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(1) 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 FormationChromatin DNA in thin, uncoiled strands.DNA replicates.DNA coils.Chromosomes DNA in 2 joined identical

chromatids.

ChromosomesChromatids - 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

Gene

Gene

Centromere

Chromatid

Chromosome

DNA supercoil

Chromosome NumberEvery species has a characteristic number of

chromosomes which is different from other species.

Example: nematode worm 2 pairsprotozoan 300

pairshuman 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 NumberHOMOLOGOUS 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. HaploidDIPLOID (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 informationBasically, 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 Mitosis46 chromosomes

Cell division

23 2311.5

11.5

11.5

11.5

5.75 5.75

2.875 2.875

1.4

1.4

.7 .7

1

2

4

8

16

32

64

128

256512

.35.175.0875

With Mitosis46 chromosomes

Cell division

46 4646 46 46 46

46 46

46 46

46 46

46 46

1

2

4

8

16

32

64

128

256512

46

46

46

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

4. Anaphase

5. Telophase

**This is a moving, continuous process. Stages are for study.

The Cell Cycle

Interphase

Mitosis

Cytokinesis

Mitosis

Interphase Period of growth and normal

cell activityEvents 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

Look for a distinct nucleus

Interphase

Mitosis: ProphaseThe 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: ProphaseLook 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: MetaphaseLook for chromosomes lined up on the “equator”

Mitosis: Anaphase•Centromeres split apart

•Chromatids move away from each other toward opposite poles

Mitosis: AnaphaseLook for chromosomes in two groups pulling apart

Mitosis: TelophaseThe 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: TelophaseLook for two groups of chromosomes with a cell plate or cleavage furrow

Stages of MitosisProphaseMetaphaseAnaphaseTelophase

P – M – A - T

CytokinesisThe 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

Differences in plant and animal cells:summary

Animals Plants

centrioles no centrioles

cleavage furrow cell plate

cell wall