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
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