Cell Division Mitosis and Meiosis

Chromatin—long, tangled strands of DNA and proteins in the nucleus Chromosomes: –Carriers of genetic material –Condensed form of chromatin –Sometimes rod-shaped –Duplicated and shaped like an “X” in cell division –Made of: DNA Proteins called “Histones”

Chromatid—half of the chromosome (individual sides) –Each part is called a SISTER CHROMATID Centromere—region on chromosome that holds sister chromotids together

Types of Chromosomes 1. Sex Chromosomes –Determine sex of organism XX= normal FEMALE XY= normal MALE 2. Autosomes –All other chromosomes Humans: 44 autosomes + 2 sex chromosomes = 46 total

Homologous chromosomes— each cell has TWO copies of each chromosome. Homologous chromosomes –One copy from mom –one copy from dad

Karyotype—picture of an individual’s chromosomes Identify gender Identify genetic diseases –Example: Down Syndrome—individuals have THREE of Chromosome 21 instead of TWO

The Cell Cycle Is the period of growth and development of cells. Two phases: –Interphase (time BETWEEN divisions) –Mitosis (time OF divisions where nucleus splits)

Interphase: Cell spends most of its life in this stage Period of cell growth DNA is copied at end of phase Some organelles are replicated

Mitosis –4 stages: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Remember----- “Please make a taco” “PMAT”

STAGES OF MITOSIS Prophase –Chromosomes become visible –Nuclear membrane breaks down, disappears –Centrioles (organelles for cell division) appear and start to move to opposite sides of cell –Spindle fibers attached to centrioles appear are made of microtubules thus are part of the cytoskeleton

Metaphase –Chromosomes move to the midline of cell (meta=middle)

Anaphase –Sister chromatids separate from chromosome at the centromere –Start moving to opposite ends of cell –Chromatids are now called chromosomes (!!)

Telophase –Spindle fibers disappear –Chromosomes return to loose chromatin state –Nuclear envelope reforms –Is like prophase, but in reverse.

Cytokinesis—division of cytoplasm, occurs after telophase –Animal cells: –Cleavage furrow forms—is a pinching inward of cell membrane –Plant cells: –Cell plate— membrane/cell wall that divides plant cells after mitosis

Daughter cells— the copies (2) of cells formed after mitosis

Prophase Metaphase Anaphase Telophase The Stages of Mitosis Interphase

Controlling the cell cycle Controlled by enzymes and other proteins Cancer—uncontrolled cell growth caused by possible environmental factors or faulty enzymes and damaged genes

Cancers form tumors (masses) that deprive other healthy cells of nutrients Metastasis—when cancer cells break free of original tumor and spread via the circulatory system to other parts. Leads to NEW tumors

Causes of cancer: Genetic and environmental Smoke, UV rays, pollutants, viruses all damage genes  cancer

Meiosis Used to make SEX CELLS only!!!!

A bit of vocab… Diploid—a cell with two of each kind of chromosomes (both homologous chromosomes) (2n) Haploid—a cell with only one of each kind of chromosome (n) Gametes—haploid sex cells (sperm and egg)

One haploid sperm + one haploid egg = diploid zygote/embryo

Why are gametes made through meiosis and not mitosis? When cells divide by mitosis the new cells have EXACTLY the same # and kind of chromosomes as the parent cell Example of if mitosis were the only kind of cell division: –46 chromosomes (mom) + 46 chromosomes (dad) = 92 total Is this human??? Well…if it were, it wouldn’t survive. Humans are defined by having only 46 chroms—rare cases allow 45 or 47, but usually no other #s!

Why meiosis is important: Species are defined by the # of chromosomes they typically have !! Meiosis maintains the # by splitting the total number of chroms. in gametes by ½!! Meiosis starts with ONE diploid cell and makes 4 haploids. Two phases: Meiosis I, Meiosis II. Each are similar to Mitosis.

Similar to mitosis… Interphase: cell replicates chromosomes. Now has 2 identical sister chromatids connected by centromere

Prophase I: –Homologous chromosomes (“homologues”) line up with each other to form a tetrad –Tetrad—two homologous chromosomes each made up of two sister chromatids (4 chroms. total) –Crossing over— the process of homologues breaking and exchanging genes. Increases genetic variation

Crossing- Over

Metaphase I –Homologues are lined up together in middle of cell

Anaphase I –Homologous chromosomes separate –Ensures each cell will receive only ONE chromosome from each homologous pair

Telophase I –Chromosomes uncoil –Cytoplasm divides (cytokinesis) to make two new cells –Each cell has ½ the amount of DNA it started with b/c it has only one chrom. from each homologue –BUT Another cell division to separate each chromosome from its identical sister chromatid is needed

Meiosis II Exactly the same phases as Mitosis, just the chromosomes don’t replicate before they start lining up Sister chromatids are separated. Results in 4 haploid cells

MEIOSIS I MEIOSIS II

How Meiosis provides for genetic variation (why we don’t all look alike!) Crossing over: rearranges genetic info. Genes from each parent don’t stay together. Mom’s gets mixed with Dad’s. Chromosomes don’t always get split the same way. They line up in different ways so each sperm/egg has a different combo of genes

When Meiosis goes wrong Nondisjunction-- failure of homologus chromosomes to separate during meiosis I. Both chromosomes will move to the same pole instead of separating. Results in one gamete having too many chromosomes (Trisomy), and one not having enough (Monosomy)

Ex. of trisomy: Down Syndrome results when chromosome pair 21 don’t separate properly –People with Down’s have 3 chromosomes for pair 21. Most embryos with monosomy won’t survive. Often times pregnancies with this problem end in miscarriage/stillbirth

Polyploidy—results from errors of meiosis. Results in extra complete sets of chromosomes (ex: 6n, 3n) –most always fatal for animals –sometimes beneficial in plants (ex: makes larger fruit)