Download Cell Cycle - Meiosis Notes

Cell Growth and Division
Chapter 5
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The Cell Cycle
• Cell
Cycle: The regular pattern of growth
, DNA duplication, and cell division that
occurs in eukaryotic cells.
• Cell cycle has 4 main stages :
•1.) Gap 1 (G1)
•2.) Synthesis (S)
•3.) Gap 2 (G2)
•4.) Mitosis (M)
The Cell Cycle
Interphase
• Interphase
• Longest part of a cell's life cycle
• Called the "resting stage" because the cell isn't
dividing
• Purpose: cells grow, develop, & carry on all
their normal metabolic functions
• There are 3 parts
• 1.) Gap 1
• 2.) Synthesis
• 3.) Gap 2
Interphase
1. G1
(1st Growth phase)
•Cells mature & increase in size
•Normal metabolic activities
• Must pass through a checkpoint
before it can go to S phase
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Interphase
2.) S (Synthesis Phase)
•DNA is replicated (copied)
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Phases of Interphase
3.)
G2 (2nd Growth Phase)
• Additional growth
•Has to pass a checkpoint
•Everything must be in order-adequate
cell size, undamaged DNA- before the
cell goes through mitosis and division.
M Phase
M Phase/(Mitosis):
• Includes two processes : Mitosis and Cytokinesis
•Mitosis (cell division) the process in which the nucleus is
divided into 2 new nuclei.
•Occurs in our somatic (body cells)
•Somatic (body) Cells: contains both sets of homologous
chromosomes (one from each parent)
•Diploid (2n) “two sets- 2 chromosomes at each pair- 23
pairs = 46 chromosomes total”
Chromosomes & Their Structure
•During the process of mitosis (cell division) DNA is
organized into structures called chromosomes
•Chromosomes: One long continuous thread of DNA
that consists of numerous genes along with regulatory
information.
•Your body cells have 46 chromosomes each.
Chromosomes & Their
Structure
• Interphase: DNA is loosely organized
• Mitosis: chromosomes are tightly
condensed.
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Chromosomes & Their
Structure
• DNA wraps around proteins called
histones, forming chromatin.
• Histones interact with each other, further
compacting the DNA.
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Chromosomes & Their
Structure
• As a cell progresses into mitosis, chromatin
further condense.
• The left and right halves of the chromosome
are two identical DNA double helixes.
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Chromosomes & Their
Structure
• Telomeres: repeating nucleotides on the end
of the chromosome that do not code for
genes.
• Make sure chromosomes don’t attach to
each other and prevent loss of genes.
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Chromosome Numbers
• Each organism has a specific
number of chromosomes
• Humans have 23 pairs of
chromosomes
• 46 chromosomes (23 from
each parent)
• Pairs 1-22= autosomes
(autosomal) chromosomes
• Pair 23= sex chromosomes
• Male- XY
• Female- XX
Mitosis – M Phase
 Division of the Nucleus
• 4 Phases: (In order – PMAT)
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
Phases of Mitosis
• Prophase:
1. Chromosomes become visible when they condense
into sister chromatids
2. Centrioles (animal cells) move to opposite ends of
cell
3. Spindle Fibers grow from the centrioles and radiate
toward the center of the cell
4. Nuclear envelope breaks down, nucleolus disappears.
Phases of Mitosis
• Metaphase:
• Spindle fibers attach to a protein structure on the
centromere of each chromosome.
• Chromosomes line up in center/middle or equator
of the cell moved by the spindle.
Phases of Mitosis
• Anaphase:
1. Spindle fibers attached to the centromere pull the
sister chromatids apart, spindle fibers shorten
•
This ensures that each cell has the SAME number of
chromosomes in each cell
2. Chromosomes move toward opposite ends of
cell
centrioles
Phases of Mitosis
• Telophase:
1. Nuclear membrane forms at each end of the
cell around the chromosomes
2. Nucleolus reform
3. Chromosomes become less tightly coiled &
appear as chromatin again
4. Signifies completion of nuclear division
Plant Cell Mitosis
Animal Cell Mitosis
Cytokinesis - Division of Cytoplasm
• Cytokinesis: division of the cytoplasm
•Cytoplasm of the cell and its organelles separate into 2 new
IDENTICAL daughter cells
•A groove (cleavage furrow) forms from outside to inside
pinching the parent cell in two – in animal cells
•In plants, a cell plate is formed
Warm-up (11/12)
• If an organism has 12 chromosomes and its
cell undergoes mitosis, how many
chromosomes will the daughter cells have?
Example Answer
• If an organism has 12 chromosomes and its
cell undergoes mitosis, how many
chromosomes will the daughter cells have?
• Daughter cells would have 12 chromosomes
(diploid cells)
Reasons for Cell Division
• Single-celled organisms: reproduction
• Binary Fission: asexual reproduction in
which the cell divides into two identical
cells.
• multicellular organisms: growth,
development, and repair.
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Cells divide at different rates
• Rate of division is linked to your body’s
need for those cells
• Rate of division is greater in embryo and
children than adults.
• Varies with tissue
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Cells divide at different rates
• Cells that rarely divide are thought to enter
a stage G0
• In G0 - cells are unlikely to divide and carry
out normal metabolic functions.
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Cell size is limited
• Cells have upper and lower size limits.
• too small—> can’t contain all the organelles
& molecules
• too big —> not adequate for the exchange
of materials
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Cell size is limited
Ratio of cell surface area to volume
• When a cell grows the volume increases
much faster than the surface area.
• Cell will not be able to move substances in
and out of the cell at sufficient
rates/quantities.
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Uncontrolled cell division-Cancer
• Regulation of the cell cycle is disrupted.
• They divide much more often than healthy
cells.
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Cancer
• Benign tumor: cancer cells remain
clustered-relatively harmless and can be
removed.
• Malignant tumor: cancer cells break away
(metastasize) from the tumor.
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Cancer
• Most cancer cells carry mutations in two
types of genes.
• 1.) oncogenes- accelerate the cell cycle
• 2.) Cell cycle breaks
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MEIOSIS
Sexual Reproduction: fusion of two gametes resulting
in genetically different offspring.
• Our gametes (sex cells- either egg, or sperm) = haploid
(1n)
• Which means “one set” of chromosomes.
• Eggs and sperm have only 1 chromosome of each pair
per cell for a total of 23.
• Egg & sperm come together to form a zygote (fertilization)
• -it will inherit one chromosome from each parent in all cells to make
them diploid.
Meiosis
• Nuclear division that creates 4 haploid cells
from one diploid cell.
• 2 rounds of division- Meiosis I and II
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Meiosis
• Homologous Chromosomes:
• two separate chromosomes one from mother
and one from father
• Same length and carry the same genes
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Meiosis I
• DNA has already been copied
• Dividing homologous chromosomes
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Prophase I
• Similar to prophase in Mitosis
• Crossing over: exchange of genetic material
between non-sister chromatids of
homologous chromosomes.
• results in genetic variation
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Metaphase I
• Homologous chromosomes line up at the
equator
• Since humans have 23 pairs of
chromosomes, meiosis may result in
8,388,608 possible combinations of
chromosomes. ( 223)
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Anaphase I
• Homologous chromosomes separate from
each other.
• Sister chromatids are still together
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Telophase I
• Cell goes through cytokinesis
• End result: 2 cells that have an unique
combination of 23 duplicated chromosomes
coming from both parents.
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Meiosis – Sexual Reproduction
Gametes
1. DNA replication prior
2. Meiosis I:
•Prophase I: Crossing over!!!
•Metaphase I
•Anaphase I
•Telophase I
3. Replication does not occur between
phases 1 & 2
4.Meiosis II -division of chromatids
** Chromosome number is reduced by
half**
•Prophase II
•Metaphase II
•Anaphase II
•Telophase II
Meiosis
• 4 haploid cells are formed which are
genetically different from each other
• Haploid cell = half the # of chromosomes
• gamete = sex cell
Example:
If an organism has 16
chromosomes in a
diploid
cell, how many
chromosomes will be in
a
sex cell produced by
meiosis?
Mitosis vs Meiosis
• Key differences:
• # of divisions: Mitosis-1/ Meiosis-2
• Meiosis- homologous chromosomes pair up
along equator.
• Anaphase I ( in meiosis) : Sister chromatids
stay together
Mitosis vs Meiosis
• Mitosis: results in 2 genetically identical
diploid cells.
• Meiosis: results in 4 genetically different
haploid cells.
Haploid cells change into gametes.
• Haploid cells from meiosis must be changed
into gametes before they can undergo
fertilization.
• Gametogenesis: Production of gametes
• Final stage differs between sexes.
Sperm: Male gamete
• Main contribution to the embryo- DNA
• Must be able to move to get to the egg.
- whip-like flagellum
- connecting neck filled w/ mitochondria
Egg- Female gamete
• More complicated process
• Begins with birth, not finished until
fertilization
• Contributes to organelles, molecular
building blocks, and other materials the
embryo needs.
Egg- Female gamete
• Only ONE of the 4 cells from meiosis
becomes an egg
• Cytoplasm is unequally divided
• Other cells become polar bodies: little
more than DNA that eventually is broken
down.
• In many species, polar bodies do not
undergo meiosis II.
Meiosis and Genetic Variation
• Major advantage to sexual reproductiongreat deal of genetic variation
• Variation results from:
• 1.) Independent assortment of
chromosomes in meiosis
• 2.) Mixing of alleles (versions of a gene)
as gametes join in fertilization.
Meiosis and Genetic Variation
Independent Assortment:
In meiosis I, chromosomes randomly pair up
along the equator- leading to more variation.
Meiosis and Genetic Variation
• Sperm- 1 of 223 (8 million) chromosome
combinations.
• Egg- 1 of 223 ( 8 million) chromosome
combinations.
• Embryo - 223 X 223 = 70 trillion
DIFFERENT combinations!