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CELL CYCLE
How Do Cells Divide?
What you will learn…
1. Why Do Cells Divide? 2. Chromosome structure 3. Cell Division in Prokaryotes 4. Cell Cycle 5. Mitosis 6. Cytokinesis 7. Control of Cell Division and Cancer 8. Meiosis 9.Why do cells need two types of cell
division? 10. Gamete Formation
1. Why Do Cells Divide?
Virchow: Cells can only come from preexisting cells In unicellular organisms, can reproduce an entire
organism Allows multicellular organisms to reproduce
asexually Basis of sexual reproduction sperm and egg Allows fertilized egg, or zygote, to develop into an
adult organism Replaces worn-out or damaged cells Enables multicellular organism to grow to adult size
http://www.pbs.org/wgbh/nova/miracle/program.html
2. Chromosome Structure
Human cells carry about 20,000 genes to make 100,000 proteins.
Almost all genes are located in the nucleus Very small amount found in mitochondria
Genes are found on DNA DNA can be in the form of Chromatin:
Diffuse mass of long, thin fibers, not seen under the microscope, less tightly coiled
Combination of DNA and protein DNA must be tightly packaged before cell
division, so it can be evenly divided between the two new cells. DNA will now be in the form of Chromosome!
http://www.dnalc.org/resources/3d/07-how-dna-is-packaged-basic.html
2. Chromosome Structure
Chromosomes Rod-shaped structure Coiled up, compact forms of chromatin Contains one long DNA molecule bearing
hundreds or thousands of genes. DNA is attached to protein molecules called
histones and other non-histone proteins DNA wraps with protein like wrapping paper on
a present giving it the X-shape Only found in eukaryotic cells (prokaryotes
have naked, circular shaped chromosomes)
2. Chromosome Structure
2. Chromosome Structure
Sister chromatids Each duplicated chromosome contains two
identical copies. Centromere
The point by which two chromatids are joined.
Chromatin Diffuse mass of long, thin fibers, not seen
under the microscope, less tightly coiled Combination of DNA and protein
2. Chromosome Structure
3. Cell Division in Prokaryotes Binary fission
Process by prokaryotes reproduce by cell division.
Steps: Duplication of chromosomes and separation of
copies. Cell elongates Divides into two daughter cells
3. Cell Division in Prokaryotes
4. Cell Cycle
In your own body, millions of cells must divide every second to maintain the total number of about 100 trillion cells.
Some cells divide once a day, and some do not at all (mature muscle cells, brain cells)
4. Cell Cycle
Starts out with Interphase Occurs when the cell is between cell division Interphase stages:
G1: Cells grow to mature size S: DNA is copied G2: Cell prepares for division
Cells exit the cell cycle via…G0: Cells do not copy DNA or prepare for mitosis, but are still alive (e.g. nervous system)
5. Mitosis
The last stage of the cell cycle when the nucleus of a cell divides to produce two new daughter cells (after cytokinesis) each with the same amount and type of chromosomes as the parent cells.
Mitosis is divided into four phases: A.Prophase B. Metaphase C. Anaphase D. Telophase
5. Mitosis
A.Prophase: What does the cell look like?
Centrioles and spindle fibers appear Nuclear envelope disappears, and chromosomes are
visible What happens to the DNA and nucleus?
Chromosomes form when chromatin tightens and coils
Nuclear membrane breaks down and disappears What two things appear near where the nucleus
was? Centrioles and spindle fibers
5. Mitosis
A. Prophase
5. Mitosis
B. Metaphase What does the cell look like?
Chromosomes move to the middle Where are the chromosomes during
metaphase? Middle of the cell
5. Mitosis
B. Metaphase
5. Mitosis
C. Anaphase: What does the cell look like?
Chromosomes move to the end of cell What happens to the chromosomes?
Chromosome splits at centromere into 2 chromatids and moves to end of cell
5. Mitosis
C. Anaphase
5. Mitosis
D. Telophase What does the cell look like?
Cell starts to pinch in Nucleus starts to reform Chromosomes are at opposite ends
What happens to the chromosomes and nucleus? Nucleus forms back around single chromatids
5. Mitosis
D. Telophase
6. Cytokinesis
What is cytokinesis? Cytoplasm and contents (other organelles)
divide What’s special about cytokinesis in plants?
Cell wall also divides with new cell plate in middle
What’s special about cytokinesis in animals? Takes place when the cell membrane pinches in
until the cytoplasm is pinched into two equal halfs
7. Control of Cell Division and Cancer Cell division is a complex process that
needs to be regulated. These regulators determine when and
how the cell should divide. External Regulators Internal Regulators
7. Control of Cell Division and Cancer External regulators:
Various proteins produced by other cells that speed up or slow down the cycle. If the cell touches other cells, than cell division
slows down. If enough space between cells and nutrients
are available, growth factors and other proteins make cells divide or speed up their cell cycle.
7. Control of Cell Division and Cancer
Internal regulators: Cyclins –
proteins that regulate the timing of the cell cycle in eukaryotic cells.
Other regulator proteins (checkpoints)– they make sure that certain things happen in the
cell before the cell moves to the next phase of the cell cycle
3 major checkpoints in the cell cycle. The age of the cell.
7. Control of Cell Division and Cancer Cancer cells
lack normal checkpoints and continue to grow without inhibition
do not respond to normal signals within the cell
are not inhibited by other cells will divide indefinitely
7. Control of Cell Division and Cancer Mutations in the genes of these
checkpoint proteins may lead to cancer: The uncontrolled growth of cells.
Tumor: an abnormally growing mass of body cells Benign tumor
If abnormal cells remain at original site Can be problematic if disrupt certain organs,
but usually easily removed by surgery Malignant tumor
If abnormal cells spread into other tissues and body parts, interrupting organ function
7. Control of Cell Division and Cancer
1. Tumor growth 2. Blood vessels feed tumor
3. Tumor cells enter blood and lymph vessels
4. Secondary tumors form in other parts of the body
Movie clips on cancer, its nature and experiments to treat it (Parts 2 and 6)
http://www.pbs.org/wgbh/nova/cancer/program.html http://www.youtube.com/watch?
v=HonoQ6mE6dY&feature=related
Tumor Progression
7. Control of Cell Division and Cancer
Treatment of Cancer: Surgical removal of tumor – Most effective when
tumor is in a defined area
Chemotherapy – Medicines that disrupt the process of mitosis in rapidly growing cells
Radiation Therapy - High energy gamma radiation is aimed at the growing tumour. This damages the DNA in rapidly dividing cells and helps to destroy the tumor.
8. MeiosisMany of the stages of meiosis closely
resemble corresponding stages in mitosis.Type of cell division that produces haploid
gametes in diploid organisms.
8. Meiosis
8. Meiosis
Like mitosis, is preceded by the replication of chromosomes.However, this single replication is followed by
two consecutive cell divisions, called Meiosis I and Meiosis II.
These divisions result in four daughter cells, each with a single haploid set of chromosomes.
Produces daughter cells with only half as many chromosomes as the parent cell.
8. Meiosis
We will be looking at chromosome pairing up: called homologous chromosomes (or
homologues) because they both carry genes controlling the same inherited characteristics.
8. Meiosis
Any cell with two homologous (the same) sets of chromosomes is called a diploid cell the total number of chromosomes is called
the diploid number (abbreviated 2n) For humans, the diploid number is 46; that is
2n=46 Almost all human cells are diploid
8. Meiosis
The exception are the egg and sperm cells, collectively known as gametes. A cell with a single chromosome set is
called a haploid cell. For humans, the haploid number
(abbreviated n) is 23; that is n=23
Prophase I – Each chromosome pairs with its
corresponding homologous chromosome to form a tetrad. The tetrads overlap and exchange some of their genetic material – crossing-over.
8. Meiosis
Crossing over in Prophase I results in great diversity because new genetic variations can result from it.
8. Meiosis
8. Meiosis
Metaphase I – Spindle fibers attach to the chromosomes.
8. Meiosis
Anaphase I – The fibers pull the homologous
chromosomes toward opposite ends of the cell.
The cells are now containing half of the genetic information from the original parent cell and are thus considered HAPLOID!
Telophase I and cytokinesis – Nuclear membranes reforms, the cell
separates into two cells.
8. Meiosis
8. Meiosis
Prophase II – Meiosis I results in two haploid (N) daughter
cells, each with half the number of chromosomes as the original cell.
8. Meiosis
Metaphase II – The chromosomes line up in a similar way
to the metaphase stage of mitosis.
Anaphase II – The sister chromatids separate and move
toward opposite ends of the cell.
Telophase II and cytokinesis – Meiosis II results in four haploid (N) daughter
cells.
http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
8. Meiosis
9.Why do cells need two types of cell division?
Mitosis Provides growth, tissue repair, and asexual reproduction Produces daughter cells genetically identical to the parent
cell Involves one division of the nucleus, and is usually
accompanied by cytokinesis, producing two diploid daughter cells.
Meiosis Need for sexual reproduction human egg and sperm cells Entails two nuclear and cytoplasmic divisions Yields four haploid daughter cells, with one member of
each homologous chromosome pair. Form tetrads; crossing over occurs.
10. Gamete Formation
In females:
In males:
10. Gamete Formation
11. Karyotype The term karyotype refers to the chromosome
complement of a cell or a whole organism. A karyotype is an ordered display of magnified
images of an individual’s chromosomes arranged in pairs, starting with the longest.
In particular, it shows the number, size, and shape of the chromosomes as seen during metaphase of mitosis.
Chromosome numbers vary considerably among organisms and may differ between closely related species.
11. Karytype
Karyotypes are prepared from the nuclei of cultured white blood cells that are ‘frozen’ at the metaphase stage of mitosis. Shows the chromosomes condensed and doubled
A photograph of the chromosomes is then cut up and the chromosomes are rearranged on a grid so that the homologous pairs are placed together.
Homologous pairs are identified by their general shape, length, and the pattern of banding produced by a special staining technique.
11. Karyotype
Male karyotype Has 44 autosomes, a single X chromosome,
and a Y chromosome (written as 44 + XY) Female karyotype
Shows two X chromosomes (written as 44 + XX)
11. Karyotype- Normal
11. Karyotype- Abnormal
12a. Chromosome mutations Can either be with…
Chromosome number Chromosome structure
12a. Mutations- Chromosome number Aneuploidy- involves a single set of
chromosomes
Polyploidy- involves a whole set of chromosomes in
12a. Mutations- Chromosome number Aneuploidy
If there is an extra or missing chromosome Caused by nondisjunctions: members of a
chromosome fail to separate. For example, trisomy 21 (Down Syndrome)
if there is a nondisjunction affecting human chromosome 21 during meiosis, the resulting gametes will carry an extra chromosome 21.
12a. Mutations- Chromosome Number
12a. Mutations- Chromosome number Polyploidy
Cells containing more than two paired homologous sets of chromosomes.
For example- triploid cell (3n) or a tertroploid cell (4n)
More common in plants, very rare in humans
12a. Mutations- Chromosome number
12b. Mutations- Chromosome Structure Abnormalities in chromosome structure:
Breakage of a chromosome can lead to a variety of rearrangements affecting the genes of that chromosome: 1. deletion: if a fragment of a chromosome is
lost. Usually cause serious physical and mental problems. Deletion of chromosome 5 causes cri du chat
syndrome: child is mentally retarded, has a small head with unusual facial features, and has a cry that sounds like the mewing of a distressed cats. Usually die in infancy or early childhood.
12b. Mutations- Chromosome Structure
2.duplication: if a fragment from one chromosome joins to a sister chromatid or homologous chromosome.
3.inversion: if a fragment reattaches to the original chromosome but in the reverse direction.
Less likely than deletions or duplications to produce harmful effects, because all genes are still present in normal number
4. translocation: moves a segment from one chromosome to another nonhomologous chromosome
Crossing over between nonhomologous chromosomes!
12b. Mutations- Chromosome Structure