Cell ReproductionMitosis & Meiosis

Why Cells Divide Surface Area/ Volume Ratio

As the cell grows, the volume increases at a greater rate than the surface area

Can't take in enough nutrients, or remove wastes

Therefore the cell must grow or divide Growth and Repair

Replace worn or damaged cellsFrequency of replacement varies:

bacteria ~ every 20 minuteshuman cells ~ every 18-22 hours

Many cells in the body don't divide

Cell Division

Cellular Reproduction When the parent cell divides, it

forms new daughter cells Organisms reproduce in two ways:

Asexual ReproductionSexual Reproduction

Sexual vs. Asexual Reproduction

Asexual Reproductionproduction of offspring from one

parent therefore genetic material is identical

to parent Sexual Reproduction

formation of a new individual from the union of 2 cells

2 parents, therefore offspring have some hereditary material from each

Types of Asexual Reproduction Binary Fission

simplest form; the cell splits in 2 Spore Formation

Begins with replicationSpores can remain inactive until conditions

are favorablemolds, fungi

Yeast reproduce by budding Vegetative Propagation

Some plants, e.g. strawberries Regeneration

planaria, star fish, etc.

Cell Division in

Prokaryotes Binary Fission: The simplest

form of cell division

The cell splits in 2

The Process of Binary Fission First the single circular chromosome

duplicates = Replication Both chromosomes attach to sites on

the cell membrane As the cell grows, a new membrane

forms between attachment sites Membrane pinches off and the new

cells separate

Sexual Reproduction The joining of 2 specialized sex cells

called gametesmale = sperm female = ovum

Process of combining gametes = fertilization

Fertilization produces a zygotehas characteristics of both parents

Human Sexual

Reproduction Male testis produces

sperm Female ovary

produces ova Each has 23

chromosomes Unite to form a

zygote with 46 chromosomes 23 pair

Develops into a fetus

Cell Division

All types of reproduction require cell division 2 processes can be used to divide the cell’s

nuclear material: Mitosis

Occurs in somatic cells (body cells) in eukaryotes As a result of mitosis each daughter cell receives

an exact copy of the chromosomes present in the parent cell

Meiosis Occurs in gametes (sex cells) As a result each daughter cell receives 1 of each

pair of chromosomes present in the parent cell

Mitosis Cell division in eukaryotic cells involves

nuclear division called mitosis Occurs in somatic cells

body cells; not sex cells As a result of mitosis, each daughter cell

receives an exact copy of the chromosomes present in the parent cell

Chromosomes contain genetic materialDNA

Chromosomes During cell division in eukaryotic

cells, the DNA is coiled into chromosomes

Every body cell of the same type of organism has the same number of chromosomeshumans = 46goldfish = 94mosquito = 6

Chromosome Structure Each chromosome is formed from

two joined strands called chromatids Each chromatid is alike

has a long arm & a short arm joined at the centromere

Chromosomes contain DNA and associated proreins

Chromosomal Proteins Each chromosome is a single DNA

molecule and associated proteins Histones –

One type of chromosomal proteinThe DNA wraps tightly around the histonesHistones help maintain the shape of the

chromosome Nonhistone proteins -

control the activity of specific regions of DNA

Picturing Chromosome Structure

Visualizing Chromosomes

Chromosome Make-up Chromosomes of somatic cells are in pairs

One of each pair comes from mother, one from father

The 2 chromosomes in a pair are homologous Alike in appearance and type of genetic

information carried Humans have 23 pairs of chromosomes

22 pairs of autosomes Autosomes are all but the sex

2 sex chromosomes ( X & Y)

Sex Chromosomes

Determine the sex of the organism

Also carry other genetic information

In humans, either X or Y

Females are XX, males are XY

Thus the male determines the sex of the offspring

Haploid vs. Diploid Cells with two copies of each

chromosome = diploid Autosomal cells are diploid Gametes (sex cells) have only one of

each type of chromosome Cells with one copy of each chromosome

= haploid

Karyotypes A picture of paired human chromosomes Used to to detect certain genetic

diseases

Mitosis The process of dividing the nuclear

material in a somatic cell in eukaryotes Necessary for cell division

Preparation for Mitosis

Interphase The time between the formation of a cell

through mitosis and the next mitosis Most of the cell cycle is interphase During this phase cell prepares by:

replicating genetic materialproducing organellesassembling structures needed for mitosis

Chromosomes & Interphase

During interphase chromosomes cannot be distinguished under the light microscope They appear as chromatin

At the start of mitosis, the chromatin thickens, and chromosomes become visible

The Cell Cycle The sequence of cell growth and division The cell cycle can last several hours to

several days Can be affected by environmental factors, like

temperature Has 4 stages:

mitosis & division of cytoplasm (cytokinesis)

The other 3 are part of interphase:G1SG2

Picturing the Cell Cycle

G1 - Growth After mitosis, a period of intense

cellular activity and growth The cell doubles in size Enzyme production is high Cells that stop growing remain in G1

S- Synthesis  Cells that divide enter S, or

synthesis, phase  The chromosomes replicate

G2 – Further Growth A second period of growth Structures used in mitosis are

assembled

The Phases of Mitosis

Mitosis is actually a continuous process But we divide it into 4 phases:

ProphaseMetaphaseAnaphaseTelophase

Prophase 60% of the period of mitosis is prophase Divided into 3 parts: early, middle, & late Chromosomes begin to coil into short rods Nucleoli break down & begin to disappear 2 pairs of dark spots called centrosomes

appear outside the nuclear membrane In animal cells, the centrosomes contain centrioles,

formed from microtubules Plant cells have no centrioles

The centrosomes move to opposite sides of the cell

Mid Prophase At the beginning of mid-prophase

spindle fibers form between the centrioles

Additional fibers radiating out from each centriole form the aster

The nuclear membrane has broken down and disappeared

The Mitotic Spindle Spindle fibers made of microtubules radiate

from the centrosomes This array of spindle fibers = the mitotic

spindle 2 types of spindle fibers: Kinetechore fibers

Attach to a disk-shaped protein called a kinetecore Found in the centromere of each chromosome Extend from the kinetechore of each chromatid to

one of the centrosomes Polar fibers

Extend across the dividing cell from one centrosome to the other

Late Prophase The centrosome pairs are at opposite

ends of the cell The centosomes are fully formed Chromosomes are attached to the

centrosomes by spindle fibers Other spindle fibers stretch across the

cell from one centriole to the other

Metaphase The chromosomes are pushed and

pulled by spindle fibers along cell's the midplanecalled the equator

Anaphase

Begins with the separation of chromatids in each chromosome

Spindle fibers appear to shorten, pulling the chromatids apart at the centomere

Each chromatid is now a chromosome 2 sets of separated chromosomes then

move through the cytoplasm to opposite poles of the cell

Telophase The last stage of mitosis After the individual chromosomes have

reached opposite poles of the cell, spindles disappear

A nuclear membrane forms around each set of chromosomes

Chromosomes return to a thread-like mass Centrioles duplicate

2 centrioles formed in each daughter cell Nucleoli re-form within each newly formed

nucleus

Cytokinesis The division of the cytoplasm Follows mitosis  Cytokinesis begins during telophase In animal cells, the cell membrane

pinches together The area that pinches in and separates

is called the cleavage furrow In plants, a cell plate is formed, dividing

the two halves

Picturing Cytokinesis

Chromosome Number Cells formed thru mitosis have the same

number of chromosomes as the parent cells

If combined in sexual reproduction, the offspring would have 2x chromosomes!

Therefore gametes have only half the number of chromosomes of somatic cellsGametes = sex cells

Meiosis

Gametes need another process for nuclear division

Meiosis reduces the number of chromosomes to 1/2 the number in somatic cells

Meiosis I & II Forming haploid

daughter cells from diploid parent cells requires two successive cell divisions:First = Meiosis I –

homologous chromosomes separate

Second = Meiosis II chromatids of

each chromosome separate

Meiosis I Preceded by replication of DNA that

forms the chromosome Synapsis = pairing of homologous

chromosomes Each pair of homologous chromosomes

twists around each other, forming a structure called a tetrad

Meiosis can be divided into same 4 phases as mitosis: Prophase, Metaphase, Anaphase, Telophase

Prophase I

 Chromatin begins to coil into short rods  Homologous chromosomes are formed  Spindle fibers appear  Nucleoli break down By the end, the nuclear membrane has

dissolved, and tetrads are visible

Crossing Over During synapsis, (prophase I) the

chromatids of homologous pairs twist around each other

A portion of one chromatid may break off and reattach, “trading” with the same piece from its homologous partner

The exchange of genes by reciprocal segments of homologous chromosomes during meiosis = “crossing-over”

Crossing over causes exchange of genetic material between maternal & paternal chromosomes

Results in genetic recombination

Genetic recombination is less likely in genes that are closer together.

Chromosome Mapping The likelihood that recombination will occur due

to crossing-over depends on the genes’ distance from each other on the chromosome

Scientists can determine how frequently genes for particular traits occur together in offspring

This can be used to create a map of the chromosome

1% recombination (crossing-over) = 1 map unit

Metaphase I Tetrads line up along the equator of the

cell Each tetrad becomes attached to

spindle fibers

Anaphase I

Homologous chromosomes that form each tetrad are pulled apart in pairs

One pair goes to one end of the cell, the other to opposite end

Telophase I Chromosomes reach ends of the cell Cell divides into 2 daughter cells

Independent Assortment During Anaphase I, one member of each

homologous chromosome pair moves to one end of the cell, the other moves to the opposite end

The separation of homologous chromosomes is random

More, or fewer maternal (or paternal) chromosomes may end up on one side or the other

Each separation is independent of the others This is the principal of independent assortment

of chromosomes Results in genetic variation

Meiosis I Summary Meiosis I is a Reductive Division It reduces the number of chromosomes

from diploid "2n" to haploid "n"

Meiosis II Similar to mitosis but not preceded by

replication of DNA 4 Stages:

Prophase IIMetaphase IIAnaphase IITelophase II

Prophase II & Metaphase II Prophase II

A new spindle forms around paired chromatids

Metaphase IIChromosomes line up along the equatorThey are attached at the centromere to

spindle fibers

Anaphase II Centromeres duplicate & the chromatids

separate Resulting single chromatids move to

opposite poles Chromatids are now called

chromosomes

Telophase II  A nuclear membrane forms around each

set of chromosomes  The spindle breaks down and

cytokinesis occurs  Result: 4 haploid daughter cells

Males vs. Females In males, all 4 daughter cells

differentiate to become sperm  In females, the cytoplasm divides

unevenly in Meiosis I The smaller cell = first polar body doesn't survive

 In Meiosis II, the division is again unequalsmaller half is second polar body

So only 1 of 4 daughter cells survivesrich in cytoplasm, has many nutrients to

nourish the young organism

Comparing Mitosis & Meiosis MEIOSIS

2 4

diploid

haploid

different

MITOSIS

1

2

diploid

diploid

identical

# of nuclear divisions:

# of daughter cells:

Parent cell type:

Daughter cell type:

Genetic likeness to parent:

Control of Cell Division Timing and rate of cell division

varies in different cell types Control of rate of division is critical Some cells require regulatory

substances to begin division = growth factors

Effect of Growth Factors

Effect of Density

Density of cells also effects the rate of division

Crowding inhibits cell division

The Restriction Point

A crucial checkpoint occurs late in the G1 phase of the cell cycle

Point of decision to divide = restriction point Cell cannot turn back after this point If it is “yes,” cell goes to S phase and copies

DNA If “no,” it goes to non-dividing state (G0)

Most cells are in G0

MPF After S, the cell will

enter G2 The “OK” signal

that causes the cell to proceed from G2 to mitosis = mitosis promoting factor (MPF)A complex of

proteins

MPF is an enzymeProtein kinase

Abnormal Cell Division Cancer cells do not respond normally to the

body’s control mechanisms for cell division Cancer cells divide excessively Can invade other body tissues When a cell divides abnormally =

transformed Abnormal cells are usually destroyed by

the immune system

Cancer

If abnormal cells are not destroyed and reproduce, they may form a mass of abnormal cells = tumorBenign tumor = abnormal cells remain at

the original siteMalignant tumor = cells spread to other

parts of the body Metastasis = spread of cancer cells in

the body

Breast Cancer Cell