Chromosomes & Cell Division Notes About two trillion cells are produced by an adult human body every day!

DNA (deoxyribonucleic acid): Stores info and tells the cells when to make proteins, and what kind to make. GENE: unit of heredity, enough DNA to instruct for the construction of one protein. Each DNA molecule is made up of thousands of genes.

Forms of DNA Chromatin - genetic material in non-dividing cell’s nucleus; DNA in thin, non-coiled strands. DNA is in this form 99% of the time because cells are generally not dividing. DNA must be in this form in order to be copied (replicated). (2) Chromosomes - genetic material in dividing cell’s nucleus; DNA in coiled, rod-shaped form. DNA must be in chromosome form in order for the cell to divide or reproduce.

Steps in Chromosome Formation Chromatin  DNA in thin, uncoiled strands. DNA replicates. DNA coils. Chromosomes  DNA in 2 joined identical chromatids.

Chromosomes Chromatids - one of two duplicated, joined, identical parts of a chromosome; found after DNA replication but before cell division. Centromere – Point at which sister chromatids are attached

Chromatid Gene Centromere Chromosome DNA supercoil Gene

Chromosome Number Every species has a characteristic number of chromosomes which is different from other species. Example: nematode worm 2 pairs protozoan 300 pairs human 23 pairs Chromosomes occur in pairs in sexually reproducing organisms. One of the pair comes from the MOM, the other comes from the DAD.

Chromosome Number HOMOLOGOUS CHROMOSOMES: two members of a pair of chromosomes that carry genes for the same traits, have the same size and shape. (ONE FROM MOM AND ONE FROM DAD)

Human Chromosomes - Karyotype

Diploid vs. Haploid DIPLOID (2N): total chromosome number in a body or somatic cell, having both chromosomes of a homologous pair. Diploid or 2N number in humans is 46 chromosomes or 23 homologous pairs. HAPLOID (1N): chromosomes number in sex cell (egg or sperm), only one chromosome from each homologous pair. Haploid or 1N number in human egg or sperm is 23, there are no homologous chromosomes.

MITOSIS the key to growth

Mitosis - the division of the nucleus into two equal halves or sets of genetic information Basically, simple cell division

Purpose: Each cell must have the complete “blueprint” of their DNA. Every time the cell divides, it must pass on a copy of each chromosome.

Without Mitosis 23 23 1 2 4 11.5 11.5 11.5 11.5 8 16 Cell division 46 chromosomes Cell division 2 23 23 4 11.5 11.5 11.5 11.5 8 5.75 5.75 16 2.875 2.875 32 1.4 1.4 64 .7 .7 128 .35 256 .175 512 .0875

With Mitosis 46 46 1 2 4 46 46 46 46 8 16 Cell division 46 46 32 46 chromosomes Cell division 2 46 46 4 46 46 46 46 8 46 46 16 46 46 32 46 46 64 46 46 128 46 256 46 512 46

Cells divide as part of growth or reproduction Every cell in the body (except 1 kind that we will talk about later) has the same DNA. Each cell will just use their part of it. (Ex. Heart cells will use heart DNA). Therefore; all cells divide using mitosis. Cells divide as part of growth or reproduction “Growth” includes: *getting larger *repair *replacement (for normal “wear and tear”)

Mitosis is part of the Cell Cycle The cell cycle is divided into 5 phases. 1. Interphase 2. Prophase 3. Metaphase Mitosis 4. Anaphase 5. Telophase **This is a moving, continuous process. Stages are for study.

Period of growth and normal cell activity Interphase Period of growth and normal cell activity Events of Interphase: The cell spends most of its life in interphase. DNA is in chromatin form, spread throughout the nucleus. DNA replicates, cell then contains two exact copies of each chromosome. Centrioles found just outside the nuclear membrane of the cell. Stages: G1 (Gap 1) – normal cell activities S (Synthesis) – DNA synthesis (replication) G2 (Gap 2) – prepares for cell division

Interphase Look for a distinct nucleus

Mitosis: Prophase The first stage of cell division Three things happen: Chromosomes coil and become visible The nuclear membrane breaks down and disappears The spindle forms *Animal cells have centrioles

Mitosis: Prophase Look for chromosomes in the center

Mitosis: Metaphase Chromatids attach to the spindle fibers at the centromere Chromosomes line up on the “equator” (in the middle of the cell)

Mitosis: Metaphase Look for chromosomes lined up on the “equator”

Mitosis: Anaphase Centromeres split apart Chromatids move away from each other toward opposite poles

Mitosis: Anaphase Look for chromosomes in two groups pulling apart

Mitosis: Telophase The fourth and final phase of mitosis The opposite of Prophase (the cell is ending its division and going back to normal) Chromosomes unwind to form chromatin Two new nuclei form with new membranes The spindle disappears

Mitosis: Telophase Look for two groups of chromosomes with a cell plate or cleavage furrow

Stages of Mitosis Prophase Metaphase Anaphase Telophase P – M – A - T

Cytokinesis The division of the cytoplasm Animal cells pinch inward forming a cleavage furrow. Plant cells divide from the inside towards the outside by forming a cell plate which becomes the cell wall.

Cytokinesis Animals: Cleavage furrow Cell plate cell wall

Animals Plants Differences in plant and animal cells: centrioles summary Animals Plants centrioles no centrioles cleavage furrow cell plate cell wall