Mitosis (and Meiosis)
Part 1: Cell Division
• Cell division is the cornerstone of
life
• Genome: a cell’s complete set of an
organism’s genetic material –
(DNA)
• Chromosome:
– Bacteria, viruses: DNA molecule
w/ most or all DNA
– Eukaryotes: DNA / protein
structure with part of the DNA
information
Chromosomes
(colored bodies)
– Prokaryotes have a single, circular
chromosome
Cell Division
• Prokaryotes (bacteria)
reproduce through cell division
– Binary fission
– Circular chromosome (DNA) is
replicated
– Replicated chromosomes are
attached to the cell membrane at
nearby sites
– As membrane expands, the
copies separate
– New cell wall forms between
copies, cell splits
Mitosis
• Eukaryotes use cell division to
create new cells
– MITOSIS: division of the nucleus
– Growth, Maintenance and Repair
– Asexual cell division
• Reproduction
– MEIOSIS: a special, two-stage type of
mitosis that results in daughter cells
with half as many chromosomes as the
parent cells
Chromosomes
– Eukaryotes have genome distributed
across many chromosomes
•
•
•
•
•
Humans: 46 (23 pairs)
Mosquito: 6 (3 pairs)
Toad: 22 (11 pairs)
Potato: 48 (24 pairs)
Dog: 78 (39 pairs)
– 46 in human somatic cells (all cells
EXCEPT reproductive cells) – 2n
– 23 in gametes (Reproductive cells sperm and egg) – n
Chromosomes (colored bodies)
– Chromosome made of a DNA and protein complex:
chromatin
– Following DNA replication, a chromosome contains
two sister chromatids attached by a centromere
Chromosome
Sister
Chromatids
Centromere
Humans:
– 46 chromosomes (2n = 46)
– 2 sets of 23 chromosomes (diploid)
• n = number of chromosomes in a set
– Fundamental number, haploid number
– n = contribution from each parent
– 23 = 1 sex chromosome, 22 autosomes
46
Humans: 46
Maternal
Paternal
Sex:
X or X
X or Y
Autosomes #:
1
1
2
2
…
…
22
22
(homologous pairs)
Mitosis and Interphase Alternate
• The cell cycle
• Cells are in interphase 90% of
the time
– G1 = Gap 1 (cell growth)
– S = chromosome synthesis
(duplication)
– G2 = Gap 2 (cell growth)
– M = Mitosis
Mitosis and Interphase Alternate
The Cell
Cycle
Chromosomes cont.
Homologous pair
(2n, diploid) in
parent cell
Chromosomes are replicated during S-phase of cell
cycle. Chromosomes and copies are separated
during mitosis.
One of each pair to
daughter cell
Copies
Mitosis is a Contiuum
• But, it’s divided into subphases for description
– Prophase
• prometaphase
– Metaphase
– Anaphase
– Telophase
Prophase
• Chromatin coils into visible chromosomes
– Under a light microscope, only the nuclear envelope
(with nucleoli) and a tangle of chromatin are visible
• Centrosomes: mitotic centers, poles for division
prometaphase
• Nuclear envelope breaks
• Microtubules (kinetochore + polar) attach
kinetochores (in centromere) to centrioles (in
mitotic center aka centrosome)
Metaphase
• Chromatids align on a plane at cell’s
equator
– Metaphase plate
(MITOTIC SPINDLE)
Anaphase
• Chromatids separate simultanously
• Sister chromatids become daughter chromosomes
Late
Telophase
• Daughter chromosomes stop moving
– Chromosomes uncoil, nucleus and nucleoli reform
• Result: two identical nuclei
Late
Cytokinesis
• Mitosis: nuclear division (karyokinesis)
• Division of cell cytoplasm after mitosis is known
as cytokinesis
End
Part 2: Gametogenesis
• Meiosis
• Produces gametes (sperm, egg)
– Gametes are haploid (n)
Meiosis
• Meiosis is reductionist division
• Highly specialized form of
mitosis
• Takes place in gonads
• Produces gametes (sperm and
egg)
– Gametes are haploid (n)
– Gametes rejoin during
fertilization
• Returns to 2n
• Sexual fertilization results in
greater variation
Chromosome number reduction
Homologous pair of
chromosomes in
diploid parent
2n
CHROMOSOMES REPLICATE
Homologous pair
Sister chromatids
2n
Meiosis I: Homologues separate into haploid daughter cells
n
Meiosis II: sister chromatids separate into four hapliod daughter cells
n
Stages of Meiosis
• Meiosis I: reduces chromosome number
– Prophase I, Metaphase I, Anaphase I, Telophase I
• Meiosis II: separates chromatids
– Prophase II, Metaphase II, Anaphase II, Telophase II
• Result: four haploid daughter cells
Stages of Meiosis: Meiosis I
Stages of Meiosis: Meiosis II
Meiosis I
• Prophase I: chromatin condenses, homologous
pairs (with copies) align
• Metaphase I: homologous pairs (and copies) align
at metaphase plate
Meiosis I
• Anaphase I: homologous chromosomes (each with
two chromatids) move to opposite ends
• Telophase I: chromosomes gather into nuclei,
original cell divides
Meiosis II
• Interphase with no DNA replication (interkinesis)
• Prophase II: chromosomes condense again
• Metaphase II: kinetochores of paired chromatids
align at metaphase plate
Meiosis II
• Anaphase II: chromatids separate to opposite poles
(=> chromosomes)
• Telophase II: chromatids gather into nuclei, cells
divide
Results
• Gametes are haploid (n) – half as many
chromosomes as parent cell
Overview
• Meiosis I (2n=6)
Interphase
Prophase I
Metaphase I
Homologous pairs
(with copies) align
Telophase
Anaphase
Overview
• Meiosis II (result: n=3)
Metaphase II
Telophase II
Anaphase II
END
Chapter 9 from ‘Life’ book