Transcript Document 7348618

Chapter 8 Cell
Reproduction
 When a cell divides, its DNA coils into
small packages, the chromosomes. The
chromosomes consist of DNA and
protein histone molecules. Histones
maintain shape and allow for compact
storage of DNA.
Typical Chromosome
Histone Proteins & DNA
Histone proteins
allow DNA to coil
and to be stored
away much
easier. The DNA
can coil and
uncoil as
needed.
Chromatin

When a cell is not
dividing, the parts
of the DNA that
need to be used
uncoil, the less
tightly coiled form
is chromatin.
Chromosomes, Chromatids &
Centromeres.

In a dividing cell,
the chromosomes
have two identical
halves, the sister
chromatids. The
two chromatids are
attached at the
centromere.
Prokaryotic Chromosomes

Single prokaryotic
chromosomes are
found in the
cytoplasm, not
bound by a nuclear
membrane, and
often attached to
the cell membrane.
Species and Chromosome
Number

Each species has a characteristic number
of chromosomes:

Adder’s tongue fern
Cat
Chimpanzee
Dog
Human
Lettuce
Orangutan


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1262
32
48
78
46
18
48
 Chromosomes are either:


1. sex chromosomes
2. autosomes
Human Sex Chromosomes
Humans have one pair of chromosomes that
determines the sex of the individual.
 (When it is said that one pair determines the
sex remember this means 2 chromosomes
out of the total of 46 chromosomes. That
means there are 44 other chromosomes.)

Sex Determination in Humans


Sex chromosomes
determine the sex of
the organism. Human
sex chromosomes are
either "X" or "Y.“
Sex is determined by
which pair of sex
chromosomes is
received from parents.
Autosomes
 All the other chromosomes in an
organism are the autosomes.
 Humans have 2 sex chromosomes and
44 autosomes for a total of 46.
Homologous Chromosomes
 Every cell, excluding sex cells, has 2
copies of each chromosome. 1 copy
from each parent. The two copies of
each autosome are called homologous
chromosomes. Homologues are the
same size and shape and carry genes
for the same traits.
Karyotypes
 A karyotype is a photomicrograph of
the chromosomes in a dividing cell.
Karyotypes are used to examine an
individual's chromosomes. The
chromosomes are photographed, cut
out, and arranged by size and shape
into pairs.
Normal Human Karyotype
Human Karyotype Lab
Some human diseases are caused by
additional, missing, or damaged
chromosomes.
 Karyotypes are used observe the
chromosomes themselves.
 Cell division is stopped at metaphase and a
picture of the chromosomes is made.

Diploid vs. Haploid
 Cells with two autosomes for each
homologous pair are diploid. Diploid
cells also have two sex chromosomes.
 Diploid is abbreviated as 2n.
 In humans, the 2n number is 46.
 Sperm and egg cells are haploid,
containing only one set of
chromosomes. They have only one
autosome and one sex chromosome
from each pair.
 Haploid is abbreviated 1n.
 In humans, the 1n number is 23.
Sperm Cell
Egg Cell

When a 1n sperm cell unites with a 1n
egg cell, a 2n new organism results. (If
reproductive cells were 2n, there
would be too many chromosomes and
the fertilized egg cell would be
nonfunctional.)
Fertilization of a Human Egg Cell
Cell Division
 Humans produce about 25 million new
cells per second (about 2 trillion new
cells per day) and all cells come from
preexisting cells.
Cell Division in Prokaryotes
 Binary fission is the mode of cell
division in prokaryotes
Binary Fission

Steps:

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1. Binary Fission begins.
2 DNA is copied.
3. The cell grows to
twice its original size.
4. The membrane
pinches inward
between the DNA
copies, a new cell wall
forms.
Cell Division in Eukaryotes
 2 kinds of eukaryotic cell division:
 Mitosis
 Meiosis
Mitosis
 One cell splits to form two identical
cells as in growth, repair, or asexual
reproduction.
Meiosis
 Occurs during the formation of
gametes (sperm and egg cells).
The number of chromosomes is
reduced by 1/2.
The Cell Cycle

The Cell Cycle is the set of repeating events
during the life span of a cell. Cell division is
only part of the cell cycle. The time
between one cell division and the next is
interphase. Interphase is divided into three
phases and cell division is divided into two
phases.
The Cell Cycle (picture)
Mitosis & Cytokinesis
 Cell division consists of mitosis and
cytokinesis.
 MITOSIS is division of the nucleus.
 CYTOKINESIS is division of the
cytoplasm.
The Phases of Mitosis
1.
2.
3.
4.
Prophase
Metaphase
Anaphase
Telophase
Interphase
 Interphase begins the cell cycle.
 Cells spend most of their time in
interphase.
 Cells are about 1/2 of their normal size
at the beginning.
The three sub phases of Interphase
 G1 – the cell grows to its mature size
 G0
– the cell cycle stops
 S - the DNA is synthesized (copied)
 G2 – the cell makes final preparations
for cell division
G1 – Growth Phase 1
 G1 begins immediately after cytokinesis.
During this period, cells grow to their
full size, new organelles are produced,
and the cell carries out normal
activities.
G0
 G0 cells reach mature size at the end of
G1 and then stop their cell cycle. They
do not copy their DNA nor do they
prepare for cell division.
 Example: Central Nervous System
cells.
S - Synthesis
 During the S phase of interphase, DNA
is synthesized. An exact copy of the
DNA is made to be used in the new cell
that will be formed.
G2 - Growth Phase 2
 During G2, the cell prepares for cell
division. Organelles and cellular
structures are duplicated. Structures
begin to move to the places they need
to be in cell division.
Prophase

Prophase begins mitosis.
 DNA condenses to chromosomes.
 These are actually double
chromosomes attached at a
centromere.
 The nuclear envelope begins to
disassemble and disappear.
Centrosomes appear next to the nucleus.
 Spindle fibers form between the
centrosomes as they move toward the
poles.
 2 types of spindle fibers
 Kinetochore fibers
 Polar fibers

Metaphase
2nd phase of mitosis.
 Chromosomes are easy to identify. It is at
this stage that most images for karyotypes
are taken.
 Chromosomes are attached to kinetochore
fibers that move them to the equator of the
cell.

Metaphase
Anaphase
Chromatids separate at the centromere.
 Chromatids are slowly pulled toward the
poles. The centromeres lead the way. This
produces a classic “A” or “V” shape to the
chromosome.
 Once the chromatids separate, they are
considered to be individual chromosomes.

Telophase

Telophase begins when the chromosomes
reach the poles.



The spindle fibers disassemble (they are made of
proteins.).
The chromosomes begin to uncoil back to their
chromatin form.
The nuclear membrane and the nucleolus
reappear in each new cell.
Cytokinesis
Cytokinesis is the division of the cytoplasm.
 Begins with an inward pinching of the cell
membrane midway between the two new
nuclei. The area that pinches inward forms
the cleavage furrow.


The microfilaments in
the cleavage furrow
eventually pinch the
cells into two new
cells.
Cytokinesis in Plants


Cytokinesis also occurs in plant cells as
they divide.
Vesicles from the Golgi join together at the
midline and form a new piece of cell wall
that separates the new cells.
Control of Cell Division
Proteins regulate the cell cycle.
 Controls occur at 3 main checkpoints:
 1. Cell Growth (G1) checkpoint
 2. DNA Synthesis (G2) checkpoint
 3. Mitosis checkpoint

Cell Growth G1 Checkpoint


If the cell is healthy and full size, proteins
will start DNA synthesis.
Proteins can stop cell division at this point
if:



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conditions are not good for DNA synthesis
the cell needs a rest
the cell goes to G0
the cell has not grown to full size
DNA Synthesis G2 Checkpoint

DNA replication is checked by proteins for
mistakes. If no mistakes are found, mitosis
begins.
Mitosis Checkpoint

Proteins check mitosis and signal the cell to
enter G1 if all the events of mitosis occurred
correctly.
Loss of Control of Cell Division:
CANCER
If a mutated gene produces a regulatory
protein, the protein may not work correctly.
 Cell growth and division may be abnormal
with the mutated protein.
 This condition may lead to cancer
(uncontrolled cell growth).

 Cancer cells do not respond to the
normal control mechanisms.
 Some cancers overproduce cells.
 Some cancers interfere with proteins
that slow or stop the cell cycle.
Overproduction of Cells
Meiosis
 Meiosis is a nuclear division process
that reduces the number of
chromosomes to one half the number
of the original cell.
 Meiosis produces sex cells:
 Male:
sperm cells
 Female: egg cells
 In humans, sperm and egg cells each
contain 23 chromosomes (1n).
 When sperm and egg cells fuse, a
zygote with 46 chromosomes (2n)
results.
Zygotes
Meiosis I
 Cells beginning to go through meiosis
enter the G1, S, and G2 phases and
divide through an initial cell division
called Meiosis I.
Meiosis II
 Meiosis II is a second cell division in
which the DNA is not copied. The 2
cells that go through this division form
four haploid (1n) cells.
Major Events of Meiosis I

Phases of meiosis I:
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I
 Cytokinesis I
Prophase I
DNA coils into chromosomes.
 Spindle fibers appear.
 The nuclear membrane and nucleolus
disappear.

Homologous
chromosomes pair
during synapsis.
 Tetrads form and
crossing over
occurs.

Crossing Over
Portions of chromatids break off and
exchange places with corresponding
portions of adjacent chromosomes.
 This process allows for the exchange of
material between maternal and paternal
chromosomes resulting in a new mixture of
the genetic material.

Crossing over
Metaphase I
The tetrads line up along the equator of the
cell.
 The orientation of the tetrad is random.
 Spindle fibers from one pole attach to the
centromere of one homologous pair and
fibers from the opposite pole attach to the
centromere of the other homologous pair.

Anaphase I


The pairs of homologous chromosomes
consisting of two chromatids joined by a
centromere each move to opposite poles of
the cell.
This is called independent assortment and
it results in greater genetic variation.
Telophase I / Cytokinesis I


The chromosomes reach the opposite
poles of the cell and cytokinesis begins.
At this time, the cells contain the haploid
number of chromosomes, but each new cell
contains two copies of each chromosome
as chromatids.
Meiosis II
 Meiosis II occurs in the cells formed
during Meiosis I.
 The events of meiosis II are NOT
preceded by DNA synthesis.
Prophase II
 The chromosomes remain in their
tightly coiled chromosome form.
 Spindle fibers begin to move the
chromosomes to the middle of the cell.
Metaphase II


The chromosomes align along the equator of
the cell.
The chromatids randomly face one of the two
poles.
Anaphase II
 The chromatids separate and move
toward the opposite poles.
Telophase II
 The nuclear membrane reforms around
the nucleus of each new cell.
Cytokinesis II
 The cytoplasm divides resulting in 4
new cells each with half of the cell’s
original number of chromosomes.
Spermatogenesis / Oogenesis
Gametes are the only cells in humans that
divide by meiosis.
 Meiosis occurs:
 In the testes of males.
 In the ovaries of females.

Spermatogenesis
Spermatogenesis occurs in the testes and
results in the production of immature
spermatids.
 Spermatids eventually develop into mature
spermatozoa.
 For every cell that enters spermatogenesis,
four healthy functioning spermatozoa are
formed.

Testes
Oogenesis
Oogenesis occurs in the ovaries of females
and results in the formation of egg cells.
 For each cell that starts oogenesis, one
healthy egg cell is formed and 3 polar bodies
are also produced.
 Polar bodies are egg cells with no or very
little cytoplasm.

Ovary

During cytokinesis I and cytokinesis II of
meiosis, the cytoplasm is unequally divided
between the egg cells. This results in one
functioning egg cell and three
nonfunctioning polar bodies. The polar
bodies eventually degenerate.

The reason that egg cells form this way is
that egg cells have to spend several days
after they have been fertilized without being
attached to the food source of the Mother.
This increased egg size allows enough
nutrients to be stored in the egg to survive
until the placenta begins to function.
Sexual Reproduction

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Sexual reproduction is the production of
offspring through meiosis and then the
union of sperm and egg.
Offspring produced by sexual reproduction
are genetically different from their parents.
Identical twins carry identical DNA.
New combinations of genes are beneficial.
Name the pictured phases.