Biology AHSGE

Standard VI- Mitosis and Meiosis

Biology AHSGE

CONTENT STANDARD 6. Describe the roles of mitotic and meiotic divisions during reproduction, growth, and repair of cells.

ELIGIBLE CONTENTA. Demonstrate an understanding of how

meiosis leads to variation.B. Describe the role of meiosis in producing

variation.C. Describe the role of meiosis in reproduction.D. Describe the role of mitosis in cell repair.E. Describe the role of mitosis in growth.F. Describe the role of both mitosis and meiosis.

The Cell Cycle When a eukaryotic cell reaches its size limit, it divides. Cell Cycle- Reproduction by a cycle of growing and

dividing. Three phases:

– Interphase- Cell growth, maturity, DNA duplication, and prep for division

• Chromatin (relaxed form of DNA) strands make copies of themselves

– Mitosis- Nuclear material divides and moves to opposite sides of the cell

– Cytokinesis- Cytoplasm divides, and two identical daughter cells form

The Cell Cycle

The Cell Cycle and Mitosis Purposes:

– Increase the number of cells as a young organism grows into the adult form

– Replace damaged cells Regulation: Controlled by cyclins (proteins) binding to

cyclin-dependent kinases (CDKs- enzymes)– Cancer- Uncontrolled division and growth of cells

– Carcinogen- Known to cause cancer

– Apoptosis- Programmed cell death

– Stem cells- Unspecialized cells that can develop into specialized cells

• Embryonic stem cells- First 100-150 cells from a fertilized egg

• Adult stem cells- Found in various tissues; may be used to maintain and repair those tissues

Mitosis- Prophase1. Chromatin strands tighten (condense) into

chromosomes- Sister chromatids- Identical halfs of a

chromosome - Centromere- Center of attachment

2. Nucleolus disappears 3. Spindle Fibers appear and attach to opposite

sides of centromeres and centrioles- Responsible for moving and arranging

chromosomes

4. Centrioles move to the poles- Spindle apparatus- Spindle fibers, centrioles and

aster fibers (star-like microtubules of centriole)

5. Nuclear envelope disappears

Mitosis- Metaphase

Sister chromatids are pulled by motor proteins along the spindle apparatus toward the center of the cell

Chromosomes line up in the middle, or equator, of the cell

Mitosis- Anaphase

Microtubules of spindle apparatus shorten, pulling the chromosomes in half

With the help of motor proteins, chromatid strands are pulled to opposite sides of the cell

Mitosis- Telophase

Chromatid strands arrive at ends of cell and decondense

Two new nuclear membranes form around chromatin

Nucleoli reappear Spindle apparatus

disassembles

Cytokinesis

Cytoplasm divides resulting in two cells with identical nuclei

In plants, a cell plate forms and new cell wall forms on each side

Binary fission- In prokaryotes, DNA is duplicated and copies attach to cell membrane. Membrane grows and pulls apart DNA molecules. Cell completes fission, resulting in two identical cells.

AHSGE Biology

Standard VI- Mitosis and Meiosis

Chromosomes

Traits- Characteristics passed to offspring from parents– Instructions found on chromosomes– Genes- segments of DNA found on each chromosome;

approx. 1500 per chromosome Homologous Chromosomes- Pairs of

chromosomes; one from each parent– Humans- 23 pairs; 23 from each parent

• 46 total chromosomes

Haploid Vs. Diploid Gametes- Sex cells with half the number of

chromosomes– Created to maintain the chromosome number from

generation to generation– Represented by symbol n– Haploid- A cell with n number of chromosomes– Human gametes- n = 23

Fertilization- Fusion of gametes– Results in 2n zygote (fertilized egg cell)– Diploid- A cell with 2n number of chromosomes– Human somatic (regular body cells) cells- 2n = 46

Meiosis Forms gametes with genetic variation

– Spermatogenesis- process of producing sperm with half the number of chromosomes• Undifferentiated- no flagellum

– Oogenesis- the creation of an ovum (egg cell)• three polar bodies all die (incomplete egg cells)

Reduction division- Reduces the number of chromosomes in half

– 2n ⃗ n (Diploid to haploid) Occurs in reproductive structures of sexual organisms Involves two divisions: Meiosis I and Meiosis II

Meiosis I

1. Cell undergoes Interphase

2. Prophase I- Homologous chromosomes

condense and pair up (synapsis)

- Crossing over- segments or genes are exchanged during synapsis

- Centrioles move to opposite poles and spindle fibers attach to centromeres

Crossing Over

Meiosis I3. Metaphase I- Homologous

chromosomes (pairs) line up at the equator

4. Anaphase I- Homologous chromosomes separate and move towards opposite poles (disjunction)

- Reduces chromosome number from 2n to n

- Independent Assortment- random segregation and assortment of chromosomes during gametogenesis (production of gametes); aids in increasing genetic diversity

Meiosis I

5. Telophase I- Individual chromosomes arrive at opposite poles

– Cytokinesis occurs

– Nuclear membrane and nucleoli reappear

– Two new cells formed, each with new genetic variation

Meiosis II

1. Prophase II- Occurring in two cells

- Chromosomes condense- Nucleolus disappears - Spindle Fibers appear

and attach to opposite sides of centromeres and centrioles

- Centrioles move to the poles

- Nuclear envelope disappears

Meiosis II

2. Metaphase II- Sister chromatids are pulled by motor proteins along the spindle apparatus to line up at the equator

Meiosis II

3. Anaphase II- Microtubules of spindle apparatus shorten, pulling the chromosomes in half and to opposite sides of the cell

Meiosis II

4. Telophase II- Chromatid strands arrive

at ends of cell and decondense

- Two new nuclear membranes form around chromatin

- Nucleoli reappear

- Spindle apparatus disassembles

- Cytokinesis occurs

- Four unique, haploid (n) gametes result

Mitosis Meiosis

One division Two divisions: Meiosis I and II

DNA replication in Interphase

DNA replication once in Meiosis I

Synapsis doesn’t occur Synapsis occurs

Two identical, diploid cells Four different, haploid cells

Somatic cells Reproductive cells

Growth and repair Gametes & genetic variation