Biology The Cellular Basis of Inheritance

Why Do Cells Divide? Cell Division is the splitting of a single cell into 2 cells. 3 life processes occur: 1.Growth: this is the increase of cell in size. Differentiation is the specialization in cells. 2.Repair: this is the ability of an organism to fix itself; humans repair their skin blood vessels and bone. Regeneration is the ability of an organism to replace a missing body part (like a starfish regrowing an arm). LinkiaRegenerating.jpg

3.Reproduction: When an organism is single-celled and that cell divides, it is reproducing. This is a form of asexual reproduction. –Bacteria and unicellular eukaryotes reproduce this way. The arm that broke off from the starfish can also reproduce asexually by cell division. It slowly regrows a new body. Asexual reproduction produces genetically identical offspring to the parents.

Sexual Reproduction: Sexual reproduction produces genetic varieties in offspring. –Plants and animals reproduce this way. This results in a recombination of chromosomes through meiosis, a specialized form of cell division.

How Do Cells Divide? The cell cycle is the sequence of phases in the life cycle of the cell The cell cycle has 2 parts: Interphase (Growth and preparation) and Cell Division Cell Division includes: Mitosis (nuclear division) and Cytokinesis (cytoplasm division).

Some Terms: Chromatin is the fibrous form of DNA and proteins that make up chromosomes. –This is what is found within the nucleus of the cell during interphase. –It is clumped DNA. Once chromosomes have been replicated, they are paired together in the form of sister chromatids. –These are identical structures that are side by side. Sister chromatids are held together by a centromere. –This is the point of attachment.

Interphase: G1, or gap 1, is characterized by growth and development. S stage, or Synthesis, is when the chromosomes are replicated. G2, or gap 2, is when the cell synthesizes organelles and other materials. This is the longest phase of the entire cell cycle. The cell is in preparation for the nucleus to divide. Cell Division:

Mitosis is the formation of 2 nuclei from 1. It occurs in 4 stages. 1. Prophase: Chromosomes condense & become visible under the light microscope Microtubules from the mitotic spindles The nuclear envelope & nucleolus break apart & disappear Centromeres attach to the spindle fibers 2. Metaphase: The chromosomes move to the center of the cell The center of the cell is called the metaphase plate

Prophase: Metaphase: state.edu/biology/images/metaphase.jpg s/dna/mitosis/images/metaphase1_ac.jpg

3. Anaphase: Centromeres divide & the spindle fibers pull 1 set of sister chromatids toward opposite poles Once chromosomes are at opposite poles, anaphase is over /files/bio%20101/Bio%20101%20Lectures/mitosis/whit efish_mitosis_anaphaseX400.jpg /mitosis/images/early_anaphase1_pc.jpg

4. Telophase: A nuclear envelope forms around each set of chromosomes Chromosomes uncoil into chromatin Mitotic spindle fibers disassemble /Bio%20101%20Laboratory/Mitosis/Photographs/whitefish_ mitosis_telophaseX400.jpg /AlliumMitosis/telophase.jpg

Cytokinesis: This is a.k.a. cytoplasm separation In animal cells, this begins in telophase as the nuclei reform. –This starts at the center of the cell and pinches inward. This is called a cleavage furrow. In plant cells, this begins in anaphase and starts in the center of the cell along the metaphase plate and grows outward. –This is called the cell plate.

Cancer Tumors: masses/clusters of cells –Benign: non-cancerous –Malignant: cancerous (usually uncontrolled dividing cells)

Metastasis: spreading of cancerous cells Treatment: surgery (removes tumor), radiation & chemotherapy (destroys cells by disrupting cell cycle) Radiation & chemo side effects: healthy cells may die, sterility, hair loss, nausea

What is Meiosis? Remember that humans have 46 chromosomes (or 23 pairs) in their cells. This means they have 2 complete sets of chromosomes. Diploid, or 2n, is a cell that has 2 complete sets of chromosomes (in humans, 46). Haploid, or 1n or n, is a cell that has only 1 set of chromosomes (in humans, 23).

Human’s sex cells, or gametes, are haploid. All human body cells are produced through mitosis whereas the sex cells, or gametes, are produced through meiosis. Gametes are sperm and egg and have only 23 chromosomes in each. –When they fuse (at fertilization), they form a zygote ( = 46). –This is how each generation remains stable.

Meiosis is a type of cellular reproduction in which the # of chromosomes are reduced by ½ so that the daughter cells are haploid (n). Homologous pairs are pairs of chromosomes. –Each of the 23 chromosomes has a matching chromosome (with 1 exception: the sex chromosomes). –Sex chromosomes are X and Y.

The Phases of Meiosis: Prior to meiosis, a diploid cell replicates its chromosomes (Interphase). Meiosis has 2 stages: Meiosis I and Meiosis II. Each has 4 phases.

Homologous Chromosomes: uschrom.jpg Karyotype: display of chromosomes in # order; informs chromosomal # abnormality, chromosomal abnormaility & genetic sex

Meiosis I: 1.Prophase I: chromosomes condense, homologous chromosomes become attached to each other, each homologous chromosome contains 4 sister chromatid (this is called a tetrad, meaning 4). 2.Metaphase I: homologous pairs align along the middle of the cell. 3.Anaphase I: homologous pairs split. 4. Telophase I and Cytokinesis: Nuclei reform and the cells split. This result is 2 diploid cells, each with 2 complete sets of chromosomes.

Meiosis II: 1.Prophase II: Spindle fibers form again & chromosomes condense. NO tetrads; NO crossing over! 2.Metaphase II: chromatids move to the center of the cell. 3.Anaphase II: chromatids are pulled to opposite poles. 4.Telophase II and Cytokinesis: Nuclei reform and cells separate. The result is 4 haploid cells.

Meiosis I  Meiosis II  GG/images/meiosis.gif

In human males, 4 haploid cells result (sperm cells) but in human females, only 1 of the 4 haploid cells forms an egg cell. The other 3 receive no cytoplasm and do not form gametes (they disintegrate). Interphase occurs only ONCE (Meiosis I), meaning chromosomes replicate only 1X.

What are the differences between mitosis and meiosis? Meiosis produces daughter cells with ½ the # of chromosomes (haploid cells), mitosis produces diploid cells. Meiosis produces daughter cells that are NOT genetically identical to each other (the homologous chromosomes separation is random); mitosis produces EXACT copies of parent cells. Meiosis produces 4 haploid cells; mitosis produces 1 cell.

Genetic Variation Variation results from the recombination of DNA (from meiosis & fertilization) and accounts for the differences between members of a population. Sources of Genetic Variation: Random separation of homologous pairs of chromosomes Random combination of haploid gametes Crossing Over (tips of homologous chromosomes switch places) occurs during prophase I (meiosis I)