Chromosomes and Cell Reproduction

• I. Cell Growth– Most living cells are between 2 and 20 µm in diameter.– How big a cell can grow is determined by physical factors

below:

1. Diffusion rate of materials into and out of the cell. (Remember diffusion only works well over short distance)2. The amount of DNA available to program the cell’s metabolism.3. The cell’s surface area-to-volume ratio.

– So, mitosis is important for cell survival by taking a large cell and dividing it into two identical smaller cells with identical diploid (2n) genetic material.

• II. Cell Division– Cell division, also called cell reproduction occurs in humans.

1. Eukaryotic organisms undergoing growth, development, and repair divide their cells by Mitosis.2. Bacteria cells undergoing reproduction divide by binary fission.

– Binary fission is a form of asexual reproduction that produces identical offspring.

– In asexual reproduction, a single parent passes exact copies of all its DNA to its offspring (clones).

3. The formation of gametes involves a third type of cell division.– Gametes, an organism’s reproductive cells, such as sperm or egg

cells. (Meiosis)– Regardless of the type of cell division that occurs, all of the DNA

(deoxyribonucleic acid) must be present in each of the resulting cells.

III. Chromosomes: The cell structure in the nucleus that carries the genetic material—DNA.

Chromosomes are the structures that carry MOST genetic information.A. Chromosome Structure:

A chromosome is a very long DNA molecule that contains many genes.Chromatin- It is the DNA and proteins in the nucleus of a non- dividing cell.

During most of a cell’s life cycle (interphase) chromosomes exist as chromatin, which are strands of DNA wrapped around protein molecules (uncoiled like tangled-up spaghetti). Chromatin is not visible during this stage. DNA can not be seen under a compound light microscope. DNA can be seen under an electron microscope.

Chromosome

Chromatin

Chromosome Structure Cont.B. Two Parts of the Chromosome:

1. Chromatid- Before the DNA coils up, DNA is copied.– The chromatin coils up into two identical parts of a

chromosome, which are called chromatids.

– AKA—sister chromatids.

2. Centromere The point at which each pair of chromatids are attached. Holds chromatid (duplicated chromo- somes) together.

A chromatid is attached to a spindle fiber by the centromere.

DRAW sister chromatids:

• IV. The Cell Cycle– The life of a eukaryotic cell is the cell cycle which includes

interphase, mitosis, and cytokinesis.– A cell can only grow so large, when it reaches a certain point it will

divide into two cells.– So, the cell is as large as it can get—then it divides and the two cells

are small. This process is called mitosis.– The cell cycle is a repeating sequence of cellular growth and division

during the life of an organism.– A cell spends 90% of its time in the first three phases of the cycle.– What phase does the cell spend the most time? Interphase

A. Interphase:• The busiest and longest part of the cell cycle.• The cell is growing and the chromosomes are replicating

(copying).Before mitosis begins, the chromosomes are copied. Each chromosome now consists of two chromatids.*Humans have 46 chromosomes (23 pairs), so now 92 chromosomes are ready for cell division. 46 will end up in 1 cell and the other 46 will end up in the 2nd cell.

• DNA is in the form of chromatin. Since DNA is in the form of chromatin, it is not visible

Interphase Has Three distinct Phases:1. G1 phase— Cellular growth.2. S phase— Cellular growth and DNA synthesis—

DNA (chromatin) is replicated.3. G2 phase— Cellular growth and preparation for

cell division mitosis.

B. Mitosis• Mitosis is the division of all somatic (body cells/non-sex cells). A

cut on your hand will repair itself using the process of mitosis.• The first step of mitosis is the chromatin coiling up into

chromosomes.• The first three steps are all involved with splitting of the nucleus.• The last step involves the splitting of the cytoplasm, known as

cytokinesis.• The end result of mitosis is the formation of two identical daughter

cells that are diploid (have a full set of chromosomes (2n).• Mitotic cell division: The parent cell divides to form 2 daughter

cells with the same number of chromosomes 2n=46.• For example: A human cell has 46 chromosomes. How many

chromosomes will it have after mitotic cell division? 46

• Here are the four stages of Mitosis:• 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase

1. Prophase (P)a. Stringy chromatin coils up into chromosomes.

Chromosomes are most easily seen during this phase because the chromatin shortens and thickens.

Each duplicated chromosome is made of the two halves called sister chromatids.

Sister chromatids and the DNA they contain are exact copies of each other and are formed when DNA is copied during Interphase.b. Nuclear membrane disintegrates (disappears)

Spindle forms between the pair of centrioles which begin to move to the opposite end (poles).

Spindles are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division.

Centrioles help anchor spindles for movement of chromosomes. The spindles pull the chromosomes to opposite sides of the cell.

DRAW the cell in Prophase:

2. Metaphase

Chromosomes line up on the equator. (Centromeres are attached to the spindle fibers.)

DRAW the cell in Metaphase:

3. Anaphase a) Centomeres split separating chromatids.

Chromatids are now called chromosomes. b) Chromosomes move toward opposite poles. Draw the cell in Anaphase

4. Telophase a) The identical chromatids, now chromosomes, reach opposite poles.b) Nuclear membrane reappearsc) Spindles disappeard) Chromosomes uncoil into threadlike pieces DNA is now called chromatin

Draw cell in Telophase

As mitosis ends, cytokinesis begins.

C. CytokinesisDivides the cytoplasm and organelles

between the two new cells.

**New daughter cells will enter into

Interphase to begin the cycle over again.

What are the differences between mitosis in animal cells and in plant cells?

1. Plant cells do NOT have centrioles2. Plant cells divide their cytoplasm with a cell plate.

Centrioles

Cell Plate

V The Cell Cycle Factors that control the cell cycle:• The cell cycle is carefully controlled. Failure of cellular control can

result in cancer.Cancer is the uncontrolled growth of cells. Cancer is a disorder of cell division. Cancer is a mistake in the cell cycle.Cancer is one result of uncontrolled cell division.• The loss of control may be caused by environmental• factors or by changes in enzyme production.

Cancerous cells affect normal cells, forming

masses of tissue called tumors that deprive

normal cells of nutrients.

In the final stages,cancer cells enter the circulatory system and spread throughout the body forming new tumors that disrupt the functioning of organs.

Lung Cancer Cells