCTION DIVISION
Early Prophase 1 the chromosomes become short, fat and are visible as two
by a centromere --> as in mitosis
Late Prophase 1 chromosomes of homologous pair lie side-by-side forming
centrioles move to opposite poles
spindle (protein threads) develop across from each centriol
crossing over takes place
Metaphase 1 bivalents move to the middle of the cell and line up on the
centromeres become attached to the spindle threads
Anaphase 1 centromeres do not split --> bivalents split and the chromo
chromatids) are pulled away from each other by the contra
threads
chromosomes move to opposite pole of cell
Telophase 1 the cytoplasm divides: cytokinesis --> forming two haploid
both new cells only have one homologous pair of chromos
Late Prophase 1 Metaphase 1 Anaphase 1 Telo
CHROMOSOMES
^ chromosomes are long, thread-like structures that form part of the chromatin network in th
they are made up of a strand of DNA wound around proteins called histones
chromosomes were discovered by chance in 1888
they absorb dye which is why they are called chromosomes -- coloured bodies
this makes them visible under a microscope
they can only be seen and individual threads
- In somatic (body) cells of diploid organisms:
the number of chromosomes in each cell is the same
chromosomes are made up of two sets: one chromosome of each pair comes from the:
mother (maternal chromosome)
father (paternal chromosome)
for each paternal chromosome there is a matching maternal chromosome
homologous pair
they are the same size, shape and have the same genes in the same place
but the alleles for each trait may not be the same
the DNA of each chromosomes replicates to form two identical threads or chromatids joine
this takes place in the interphase of a cell cycle
these threads only become visible when they shorten and thicken as the cell divides
replication of DNA is important to ensure that each daughter cell receives a full complement o
, the DNA of the parent cells is replicated in interphase preceding both mitosis and meiosis
in meiosis, replication is followed by two divisions:
1.) meiosis 1 --> reduction division
results in two cells being formed each with half the number of chromosomes from
2.) meiosis 2 --> copying division
two haploid cells each dividing again by mitosis to form 4 haploid cells
________________________________________________________________
MEIOSIS 1 --> REDUCTION DIVISION
Early Prophase 1 the chromosomes become short, fat and are visible as two
by a centromere --> as in mitosis
Late Prophase 1 chromosomes of homologous pair lie side-by-side forming
centrioles move to opposite poles
spindle (protein threads) develop across from each centriol
crossing over takes place
Metaphase 1 bivalents move to the middle of the cell and line up on the
centromeres become attached to the spindle threads
Anaphase 1 centromeres do not split --> bivalents split and the chromo
chromatids) are pulled away from each other by the contra
threads
chromosomes move to opposite pole of cell
Telophase 1 the cytoplasm divides: cytokinesis --> forming two haploid
both new cells only have one homologous pair of chromos
Late Prophase 1 Metaphase 1 Anaphase 1 Telo
