Lecture 7Mitosis & Meiosis

Cell Division

Essential for body growth and tissue repair

Interphase G1 phase

Primary cell growth phase S phase

DNA replication G2 phase

Microtubule synthesis

Mitosis

Nuclear division

Cytokinesis Cytoplasmic division

Chromosomes: Carriers of Inherited Traits

Chromosomes were first observed by the German embryologist Walther Fleming in 1882

Chromosomes exist in somatic cells as pairs Homologous chromosomes or homologues

The number of chromosomes varies enormously from species to species The Australian ant Myrmecia spp. has only 1 pair Some ferns have more than 500 pairs

Human Chromosomes

Humans have 46 chromosomes

This display is termed a karyotype

The 23 pairs of homologous chromosomes can be organized by size

Basic Terminology

Diploid cells have two copies of each chromosome Replicated chromosomes consist of two sister chromatids

These are held together at the centromere

Chromosomes

Chromosomes are composed of chromatin Complex of DNA (~ 40%) and proteins (~ 60%)

A typical human chromosome contains about 140 million nucleotides in its DNA This is equivalent to

About 5 cm in stretched length 2,000 printed books of 1,000 pages each!

In the cell, however, the DNA is coiled

Chromosomes: Packaged DNA

The DNA helix is wrapped around positively-charged proteins, called histones 200 nucleotides of DNA coil around a core of eight histones, forming a

nucleosome The nucleosomes coil into solenoids Solenoids are then organized into looped domains The looped domains appear to form rosettes on scaffolds

Mitosis

The phases of mitosis are:

Prophase

Xm

Xf

Xm

Xf

Xm

Xf

Condense

Metaphase

Xm

XfXmXf

Xm

Xf

Line Up

Anaphase

ImIf

ImIfImIfIf

Im

IfImIfIm

Separate

Telophase

Im IfImIfIm

If

If ImIfIm

If Im

Divide

Cytokinesis Cleavage furrow formed in late anaphase by contractile ring Cytoplasm is pinched into two parts after mitosis ends

Early Prophase

Early prophase

Early mitotic spindle

Pair of centrioles

Centromere

Aster

Chromosome, consisting of two sister chromatids

Figure 3.32.2

ProphasePLAYPLAY

Asters are seen as chromatin condenses into chromosomes

Late prophase

Fragments of nuclear envelope

Polar microtubules

Kinetochore

Kinetochore microtubule

Spindle pole

Late Prophase

Figure 3.32.2

PrometaphasePLAYPLAY

Nucleoli disappear Centriole pairs separate

and the mitotic spindle is formed

Metaphase

Metaphase plate

Spindle

Metaphase

Figure 3.32.4

Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell

This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate

MetaphasePLAYPLAY

Anaphase

Figure 3.32.5

Daughter chromosomes

Anaphase

Centromeres of the chromosomes split

Motor proteins in kinetochores pull chromosomes toward poles

AnaphasePLAYPLAY

Telophase and Cytokinesis

Telophase and cytokinesis

Nucleolus forming

Contractile ring at cleavage furrow

Nuclear envelope forming

New sets of chromosomes extend into chromatin

New nuclear membrane is formed from the rough ER

Nucleoli reappear Generally cytokinesis

completes cell division

TelophasePLAYPLAY

Controlling the Cell Cycle

1. Cell growth is assessed at the G1 checkpoint

2. DNA replication is assessed at the G2 checkpoint

3. Mitosis is assessed at the M checkpoint

G0 is an extended rest period

The eukaryotic cell cycle is controlled by feedback at three checkpoints

Control of the cell cyclePLAYPLAY

During fetal development, many cells are programmed to die

Cell Death

Human cells appear to be programmed to undergo only so many cell divisions About 50 in cell cultures

Only cancer cells can divide endlessly

Programmed cell death

Fingers and toes form from these paddle-like

hands and feet

What is Cancer?

Cancer is unrestrained cell growth and division The result is a cluster of cells termed a tumor

Benign tumors Encapsulated and

noninvasive

Malignant tumors Not encapsulated and

invasive

Leave the tumor and spread throughout the body

Can undergo metastasis

What is Cancer?

Most cancers result from mutations in growth-regulating genes

There are two general classes of these genes 1. Proto-oncogenes

Encode proteins that stimulate cell division If mutated, they become oncogenes

2. Tumor-suppressor genes Encode proteins that inhibit cell division

Cancer can be caused by chemicals, radiation or even some viruses

Cancer, cell division, and p53 protein

How tumor suppression genes workPLAYPLAY

Receiving the signal to divide

Passing the signal via a relay switch

Amplifying the signal

Releasing the “brake”

Checking that everything is ready

Stepping on the gas

Stopping tumor growth

New molecular therapies for cancer

Potential cancer therapies are being developed to target seven different stages in the cancer process Stages 1-6

Prevent the start of cancer within cells Focus on the decision-making process to divide

Stage 7 Act outside cancer cells Prevents tumors from growing and spreading

Diversity is what allows gene populations to survive changes in their environment The greater the diversity, the more likely some individuals will have

the traits to survive a major change

Genetic diversity is the raw material that fuels evolution

No genetic process generates diversity more quickly than sexual reproduction

Importance of Generating Diversity

Contains two sets of chromosomes

Two Types of Reproduction

Sexual reproduction Maximizes diversity Involves the alternation of

meiosis and fertilization Meiosis halves the number

of chromosomes from each parent cell

Fertilization restores the number of chromosomes

Asexual reproduction Creates a faithful copy Does not involve

fertilization

Contain one set of chromosomes

The Sexual Life Cycle

The life cycles of all sexually-reproducing organisms follows the same basic pattern Haploid cells or organisms alternate with diploid cells or organisms

There are three basic types of sexual life cycles

Spend most of life in haploid form

Spend most of life in diploid form

Spend half of life in haploid form and half in

diploid form

The Sexual Life Cycle in Humans

Discovery of Meiosis

Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887

Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells

The fusion of gametes is called fertilization or syngamy It creates the zygote, which contains two copies of each

chromosome

Mitosis

Meiosis

Prophase

Xm

Xf

Xm

Xf

Xm

Xf

Condense

Metaphase

Xm

XfXmXf

Xm

Xf

Line Up

Anaphase

ImIf

ImIfImIfIf

Im

IfImIfIm

Separate

Xf

Xm

Xm

Xm

Xf

Xf

If

If

ImIf

If

Im

Im Im

Im

Im

IfIf

Xm Xf

XmXf

XmXf

ImIm

If

Im

If

Im

ImIm

IfIf

If If

Xf

Xm

Xm

Xm

Xf

Xf

XmfX

mXXf

XmfXMeiosis I

mXXfXmfXXmfX

Meiosis IIXf

Xm

Xm

Xm

Xf

Xf

Telophase

Im IfImIfIm

If

If ImIfIm

If Im

Divide

Unique features of meiosisPLAYPLAY

Meiotic Cell Division: Meiosis I

Figure 27.7 Tetrads line up at the spindle equator during metaphase I In anaphase I, homologous chromosomes still composed of joined

sister chromatids are distributed to opposite ends of the cell At the end of meiosis I each daughter cell has:

Two copies of either a maternal or paternal chromosome A 2n amount of DNA and haploid number of chromosomes

In telophase I: The nuclear membranes re-form around the chromosomal masses The spindle breaks down The chromatin reappears, forming two daughter cells

Prophase I, Metaphase I, Anaphase I, Telophase IPLAYPLAY

Meiotic Cell Division: Meiosis II

Mirrors mitosis except that chromosomes are not replicated before it begins

Meiosis accomplishes two tasks: It reduces the chromosome number by half (2n to n) It introduces genetic variability

Meiosis IIPLAYPLAY

Differences Between Mitosis & Meiosis

Purpose

Where it occurs Body cells Special reproductive organs

Number of divisions after one chromatin replication event

One Two

Number of daughter cells

Two Four

Homologous chromosomes in daughter cells

Two (diploid) One (haploid)

Makeup of daughter cells vs. parent cell

Same as parent cell Different from parent cell

Mitosis Meiosis

Individually Homologous pairs in Meiosis I,Individually in Meiosis II

How chromosomes Line up at metaphase

Differences between meiosis &. mitosisPLAYPLAY

Create new cells(faithful copies)

Create new individuals(diversity)

Evolutionary Consequences of Sex

Sexual reproduction increases genetic diversity through three key mechanisms

1. Independent assortment

2. Crossing over

3. Random fertilization

In humans, a gamete receives one homologue of each of the 23 chromosomes Humans have 23 pairs of chromosomes

223 combinations in an egg or sperm 8,388,608 possible kinds of gametes

Independent assortment

Three chromosome pairs23 combinations

DNA exchanges between maternal and paternal chromatid pairs

Crossing over

This adds even more recombination to independent assortment that occurs later

The zygote is formed by the union of two independently-produced gametes

Therefore, the possible combinations in an offspring

8,388,608 X 8,388,608 =

70,368,744,177,664 More than 70 trillion!

And this number does not count crossing-over

Random fertilization