Download Bio1A Unit 2-1 Cell Division Notes File

Cell Division:
Process where cell divides into 2 daughter cells
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Replacing lost or damaged cells
Growth
Genes passed on from cell to cell
Some organisms reproduce this way (asexual)
Mitosis - Most cell division results in daughter cells with identical genetic information, DNA
Meiosis - A special type of division produces nonidentical daughter cells (gametes, or sperm
and egg cells)
Cell Cycle
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Orderly sequence of events
It is the time from one complete cell division process to the next
These cycles vary
Prokaryotes cell cycles are simpler than Eukaryotes
Prokaryotes
• Simple such as bacteria or blue-green algae
• They have fast cell cycles
• Remember chromosomes are not in a membrane
bound nucleus
• Some bacterial can double every 20 minutes
• Called binary fission – asexual reproduction
• Identical to parent cell
The Cell Cycle is Regulated
• The sequential events of the cell cycle are directed by
a distinct cell cycle control system, which is similar to
a clock
• The cell cycle control system is regulated by both
internal and external controls
• The clock has specific checkpoints where the cell
cycle stops until a go-ahead signal is received
Eukaryotes: The Cell Cycle is Regulated
• The sequential events of the cell cycle are directed by
a distinct cell cycle control system, which is similar to
a clock
• The cell cycle control system is regulated by both
internal and external controls
• The clock has specific checkpoints where the cell
cycle stops until a go-ahead signal is received
Checkpoints
Cellular events, on a molecular level, that must be
fulfilled before the next even can occur.
• G1 checkpoint often most important one
• If G1 checkpoint passed cell cycle is usually completed
• Otherwise  G0 phase
Examples:
• Intracellular - kinetochores not attached to microtubules  causes signal that delays
anaphase
• Extracellular - growth factors platelet-derived growth factor (PDGF) stimulates the
division of human fibroblast cells
Cyclins and Cyclin-Dependent Kinases (CDKs)
• regulatory proteins in cell cycle control
• The activity fluctuates during the cell cycle
• Combine to form MPF (maturation-promoting factor) complex that triggers Mitosis
Failure of checkpoints and cell cycle control
Lead to  Cancer = uncontrolled growth
Phases of the Cell Cycle
Interphase – period of growth and cell function
• G1 - growth
• S – DNA Replication – DNA doubles
• G2 – checkpoint
Mitotic (M) phase- Chromatid segregate
• Prophase
• Metaphase
• Anaphase
• Telophase
Cytokinesis
Chromosomes
Eukaryotic Chromosomes
Chromosomes (single piece of DNA) duplicates (S)
2 sister chromatids with identical genes
• Held together by centromere (still one piece! = 1 chromosome)
• When cell divides, one chromosome goes to each daughter cell
arm
Duplication /
DNA synthesis
Centromere
Sister
chromatids
Separation of
sister chromatids
Centromere
Telomere
Chromatid – 1 half of duplicated chromosome – packaged for mitosis/meiosis
Chromatin – Packaged DNA
•Euchromatin – relatively lightly packaged
•Heterochromatin – relatively heavily condensed
Aster
= DNA sequence
Centrosome
= DNA binding protein
& motor protein
Centrioles
Mitosis – Division of DNA
• Unique to eukaryotes
• Very accurate: error in chromosome distribution ≈ 1 in 100,000 cell divisions
Stages of Mitosis:  PMAT
Prophase
• Chromosomes condense
• Nuclear membrane disappears
• Centrosomes move to polar ends and form asters (microtubules extend)
Metaphase
• Mitotic spindle fully formed
• Centromeres line up “single file” at imaginary equatorial plate
• One spindle microtubule attaches to each sister chromatid
Anaphase
• Chromatids separate & move towards opposite poles
Telophase – reverse of prophase
• Chromosomes have reached poles
• Chromatids uncoil back into chromatin
• Nuclear envelope forms
• Spindle disappears
• Cytokinesis occurs
• Cleavage furrow forms
Cytokinesis - division of cytoplasm,
distinct from mitosis but occurs at the same time as telophase
Human Chromosomes
Autosomes
– Non-sex chromosomes
– 22
Sex chromosomes
– Determine gender
– Normal female = XX
– Normal male = XY
You get 23 ch’s from each parent
Different organisms have diff # Ch’s
karyotype - ordered display of metaphase
chromosomes (somatic cell). DNA is doubled
• Pairs compensate for defective genes
One chromosome may have a functional gene that compensate for a defective gene on
the other chromosome
• Generates diversity among offspring when eukaryotes sexually reproduce
Meiosis
Generation of diverse gametes
Gametes: Cells for sexual reproduction
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Produced in specialized organs, ovary or testes
Gametes (sperm or egg) contains a single set of chromosomes, and are haploid (n)
Meiosis  reduction in chromosome number
For humans, the haploid number is 23 (n = 23)
Each set of 23 consists of 22 autosomes and a single sex chromosome
• In unfertilized egg (ovum), sex chromosome is X
• In sperm cell, the sex chromosome either X or Y
Homologous (=same) chromosomes
Gene 1
Gene 1
Eye color
Blue
Eye Color
Brown
Gene 3
Gene 3
Gene 4
Gene 4
Height
Tall
Height
Tall
Gene 6
Gene 6
What makes them the same /
homologous?
•chromosomes have many genes
•homologous chromosomes have the same genes, but
different versions of that gene (alleles)
•So they are not exactly the same, that’s why we say
homologous and not “same”
Same but not identical
Example:
• all chromosome 1’s in this organism (shown above) will have an eye color gene and a height
gene.
• However, although all ch1’s have the eye color gene. Each ch1 might (or might not) have a
different version. The first ch1 has blue eye color, the other ch1 has brown eye color.
• All ch2’s (not shown) will have a genes that are not in ch1, perhaps a gene for skin tone.
n, ploidy, (& C)
Ways of describing copy number
n = # of non-homologous ch’s
Ch 1, 2, 3,……8  n = 8
Ploidy – sets (versions) of n
For every chromosome
1a, 1b, 1c, 1d  4n or tetraploid
= 4 different versions of ch 1
n=1
c=1
DNA Content (C)
(aka chromosome number)
C = amount of DNA (strands, pg…)
Definition: Gametes have 1C of DNA
n=1
2c=2
The way ploidy is stated is along with “n”
Because n is always the same we say ___n is the ploidy.
1n for
2n for
3n for
4n for
1 version of each ch
2 versions of each ch
3 versions of each ch
4 versions of each ch
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haploid
diploid
triploid
tetraploid
(note: doesn’t quite make
sense in math terms, but oh
well)
n=3
Human Life Cycle
Haploid sperm fertilizes haploid egg
Resulting zygote is diploid
Growth, development
Human adult
– Some cells go through
meiosis
– Gametes (sperm & egg)
Key
Haploid (n)
Diploid (2n)
Mitosis
n
Haploid multicellular organism
(gametophyte)
n
n
n
n
Spores
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
Mitosis
Gametes
FERTILIZATION
2n
Mitosis
Zygote
• Plants and some algae exhibit an alternation of
generations
• This life cycle includes both a diploid and
haploid multicellular stage
• The diploid organism, called the sporophyte,
makes haploid spores by meiosis