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Chapter 8 Cell Reproduction
http://www.pisgahscience.com/bioonline/
• Which of these
pictures shows an
organism with cell
division taking
place?
I. Introduction - Cell Division:
• Occurs more often in young, growing
organisms or when damaged tissue is
repaired.
• Some tissues like skin & intestinal cell
grow throughout life while some- like
nerve cells, do not divide again after
full size achieved.
• Genetic information must be divided
between cells.
• How does this happen?
Cell Division & Reproduction
-allows for growth &
reproduction of individuals
-passes on genetic
information to future
generations.
In this chapter:
• Chromosome Structure
– Tightly coiled DNA molecules
• Cell Division– Mitosis (nuclear division)
– Cytokinesis (division of cytoplasm)
• Cell Reproduction–Meiosis- Process reduces # of
chromosomes to half of original cell to
produce gametes.
II. Chromosome Structure
• Chromosomes are rod-shaped
structures made of DNA wrapped
around a protein called HISTONE
http://www.firedbydesign.com/1949/cell-animation/
Chromosome Structure
• When a cell is NOT
dividing, the DNA
is loosely coiled in
the cell NUCLEUS
& is called
CHROMATIN.
When eukaryotic cells
are ready to divide,
the DNA coils tightly
around proteins called
HISTONES & forms
CHROMOSOMES.
Chromosomes
• Consist of 2
identical halves
• Each half is called a
CHROMATID.
• Are attached to
each other at the
CENTROMERE.
http://www.micro.utexas.edu/courses/levin/bio304/genetics/chromosome.gif
http://www.gla.ac.uk/cancerpathology/genemech/awest/chromatin.jpg
Chromosome Structure
How many Chromosomes in an Organism?
• Look at this table- Does the # of chromosomes
show how complex the organism is?..........NO!
B. Chromosome Numbers
In various species:
•
•
•
•
•
•
•
•
•
Adder’s Tongue Fern
Carrot
Cat
Chimpanzee
Human
Dog
Potato
Earthworm
Fruit fly
1,262
18
32
48
46
78
48
36
8
•
Garden pea
20
• Lettuce
18
• Can different organisms can have same #
chromosomes? …….. YES.
• Can any conclusion about relatedness be
made because of this? NO.
Sex chromosomes or Autosomes.
• Sex Chromosomes (X, Y) - determine sex
& carry genes for other characteristics.
• Autosomes- all of the other chromosomes in
an organism.
• Example: Humans have 2 sex chromosomes &
remaining 44 are autosomes. Humans
have 46 chromosomes!
Sex chromosomes:
• Females are XX
• Males are XY
• Males determine the sex of the babythe sperm may contain either an X or a Y;
eggs can only have Xs.
Homologous Chromosomes
• Most organisms have 2 copies of
each chromosome.
• These pairs are called homologous
chromosomes or “homologues”.
geneticsmodules.duhs.duke.edu/Design/page.asp...
2 copies of each gene
• On each homologue- there are a genes
for the same traits
• For Example
– if one of the homologous chromosomes has a
gene for eye color, so will the other.
http://course1.winona.edu/sberg/ILLUST/homolog1.gif
Homologous chromosomes
•The 2
chromatids
•are exact copies of each
other- they’re identical.
• attached by a centromere.
Homologous
chromosomes
•The
•ARE NOT exact copies.
(One set is from mom, one from dad)
•They have genes for the same
traits
www.emunix.emich.edu
The large red & blue chromosomes
are homologous.
The small purple & green
chromosomes are homologous.
Copied from: www.stanford.edu/group/hopes/basics/dna/b5.html
Homologous chromosomes in humans
www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=g...
Karyotype
• A picture (photomicrograph) of chromosomes.
• Notice that humans have 46 chromosomes
– 22 homologous pairs of autosomes
– Plus 2 sex chromosomes. (Is this person male or female?)
Karyotype Copied from: http://homepages.uel.ac.uk/V.K.Sieber/human.htm
A human karyotype
Diploid & Haploid cells
• Diploid- cells having 2 sets of chromosomes.
• Haploid- have only 1 set of chromosomes
These are Sperm cells & Egg cells.
**What is the number of chromosomes in a
human haploid cell?
(23)
• Diploid is usually abbreviated 2n
( where “n” is the number of chromosomes in that
organism)
(in humans, the diploid- or 2n number is 46
• Haploid is 1n.
• When an egg cell (1n) combines with a sperm cell
(1n), the new cell will be diploid (2n)
**What would happen if haploid cells were not
used in sexual reproduction?
III. Cell Division
A.Prokaryotic Cell Division
•
•
Prokaryotes are bacteria that lack nuclei &
membrane bound organelles.
Have a single circular stand of DNA
–
•
molecule is not coiled & do not form chromosomes.
Division is much simpler than in eukaryotic
cells
• Binary fission - how prokaryotic cells divide.
Binary fission:
- Is the division of a prokaryotic cell into 2
identical offspring cells. (creates clones)
Cell Reproduction:
Mitosis & Meiosis
B. 2 kinds of cell division in eukaryotic cells:
• Mitosis occurs in cells for
growth, development, repair
or asexual reproduction.
• Meiosis occurs during formation of gametes
for sexual reproduction
www.ccs.k12.in.us/.../Humanembryology.htm
2. The Cell Cycle
• The cell cycle is the repeating set of events in
the life of a cell. 2 main parts; further divided:
• INTERPHASE – Time between cell divisions
– G1 phase
– S phase
– G2 phase
• CELL DIVISION- When chromosomes &
cytoplasm are equally divided.
– Mitosis - nuclear division
– Cytokinesis - cytoplasm division
http://137.222.110.150/calnet/cellbio/page6.htm
3 Stages of Interphase
• G1 phase- cell grows to mature size.
– “G1” stands for the time gap
– after cell division & before DNA replication.
• S phase- cell’s DNA is copied (synthesized).
– (“S” stands for synthesize, meaning to “make”)
• G2 phase- the cell prepares for cell division.
– “G2” is the time gap between DNA synthesis & cell
division.
Some cells Exit the cell cycle:
• Go phase- state where cells do not copy DNA or
prepare for cell division. (example- CNS cells)
C. Mitosis
- is division of the nucleus
- results in:
- two offspring cells
- Diploid # chromosomes
- genetically identical (clones)
of the original cell.
Stages of Mitosis
A process divided into 4 phases:
1. PROPHASE
2. METAPHASE
3. ANAPHASE
4. TELOPHASE
Memory aids: "People Meet And Talk", or "PMAT"
or
“Pro met Anna (on the) telophone”
or make up your own!
What happens during each phase of mitosis?
1. PROPHASEof DNA into chromosomes.
-The nucleus starts to disappear.
-Centrosomes appear (In animal cells - coiling
centrioles appear also) & move to opposite
poles of the cell.
-Spindle fibers, made of microtubules, radiate
from the centrosomes & forms the mitotic
spindle & will be used to equally divide the
chromosomes in the next phase.
1. Prophase
• Can you list some things
that happen in prophase?
• Find the structures in
the drawing:
– Nuclear membrane
– Homologous Chromsomes
– Chromatids
– Centromere
– Centrosomes
– Spindle fibers; (look in
book for 2 types of spindle fibers.)
Drawings from: www.tqnyc.org
2. METAPHASE
• chromosomes are
moved to the center of
the cell
• Note how they line up.
• Chromosomes are easiest
to ID under a microscope
during metaphase -most
karyotypes are made
from cells in metaphase.
3. ANAPHASE
- the chromatids separate
at the centromere & move
to opposite poles.
- after they separate,
each chromatid is now
considered to be a
separate chromosome.
(Tricky question- what “n” is
a cell in anaphase? 4 )
4. TELOPHASE
After the chromosomes
reach the opposite ends
of the cell:
• the spindle fibers
disassemble,
• the chromosomes return
to a less tightly coiled
state,
• the nuclear envelope
reforms in each newly
forming cell.
http://jabberwiki.wikispaces.com/file/view/mitosis1.gif/53565174
D. Cytokinesis
• Division of the cytoplasm:
• In animal cells- a cleavage furrow
forms as the area of the cell membrane
pinches the cell into 2.
• In plant cells- a cell plate forms a
new cell wall as vesicles from the golgi
apparatus join together at the midline of
the 2 dividing cells.
E. Control of Cell Division
• A cell spends most of its time in interphase.
• What triggers a cell to leave interphase & divide?
• feedback from Checkpoints:
– 1. Cell growth checkpoint (G1)- if the cell is healthy &
grows to mature size, protiens will intiate DNA synthesis.
– 2. DNA synthesis checkpoint (G2) proteins check
results of DNA replication , signal OK
– 3. Mitosis checkpoint, if ok- growth cycle starts.
Cell Division Errors
• If mutations occur
• proteins may not function properly
• growth may not be controlled
When control is lost = cancer.
IV. Meiosis
• How it is different than mitosis:
– reduces the number of chromosomes in new
cells to half the number in the original cell.
• Produces four haploid cells
(gametes)
• Different than two diploid cells as in
mitosis.
Beginning meiosis:
• Cells begin meiosis just like they begin
mitosis, after interphase.
• Cells must divide twice to reduce the
number of chromosomes & so we have
Meiosis I & Meiosis II.
• Remember- the whole point of meiosis
is to make gametes (which are 1N)
A. Meiosis- 8 phases
• MEIOSIS I
1. PROPHASE I
2. METAPHASE I
3. ANAPHASE I
4. TELOPHASE I
• MEIOSIS II
5. PROPHASE II
6. METAPHASE II
7. ANAPHASE II
8. TELOPHASE II
MEIOSIS I -what happens in each phase?
1. PROPHASE I
-Spindle fibers appear & the nuclear membrane
disappears as in mitosis.
NOTE IN MEIOSIS I -HOMOLOGUES LINE UP NEXT
TO EACH OTHER.
-This pairing of homologous chromosomes, which
does NOT occur in mitosis, is a synapsis.
-Each pair of homologous chromosomes is a
tetrad.
Genetic Recombination
During synapsis, chromatids within
a pair of homologues may twist
around each other.
Parts of the chromatids may break
off & attach on the homologous
chromosome.
This is called crossing-over.
Crossing over permits exchange of
genetic materials
= genetic recombination.
•http://regentsprep.org/Regents/biology/units/evolution/crossovr.gif
Important to understand:
• Crossing-over,
– which is when portions of homologous
chromosomes line up to form tetrads in
synapsis.
– exchange genetic material,
– occurs during prophase I
– results in genetic recombination.
2. METAPHASE I
•
Pairs of homologous chromosomes (tetrads) line
up on metaphase plate.
3. ANAPHASE I
•
•
Tetrads split- reducing the cells to haploid
The chromosomes have randomly separatedthis is called Independent Assortment.
4. TELOPHASE I
•
2 new cells begin to split off. Cytokinesis occurs.
MEIOSIS II
Meiosis II occurs in both cells made in meiosis I
5. PROPHASE II- Chromosomes condense, nuclear
membrane breaks down, spindle fibers form. NOT
preceded by replication or recombination.
6. METAPHASE II- Chromosomes line up on
metaphase plate
7. ANAPHASE II- The chromosomes divide at the
centromeres
8. TELOPHASE II- Chromosomes begin to uncoil,
nuclear envelope reforms, followed by cytokinesis.
Stages of Meiosis, continued
The biggest differences between
mitosis & meiosis occur at:
*prophase I (Meiosis I has synapsis, tetrads &
crossing-over. Mitosis does not.)
*metaphase I (Meiosis I has tetrads lining up on
metaphase plate.)
*anaphase I (Meiosis –the centromeres do not
separate & haploid number chromosomes results.
animation
Biologygmh.com
• http://glencoe.mcgrawhill.com/sites/0078695104/student_view0/
unit3/chapter10/concepts_in_motion.html#
B. What are Gametes?
Gametes are 1N cells for sexual reproduction
Meiosis -occurs to form haploid gametes for
sexual reproduction.
-
Spermatogenesis (production of sperm)
-occurs in humans in testes
-forms 4 haploid spermatids (develops 4 sperm cells)
-
Oogenesis (produces 1 egg cell, 3 polar bodies)
- All of the cytoplasm goes to 1 egg cell
- 3 useless polar bodies are formed from the other nuclei
& they eventually disintegrate.
Meiosis in Male and Female Animals
Sexual reproduction
• is the formation of offspring through
meiosis and the union of a sperm and
an egg.
• Offspring produced by sexual
reproduction are genetically different
from the parents
Questions
1. Which of the following statements about
prokaryotic chromosomes is true?
A. Prokaryotes have at least two
chromosomes.
B. Prokaryotic chromosomes consist of a
circular DNA molecule.
C. Prokaryotic chromosomes include histone
and nonhistone proteins.
D. Prokaryotic chromosomes are made of DNA
wrapped tightly around histone proteins.
2. Crossing-over occurs during which
process?
A. mitosis
B. meiosis I
C. meiosis II
D. interphase
3. Which type of cell division is shown in the
diagram?
A. mitosis
B. meiosis
C. binary fission
D. sexual reproduction
• 4. What are the differences between
mitosis & meiosis?
• 5. Explains the difference between
cytokinesis in plant & animal cells.
Your body contains over 200
different cell types!
• Cells are specialized into different types.
• The adult human body is made up of
about 60-90 trillion cells. That's a lot of
cells!!!
• If you lined up all the cells in a human
body end-to-end, you could actually circle
the earth 41/2 times!
Stem cells
• Tissue precursor cells that have the ability
to self-renew and differentiate into more
specific cell types.
• They are important because they can
replace dying, old or damaged cells.
• These cells are found in human embryos,
fetuses, children and adults, i.e. at all
stages of development and in most tissues
but it is the embryonic cells which have
raised the most controversy.
• Early human embryos (5 - 6 day old
blastocysts) have an outer cell layer
from which the placenta develops, and
an inner cell mass, in the region of 200
cells, which gives rise to the fetus.
This inner cell mass is the source of
embryonic stem cells.
Growing stem cells is making
clones of those cells.
• Mixed reactions to the prospect of
cloning for biomedical research.
• Supported by some for its medical
promise
• Opposed by others who view it as
intentional exploitation and destruction
of human life created specifically for
research purposes.
• Bioethical problems-
A mere speck nestled in
the eye of a needle, a
five-day-old embryo
(photographed using an
electron microscope)
contains controversial
stem cells. Photograph by Yorgos Nikas,
M.D.
Stem cell research has been living up to
its reputation for being fast-paced.
In the few weeks since the July National
Geographic cover story went to press,
several important advances were
reported—along with some significant
political milestones.
>> See Story from National
Geographic
Stem cell research: the US politics
• 2006- The Senate opens debate on bill that matches H.R.
810, which would ease Bush's restrictions. Matching bill
passed the House May 24, 2005. Senate also opens debate
on two other stem cell bills. One would encourage research
into creating stem cell lines without destroying human
embryos and the other would ban the creation of a fetus
solely for the purpose of destroying it and harvesting its
body parts. President Bush says despite divided GOP, he
will not ease policy and if passed will likely veto. Post Story
• 2006- In the first veto of his presidency, Bush vetoes the
stem cell bill at a White House ceremony where he was
joined by children produced by what he called "adopted"
embryos. Post Story
Embryonic stem cell advance
Updated: 7:34 p.m. ET June 6, 2007
http://www.msnbc.msn.com/id/19067616/
• Reprogrammed mouse cells avoid controversial
destruction of embryos
• NEW YORK - In a leap forward for stem cell
research, three independent teams of scientists
reported Wednesday that they have produced the
equivalent of embryonic stem cells in mice using skin
cells without the controversial destruction of
embryos.
• If the same could be done with human skin cells — a
big if — the procedure could lead to breakthrough
medical treatments without the contentious ethical
and political debates surrounding the use of embryos.
• Experts were impressed by the achievement.
Stay Tuned• Remember- science is a DYNANIC field- it
keeps changing daily.
Stem Cells: Promise & Difficulty
-Read:
-page 160 (chapter 8 in Holt Textbook)
-answer questions 1 ,2 & 3 with a
partner
- See reading assignment & questions
in packet handout.