How Do Cells Divide?

What you will learn…

1. Why Do Cells Divide?2. Chromosome structure3. Cell Division in Prokaryotes4. Cell Cycle5. Mitosis6. Cytokinesis7. Control of Cell Division and Cancer8. Meiosis9.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

sizehttp://www.pbs.org/wgbh/nova/miracle/

program.html

Cell size

Cells are small, because they have to be able to carry materials from one side of the cell to the next in a short period of time.

Cells must have a large enough surface area to be able to take in nutrients and oxygen and release waste quickly.

2. Chromosome Structure

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 histonesDNA wraps with protein like wrapping paper

on a present giving it the X-shapeOnly 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:

If the cell touches other cells, then cell division slows down.

If enough space between cells and nutrients are available, cells divide or speed up their cell cycle.

7. Control of Cell Division and Cancer Internal regulators:

1.Cyclins – proteins that regulate the timing of the cell

cycle in eukaryotic cells. 2.Check points

Proteins that 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. 3.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

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

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

Tumor Progression

7. Control of Cell Division and Cancer Treatment of Cancer:

1.Surgical removal of tumor – Most effective when tumor is in a defined area

2.Chemotherapy – Medicines that disrupt the process of mitosis in rapidly growing cells

3.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, with only half as many chromosomes as the parent cell

Each new cell is known as haploid

8. Meiosis

Starts with diploid cells with what are called homologous chromosomes (or homologues) because they both carry genes controlling the same inherited characteristics

These diploid cells divide to form 4 haploid cells

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

8. Meiosis

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

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!

8. Meiosis

Telophase I and cytokinesis – Nuclear membranes reforms, the cell

separates into two cells.

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.

8. Meiosis

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

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:

10. Gamete Formation

In males:

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. Mutations- Chromosome Number

Nondisjunction Members of a chromosome fail to separate. Can lead to an abnormal chromosome

number in any sexually reproducing diploid organism.

For example, if there is nondisjunction affecting human chromosome 21 during meiosis I, half the resulting gametes will carry an extra chromosome 21. Then, if one of these gametes unites with a

normal gamete, trisomy 21 (Down Syndrome) will result.

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