Download Mitosis

Cellular Reproduction
(Cell Division)
Mitosis and Meiosis
Cell Division
• Cells must divide after they reach a certain
size.
• Otherwise, their surface area becomes too small
for their volume (Surface:Volume ratio)
• Before they divide, they must make copies
of all the information they contain
• It’s sort of like backing up your hard drive so you
can copy the files to a new computer.
• only using chromosomes instead of hard drives.
• That’s what the process of MITOSIS is all
about... Copying information.
The Cell Cycle
(the life cycle of the cell)
• The cell spends more
than ¾ of its life cycle
growing and preparing to
divide (INTERPHASE)
• While it is growing, it also
begins to replicate its
DNA
• When it is ready to
divide, MITOSIS, the
process of dividing its
DNA, begins.
• When the DNA has been
evenly split, Cytokinesis
(division) occurs.
DNA and Chromosomes
• In humans, the information needed to build
all the proteins and enzymes you need for
your cells to function is stored in a
chemical called DNA (Deoxyribose
Nucleic Acid)
• This DNA is found in the nucleus of every
cell of your body (except red blood cells)
• Every cell has (in theory) the information
needed to make an entire body.
Anatomy of a Chromosome
• Chromosomes hold the DNA in the
nucleus, all coiled up.
Nucleosome
Chromosome
DNA
(double helix)
Coils
of DNA
Supercoils
Histones
(proteins that give DNA more strength)
YouTube Videos
• Zoom in on DNA
• Wrapped up DNA
• Nucleosomes & chromatin
Terminology preview
• Chromosome: a structure composed of DNA and some proteins
that contains the genetic information. Just before division a chromosome
may contain two sister chromatids.
• Chromatid: a half a chromosome– before a cell divides its
chromosomes appear to be “double width” and is made of two chromatids.
• Gene: The portion of a chromosome that contains enough DNA to code
for one specific trait.
• Diploid: A cell that contains a full “double set” of chromosomes (half
from the mother, half from the father). Normal body cells are diploid.
• Haploid: a cell that contains only one set of chromosomes (half the
normal number). Egg and sperm cells are haploid
• Homologous :
In a diploid cell the chromosomes are “paired”.
Each pair is said to be homologous, meaning they resemble each other.
Enrichment: A bunches of “chromes” and “somes”
• In addition to the words defined before, you will
find lots more words that either start with
“chrom” or end with “some”
• Chromatid, Chromosome, Chromatin, Chromatography
• Chromosome, Centrosome, Lysosome, Ribosome.
• “Chromo” means “coloured”. Since DNA picks
up dyes easily, anything made of DNA can be
easily identified under the microscope. Many
things made of, or containing, DNA start with
chrom- or chromo-.
• “Soma” or “some” means “body”. In biology this
is usually a tiny body, so organelles or cell
structures often end in -some.
Cell Division in Eukaryotes
• Eukaryotes (organisms with complete
cells) have two types of cell division:
– Mitosis (normal cell division)
– Meiosis (cell division for reproduction)
• The examples are all from eukaryotes.
• Prokaryotes (simple cells like bacteria)
undergo a similar, but slightly simpler,
process called “binary fission”
– This will not be studied in any detail this year,
just think of binary fission as “mitosis-light”
Mitosis and normal cells
• Normal body cells are Diploid, which
means that they have a double set of
chromosomes
• In humans, this is 2x23=46 chromosomes
• 46 is the “human diploid number”
• In Mitosis, all chromosomes are duplicated
before the cell divides.
• In humans, this means that each cell starts with 46
chromosomes before it divides, and the new cells
each end up with 46 chromosomes after division.
Steps in the Cell Cycle
I
PM
AT
= INTERPHASE
Remember:
Preparing for Mitosis
= PROPHASE
= METAPHASE
= ANAPHASE
= TELOPHASE
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Mitosis
5 main
phases of
a cell’s life
Animated Diagram of Mitosis
The next slide shows an animated
version of the main steps of
mitosis. Just watch and follow what
happens. More detailed
descriptions will follow.
Mitosis
Centrosome (centrioles)
Nucleus
Chromosomes
Cytokinesis:
Prophase:
Metaphase:
Interphase:
Anaphase:
Telophase:
The
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MITOSIS in Detail
• The following slides show more detail of
how mitosis occurs. The photographs are
of a plant cell. Animal cells are similar, but
the centrosomes are visible in animals, but
not in plant cells.
INTERPHASE
• Interphase is the time during which a cell
matures, grows, and duplicates its DNA
• The cell spends most of its life in
interphase
• Your book identifies 3 parts of interphase:
– G1: The cell grows and matures
– S: The DNA replicates
– G2: The cell grows some more, in preparation
for mitosis.
Photograph of Interphase
PROPHASE
• DNA coils up to form chromosomes
• each chromosome has 2 chromatids
• Two centrosomes appear.
• These are fibre anchoring points near the nucleus.
• In animal cells there are tiny organelles (centrioles) inside the
centrosomes. Plant cells don’t have visible centrioles.
• The centrosomes move towards opposite ends
• In this plant cell you won’t see them.
• The nuclear membrane disappears (breaks up)
• The centrosomes produce “spindle fibres” made
of 2 varieties of microtubules.
• Kinetochore fibres: attach to the chromosomes
• Polar fibres: extend from one centrosome to the other.
Photo: Early Prophase
Chromosomes
METAPHASE
• During metaphase the kinetochore
(spindle) fibers move the chromosomes to
the center of the dividing cell.
• The chromosomes line up along the
equatorial plane (middle) of the cell.
• Kinetochore fibers are attached to each
chromatid, near the centromere.
Photo: Metaphase
Spindle (fibers)
Centrosome location
(this is a plant cell, so the
centrioles are not visible)
Chromosomes
Equatorial Plane
ANAPHASE
• The chromatids separate
• They move towards opposite ends of the
cell
• From this point on, each chromatid is
considered to be a new chromosome.
Photo: Anaphase
Centrosome location
Chromosomes
TELOPHASE
• Chromosomes reach the opposite ends of
the cell.
• Chromosomes begin to uncoil, changing
back into chromatin.
• Nuclear membranes begin to form on each
side of the cell
• CYTOKINESIS begins.
CYTOKINESIS
• As TELOPHASE ends, the cell divides into
two new cells.
• A cleavage furrow forms, and pinches the
cell apart.
• There are now two cells, each with a full
set of chromosomes (ie. Diploid)
Photo: Telophase & Cytokinesis
Cleavage
furrow
Nucleus
forming
Cells Divide
Videos
Serious MITOSIS Description
Mitosis introduction Mind Bites
The most annoying MITOSIS song ever!
MITOSIS animation.
Silent MITOSIS animation
MEIOSIS
1/
2
• Meiosis is a process of cell division that
reduces the number of chromosomes in a
cell by half.
• Meiosis occurs in the reproductive organs.
• In humans, meiosis produces ova (egg) or
sperm cells. This is necessary for sexual
reproduction.
M
M
• Visually, meiosis resembles
mitosis
(phases: P.M.A.T.) but twice!
• Prophase I, Metaphase I, Anaphase I, Telophase I
• Prophase II, Metaphase II, Anaphase II, Telophase II
Why is Meiosis so Important?
• It’s only important to organisms that
reproduce sexually!
• If meiosis didn’t occur, then the egg and
sperm cells would have the same number
of chromosomes as the body cells
• When fertilization occurred, there would be
too many chromosomes!
Meiosis, First Division.
Homologous Chromosomes Line up
• Remember:
• Humans have 46 chromosomes in each cell.
• These are actually 23 pairs of homologous
chromosomes (23 maternal, 23 paternal)
• During the first metaphase of MEIOSIS, the
chromosomes line up in homolgous pairs.
• When anaphase starts, the chromatids
don’t break apart (unlike mitosis), instead
whole chromosomes (including both
chromatids) move to each side of the cell.
Meiosis, Fist Division (continued)
Crossing Over
• Normally the genes on one chromosome
travel together...
• but sometimes during meiosis, a gene
from one chromosome may switch places
(or cross over) with the corresponding
gene on the homologous chromosome
that lines up beside it during the first
division of meiosis.
• Crossing over helps randomize the distribution of
genes from parent to offspring
Meiosis, Second division
Chromatids finally split.
• After the first division of meiosis, both
human cells have 23 chromosomes
(instead of the normal 46), but each
chromosome still has two chromatids.
• A second division occurs, this time the
chromatids do split (more like mitosis),
giving four cells with 23 chromosomes
each.
For Diagrams of Meiosis, See pages 154 and 155
Oogenesis
Spermatogenesis
After Meiosis:
Production of Gametes
• In the male, each cell that undergoes
meiosis produces four spermatids, that
later grow tails (flagella) and become
sperm cells.
• In the female, each cell that undergoes
meiosis produces one large ovum and
three tiny polar bodies. The ovum survives
to become an egg, and the three polar die
and disintegrate.
Diploid Body Cells
Haploid Gamete Cells
Mitosis
 Single division
 Double division
Meiosis
Diploid Body Cells
More about Human Chromosomes
• This picture shows the
chromosome map (or
karyotype) of a normal
human, showing the
chromosomes as they would
appear immediately after
mitosis.
• It consists of 22 numbered
pairs of neatly matched
homologous chromosomes
(called the autosomes)
• The last pair (called the sex
chromosomes) is not
numbered, and are only
neatly matched in females
(XX). In males, they are
mismatched (XY)
Assignments
• Read chapter summaries
– Page 157
• Do End of Chapter Questions
– Page 158 # 1 to 20
End of Chapter Questions
1. A chromosome is made of DNA and histone
proteins, tightly wound into rod-shaped
structures. Homologous chromosomes are
pairs of morphologically similar chromosomes.
2. Mitosis is normal division of the cell nucleus.
Meiosis is division of the cell nucleus to form
gametes (sex cells). Cytokinesis is the division
of the cell cytoplasm into two new cells.
3. Autosomes are the normal, paired homologous
chromosomes. Sex chromosomes are a
dissimilar pair of chromosomes used in sex
determination (XX or XY)
• 4. Kinetochore fibres attach to the center
(or centromere) of the chromosomes
during metaphase (of mitosis). Polar
fibres connect one centrosome to the
other. Both are types of spindle fibres.
• 5. A Diploid cell has a complete (double)
set of matching homologous pairs of
chromosomes (2 of each type). A Haploid
cell has only one of each of the
homologous chromosomes. (1 of each
type).
6. D) Prokaryotic chromosomes consist of a
circular DNA molecule.
7. C) A chromatid one of two identical parts that
make up a (mature) chromosome.
8. B) Every species has a distinctive number of
chromosomes per cell.
9. D) Binary fission is prokaryotic cell division
10.A) Mitosis can increase the number of body
cells without changing the information
contained in the cells’ DNA
11.D) Interphase is a time of cell growth and
development (not A, since there is no G3 phase)
12. A) Cytokinesis differs in plant and
animal cells.
13. A) Spermatogenesis produces four haploid
cells (called sperm)
14. B) Oogenesis requires meiotic cell divisions.
15. D) Crossing over occurs during
meiosis I