Biology - Cell Cycle and Cell Division

Chromosomes

contain the genetic information (DNA) that is passed from parent to offspring.  

Located in the Nucleus

Chromosomes have two parts:

Chromatid:

• Contain genes for traits.

• Chromosome has two identical copies. Separated during Mitosis

Centromere:

• Holds chromatids together

How Chromosome Replication Works:

1. A chromatid makes an identical copy of itself called a sister chromatid

2. When there are pairs of these sister chromatids for the same trait, they are called Homologous Pairs

Human Chromosomes

• Human diploid cells have 46 CHROMOSOMES – 2 COPIES OF 23 DIFFERENT CHROMOSOMES

• Karyotype -chart that shows the physical features of chromosome pairs

46 … or … 23? (dip vs. hap)

Diploid (2n)

• 1. 23 homologous pairs of (human) chromosomes (46 total, full number)

• 2. Homologous Pairs- One copy from your mother, and one from your father

• 3. Somatic (body) cells (every cell except sperm and egg)

Haploid (n)

• 1. 23 individual chromosomes (half the full number)

• 2. Number of chromosomes in a gamete

• 3. Gametes (Sex Cells: sperm and egg)

Two Types of Cells In Organisms

Somatic Cells

1. Body Cells

2. Diploid (2n)

3. Divide by mitosis to make exact copies

4. Identical Cells

5. In every tissue except reproductive tissue

Germ Cells

• I. “Germinating” (reproductive) cells; 

first division by meiosis (Diploid->Haploid)

• II. Divide by meiosis to make gametes; found in testes & ovaries

• Diploid to Haploid!!!

Cells Multiply By Dividing

• Cells Divide for:

1. Repair

2. Growth,

3. Volume Management

• Eukaryotes Divide By:

  • Mitosis

  • Meiosis

• Prokaryotes Divide By:

  • Prokaryotic (binary) fission

Why do cells divide?

1. They become too large

• The volume of the cell increases faster than the surface area

• Cells need large surface area (cell membranes) to move things in and out

2. To make more cells so the organism can grow

• Example:  a growing baby needs more skin cells!

3. To replace old, worn out cells

• Cells reproduce at different times depending on cell jobs

The Cell Cycle for Somatic Cells

• 2 Main Parts

  1. Interphase (life, growth)

  2. Mitosis (division of nucleus)

Consider cytokinesis (division of cytoplasm) as part of mitosis

Interphase

What Happens:

• Interphase is the phase the cell normally in, a “Living Phase”

• 1. DNA  is Copied and prepares to divide

• 2. Longest phase of the cell cycle

DNA called “Chromatin” or uncondensed genetic material at this phase

Phases/Steps of Interphase

G0 Interphase (cell is at rest)

G1 (Gap1/Growth 1): First growth phase - proteins are created, mitochondria and chloroplasts divide, volume of cytoplasm increases

S (Synthesis): DNA is Synthesized and replicates

G2 (Gap 2/Growth): Phase -DNA is checked for errors; Cell is growing, getting ready for division

The Cell Cycle Control System

Cell control system has specific checkpoints where the cell cycle stops until a go-ahead signal is received.

G1 Checkpoint

G2 Checkpoint

M Checkpoint

1. For many cells the G1 (Gap1) checkpoint is most important

2. If the cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phase and cell will divide.

If the cell does not receives the go-ahead signal. cell will exit the cycle, switch into a non-dividing state called G0

Cells that do not divide

Some cells do not divide in adults:

1. Brain cells

2. Nerve cells

3. Muscle cells

Stem Cells are the focus of much research to find ways to create new cells of these types. Stem cells are cells that can produce or become any type of cell. 

Stages of Mitosis:

• Prophase

  • 1. Chromatin thickens into chromosomes

  • 2. Nuclear envelope disintegrates

  • 3. Centrioles migrate to opposite poles

  • 4. Centrioles begin producing spindle fibers

• Metaphase

  • 1. Chromosomes line up along metaphase plate

  • 2. Spindle fibers attach to centromere of chromosomes

• Anaphase 

  • 1. Chromosomes are separated

  • 2. Sister chromatids pulled apart to opposite poles

• Telophase and Cytokinesis

  • 1. Nucleus reforms

  • 2. Chromosomes begin to relax into chromatin (back to G1)

  • 1. After the Telophase occurs, the cell splits into two separate daughter cells.

  • Animal and Plant Cells do this in different ways.

Cytokinesis in Plant vs. Animal Cells:

Plant cells How it works:

• Instead of the cell being torn in half, a new cell wall is created between the two nuclei

• This is called a Cell Plate

Animal cells How it works:

• The cytoplasm of the cell starts to pinch in the center

• This is called a Cleavage Furrow

• This continues until the cell splits in two

Cancer cells

1. In cancer, mitosis is uncontrolled

2. Cancer cells divide constantly and form clumps called tumors.

Benign tumors:  remain clumped; can be removed.

• Benign tumors are usually not life-threatening unless they interfere with vital processes.

Malignant tumors: break apart (metastasize) and can form tumors in other parts of the body.

How Cancers Develop and Spread

Cancer develops only in cells with damaged genes (mutations).

Mutations can be inherited or caused by exposure to:

–Low-dose radiation

–Drugs

–Toxic chemicals

–Infection with certain viruses(ex. HPV)can cause mutations.

Cancer screening

Cancer screening is an examination to detect cancer before a person has symptoms.

–Visual examination

–Self-examination

–Clinical (physician) examination

–Laboratory testing

–Scans (MRI, CAT)

Therapeutic Strategies for Treating Cancer 

Slash – Surgical Removal​

Burn – with Radiation​

Poison – with Chemotherapy​

Since cancer is uncontrolled cell division, all chemotherapeutic treatments involve the cell cycle.​

·  Phase-specific chemotherapies​

·  Prevent cells from entering S-phase​

·  Block the S-phase​

·  Block the M-phase (mitosis)

Reducing Cancer Risk

• Don’t smoke or chew tobacco

• Don’t drink excessive amounts of alcohol

• Avoid unnecessary exposure to ionizing radiation, such as x-rays and UV light

• Don’t lie in the sun or tanning beds

• Avoid exposure to toxic chemicals and fumes

• Avoid asbestos dust and radon gas

Cell Self Destruction/Death

Apoptosis is programmed cell death.

–It’s normal for healthy cells

–cell produces self-destructive enzymes

–Example:

Webbing between fingers in fetus

Meiosis Involves Two Divisions

In meiosis:

• Start with one diploid germ cell and end with four haploid gamete daughter cells

• Each cell has a different variation

Meiosis I

• Splits homologous pairs of chromosomes

Meiosis II

• Splits sister chromatids apart

Meiosis stages

1. PROPHASE I

   • 1. Nuclear envelope disintegrates

   • 2. Centrioles migrate, produce spindle fibers

   • 3. Homologous chromosomes pair up

   • 4. *** Homologous pairs CROSS OVER!(CROSSING OVER = Exchange of segments of genes between homologous chromosomes to create genetic variation)

2. METAPHASE I

• 1. Homologous pairs of chromosomes line up along metaphase plate

• 2. Spindle fibers attach to centromeres

3. ANAPHASE I

   • 1. Homologous pairs pulled apart to opposite poles

   • 2. This is done at random (note light blue and dark blue pairings)

4. TELOPHASE I

• 1. Chromosomes (no longer homologous pairs!) prepare for meiosis II

• 2. Cytoplasm begins to split into first 2 (of 4) daughter cells

• 3. At end of Meiosis I, cells are now haploid!

5. PROPHASE II

   • 1. Centrioles migrate, produce spindle fibers

6. METAPHASE II

   • 1. Chromosomes line up along metaphase plate

7. ANAPHASE II

   • 1. Sister chromatids pulled apart to opposite poles

8. TELOPHASE II

• 1. Nuclear envelopes reform around daughter cells and is then divided into 4 separate daughter cells

• 2. Each of the 4 daughter cells is a new gamete with genetic information different from either parent!

Formation of Gametes

FERTILIZATION

• Male and female gametes unite

• Fusion of two haploid nuclei produces diploid nucleus

• Egg + Sperm = Zygote (new life in one diploid cell)

                        n + n = 2n

MEIOSIS AND SEXUAL REPRODUCTION GUARANTEE VARIATION AND DIVERSITY! In Three Ways

1. 1. Independent Assortment

   • Homologous Chromosomes separate randomly

2. 2. Crossing Over

   • Homologous pairs exchange gene segments

3. 3. Random Fertilization

   • Which sperm meets which egg? (game of chance)

Mitosis vs. Meiosis

Mitosis

Cell division of body cells (somatic)

Produces 2 diploid cells (2n)

Each new cell contains copies of all chromosomes

One cell division

All cells are identical (photocopies)

Meiosis

Cell division that produces gametes

Produces 4 haploid cells (n)

Each gamete contains just one chromosome of each pair

Two cell divisions

All cells are different (diversity)