Cell Division

L3 BiologyL3 Biology

Why do cells divide?

GrowthGrowth

Repair/regenerationRepair/regeneration

ReproductionReproduction asexualasexual

The Cell Cycle – life cycle of a cell that will divide:

Cells go through a life cycle

First growth phase

9 hr

Synthesis phase – DNA is copied – 10 hr

Second growth phase – 2 hr

Cell division – mitosis – 1 hr – followed by division of the cytoplasm

interphase

Chromosomes Invisible most of the time - Only visible Invisible most of the time - Only visible

during cell division (mitosis or meiosis)during cell division (mitosis or meiosis) During S-phase – the DNA replicates During S-phase – the DNA replicates

(makes an exact copy of itself)(makes an exact copy of itself) This means the cell has twice as much DNA This means the cell has twice as much DNA

in it after replicationin it after replication Once a chromosome has replicated, it Once a chromosome has replicated, it

shortens and thickens and can now be seen shortens and thickens and can now be seen in our microscopes.in our microscopes.

One Chromosome

Sister Sister

ChromatidsChromatidsEach strand is an identical Each strand is an identical

copy of the other onecopy of the other one

CentromereCentromereWhere the two chromatidsWhere the two chromatids

Are attached to each other – Are attached to each other –

This is different for each This is different for each

chromosomechromosome

DNA

Chromosome Number Each species has the same number of chromosomes in Each species has the same number of chromosomes in

all their cells that are made by mitosis. This is the all their cells that are made by mitosis. This is the diploid numberdiploid number (2n). In humans this number is 46. So (2n). In humans this number is 46. So cells of your skin and muscle and liver each have 46 cells of your skin and muscle and liver each have 46 chromosomes in them. Look how many chromosomes chromosomes in them. Look how many chromosomes are in the cells of these creatures:are in the cells of these creatures:

2n = 38

2n = 78

2n = 94

2n = 42

When cells divide by When cells divide by mitosismitosis, each daughter cell , each daughter cell receives the same number of chromosomes as receives the same number of chromosomes as its mother cell has.its mother cell has.

In order to do this, the chromosomes must be In order to do this, the chromosomes must be copied first, then one of each copy is placed in copied first, then one of each copy is placed in the new cells. the new cells.

46

46

46

46

46

46

92

Humans have 22 pairs of similar

chromosomes; one pair of sex chromosomes

Sex Chromosomes

Similar in females: XXSimilar in females: XX

Not similar in males: XYNot similar in males: XY

Mitosis

Cell division resulting in nuclei identical Cell division resulting in nuclei identical to parent cell – to parent cell – asexual reproductionasexual reproduction for for some organisms. some organisms.

Four phases: Four phases: ProphaseProphase

MetaphaseMetaphase

AnaphaseAnaphase

TelophaseTelophase

Prophase

Mother cell

Chromosomes become visible, nucleus disintegrates, spindle fibers form, centrioles move to poles

Chromosomes line up in the middle

Chromatids separate and move to opposite poles

Chromosomes become invisible, nuclear membranes form around new nuclei, spindle fibers disappear.

Prophase

Chromosomes become visibleChromosomes become visible Spindle fibers formSpindle fibers form Nuclear membrane disintegratesNuclear membrane disintegrates Nucleolus disintegratesNucleolus disintegrates In animal cells, centrioles move to opposite In animal cells, centrioles move to opposite

ends of the cell (poles) and spindle fibers ends of the cell (poles) and spindle fibers attach to them. attach to them.

Metaphase Chromosomes line up single file at the Chromosomes line up single file at the

middle of the cell middle of the cell

Anaphase

Sister chromatids are pulled apart toward Sister chromatids are pulled apart toward opposite polesopposite poles

Telophase

Nuclear membrane forms around Nuclear membrane forms around each group of chromosomeseach group of chromosomes

Nucleolus reappears in each nucleusNucleolus reappears in each nucleus Spindle fibers disappearSpindle fibers disappear Chromosomes become invisible again as chromatinChromosomes become invisible again as chromatin Cytokinesis begins in plant cell by formation of cell Cytokinesis begins in plant cell by formation of cell

plate; cleavage furrow in animal cell completely plate; cleavage furrow in animal cell completely separates the two nuclei into two different cells. separates the two nuclei into two different cells.

Cell plate

Occurs after nucleus has Occurs after nucleus has

been duplicatedbeen duplicatedBegins in anaphaseby the Begins in anaphaseby the

formation of a formation of a cleavage furrowcleavage furrow

Cytokinesis in an animal cell

Begins in telophase in plant cells Begins in telophase in plant cells

by the formation of a by the formation of a cell plate.cell plate.

Find the different stages of mitosis in these onion root cells:

CB

AD

E

Meiosis

Cell division producing cells that have half the Cell division producing cells that have half the number of chromosomes of the mother cellnumber of chromosomes of the mother cell

Produces Produces gametesgametes – eggs and sperm – eggs and sperm Occurs so that fertilization doesn’t increase the Occurs so that fertilization doesn’t increase the

number of chromosomes in each generation.number of chromosomes in each generation.

Why is cell division important to understand?

Cancer is uncontrolled cell division – cells then Cancer is uncontrolled cell division – cells then spread to other parts of the body.spread to other parts of the body.

Gametes = sex cells

Eggs or spermEggs or sperm Have half the normal number of chromosomes Have half the normal number of chromosomes

= 23 in humans= 23 in humans Combine DNA from two different parents, Combine DNA from two different parents,

producing offspring that is different from each producing offspring that is different from each parentparent

Skin cancer cellsThis is how skin cancer looks – a change in a mole is the first symptom. If you know how your moles usually look, you can identify any changes.

This is the worst kind of skin cancer – Malignant melanoma – it has often metastesized by the time it is diagnosed.

Meiosis = Gametogenesis – the formation of eggs and sperm

OogenesisOogenesis

Production of an eggProduction of an egg

One mother cell One mother cell produces one egg cell produces one egg cell and three polar bodies and three polar bodies that diethat die

SpermatogenesisSpermatogenesis

Production of spermProduction of sperm

One mother cell One mother cell produces 4 equally produces 4 equally sized sperm cellssized sperm cells

One egg4 sperm

Sperm formation

Egg formation

FertilizationThe sperm unites with the egg forming a zygote (fertilized egg).

The zygote then divides by mitosis to produce the trillions of cells that make up a multicellular body like yours.

The End