Cell Reproduction

Mitosis and Meiosis

Two types of Reproduction

Sexual Reproduction the production of new

living organisms by combining genetic information from two individuals of different types (sexes).

Asexual Reproduction  is a mode

of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only.

Sexual Reproduction Every cell in your body has 46 chromosomes

(or 23 sets) this is called a diploid cell. Sexual reproduction includes two sex cells

called a egg and sperm. Sex cells (gametes) have 23 chromosomes;

this is called haploid cell. (half the number of chromosomes)

The sex cells combine creating a full set of chromosomes for an animal or plant. This is called fertilization.

Sexual reproduction introduces new gene combinations in a population through genetic recombination.

Sexual Reproduction in a Flowering plant.

Eggs in the ovary are fertilized through the pollen tube forming a zygote which in turn forms a seed.

Pollen are the sperm cells which will fertilize the eggs.

Variations in Genes

Combination of Genes

Combination of Genes different results.

Plants and animals receive genes from both the mother and the father producing

offspring with a different genetic combination.

Asexual Reproduction

Binary FissionProkaryotic fission, which is binary fission, is a form of asexual reproduction and cell division used by all prokaryotes, (bacteria and archaebacterial), and some organelles within eukaryotic organisms (e.g., mitochondria).

BuddingBudding is a form of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. The new organism remains attached as it grows, separating from the parent organism only when it is mature, leaving behind scar tissue.

Vegetation ReproductionVegetative reproduction is a type of asexual reproduction found in plants where new independent individuals are formed without the production of seeds or spores. 

Fragmentation/RegenerationFragmentation also referred to as regeneration is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. 

Spore Formationparent organism produces spores:single cells that can develop intonew individuals by repeated mitosis

The Great DivideThe Great Divide

05/04

To Be To Be Answered…Answered…THINK:

How many cells are you composed of?

When an organism grows bigger do you get more cells or just bigger cells or both?

When do your cells divide the fastest? Slowest?

Do cells ever stop dividing?

Are all cells capable of division and replacement?

Why Would a Cell Why Would a Cell Divide?Divide? As cells absorb nutrients and get larger,

the volume of the cell increases faster than the surface area

This means that a cell can no longer absorb nutrients and get rid of wastes fast enough to support its demands (volume)

So what’s a cell to do?

Solution: divide in 2!

Surface area for exchange not great enough to support cell’s needs

When Would a Cell When Would a Cell Divide?Divide? Growth

Repair or Replacement

Cancer

Different cells divide at different rates:

Most mammalian cells = 12-24 hours

Some bacterial cells = 20-30 minutes

Getting Older…Getting Older… All cells are only allowed to complete a

certain number of divisions

Then they die (programmed cell death)

How does cell division change over a lifetime?

Childhood = cell division > cell death

Adulthood = cell division = cell death

The Later Years = cell division < cell death

The Cell CycleThe Cell Cycle

Stages of the Cell Stages of the Cell CycleCycle 2 stages = interphase (growth &

replication of DNA) & mitotic phase (division of cell into 2 daughter cells)

Cell spends about 90% of the time in interphase

Cell Cycle TidbitsCell Cycle TidbitsHow long is one cell cycle?

Depends on the cell- skin cells = ~24 hours, nerve cells = never after maturity, cancer cells = very short

Remember: every cell only has a certain # of divisions it can undergo, then it dies = apoptosis (programmed cell death)

InterphaseInterphase Divided into 3 phases:

G1 (1st gap) = small cell is absorbing nutrients, growing & doing its job (i.e. making proteins)

S (synthesis) = cell is continuing to grow & duplicates its DNA (i.e. chromosomes) in preparation for making duplicate cells during mitosis

G2 (2nd gap) = cell keeps growing & doing its job (i.e. making proteins); it grows too big…solution = divide in 2

Mitosis: A Closer LookMitosis: A Closer Look

Prior to entering the mitotic phase, the cell has just come out of interphase

Replicated DNA during S (synthesis)

2 complete sets of chromosomes that must be distributed equally between 2 cells = mitosis

Interphase

The Mitotic PhaseThe Mitotic Phase Equal distribution of 2 sets of chromosomes

(DNA) into 2 identical daughter cells

Divided into 4 stages of Mitosis:

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

ProphaseProphase

Chromatin condenses visible chromosomes

Appear as sister chromatids held together by centromere

Nuclear membrane dissolves

The centrioles migrate to opposite poles & spindle fibers form between them

MetaphaseMetaphase

Chromosomes line-up on the metaphase plate

Centromeres are attached to spindle fibers

AnaphaseAnaphase

Centromeres divide

Spindle fibers contract

Result = sister chromatids are pulled away from one another towards the poles

TelophaseTelophase

The chromosomes reach the poles

Nuclear membranes form around the 2 new nuclei

CytokinesisCytokinesis The cytoplasm

distributed equally between the 2 new cells

In animals, a cleavage furrow forms from outside

In plants, a cell plate forms from inside out

PlantAnimal

What Mitosis Actually Looks What Mitosis Actually Looks LikeLike

InterphaseProphase Metaphase

Anaphase Telophasehttp://www.sci.sdsu.edu/multimedia/mitosis/mitosis_gif2.html

http://science.nhmccd.edu/biol/bio1int.htm

Summary of MitosisSummary of Mitosis

Plant Cell Mitosis

What Happens After What Happens After Mitosis?Mitosis?

The cell returns to interphase

Chromosomes uncoil back into chromatin

The cycle repeats itself over & over…

At What Stage Are Our Cells At In The Cell At What Stage Are Our Cells At In The Cell Cycle?Cycle?

Different cells can be in different stages

Interphase

Mitosis:

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Can You Identify the Stages of Can You Identify the Stages of Mitosis?Mitosis?

Put the following mitosis stages in the correct sequence

The GuaranteeThe Guarantee

The product of mitosis is 2 cells

The daughter cells are identical to each other & to the mother cell

Mother cell

Identical daughter

cellsWhy is this so important?

The Daughter CellsThe Daughter Cells

In humans, the 2 daughter cells will have 46 chromosomes (23 pairs)

1 chromosome originally from mom & 1 from dad

Each chromosome is said to have the same gene sequence

Identical daughter

cells

Review of Asexual ReproductionReview of Asexual Reproduction

Mitosis is a form of asexual reproduction

New individuals are produced by 1 parent & thus, are identical to their parent

Mother cell

Identical daughter

cells

Runners produces by strawberries

Cuttings from plants

Budding by hydra & yeast

https://www.youtube.com/watch?v=JcZQkmooyPk

Meiosis – A Source of DistinctionMeiosis – A Source of Distinction

Why do you share some but not all characters of each parent?

What are the rules of this sharing game?

At one level, the answers lie in meiosis.

Meiosis does two things -Meiosis does two things -

1) Meiosis takes a cell with 1) Meiosis takes a cell with two copies two copies of every of every chromosome (diploid) and makes cells with a chromosome (diploid) and makes cells with a single copy single copy of every chromosome (haploid). of every chromosome (haploid).

This is a good idea if you’re going to combine This is a good idea if you’re going to combine two cells to make a new organism. This trick two cells to make a new organism. This trick is accomplished by is accomplished by halvinghalving chromosome chromosome number. number.

In meiosis, In meiosis, one diploid cells produces four one diploid cells produces four haploid cells.haploid cells.

Why do we need meiosis?Why do we need meiosis?

• Meiosis is necessary to halve the number Meiosis is necessary to halve the number of chromosomes going into the sex cellsof chromosomes going into the sex cells

Why halve the chromosomes in gametes?Why halve the chromosomes in gametes?• At fertilization the male and female sex At fertilization the male and female sex

cells will providecells will provide ½ of the chromosomes ½ of the chromosomes each – so the offspring has genes from each – so the offspring has genes from both parentsboth parents

2) Meiosis scrambles the 2) Meiosis scrambles the specific forms of each gene specific forms of each gene that each sex cell (egg or that each sex cell (egg or sperm) receivessperm) receives. .

This makes for a lot of genetic This makes for a lot of genetic diversity. This trick is diversity. This trick is accomplished through accomplished through independent assortmentindependent assortment andand crossing-overcrossing-over..Genetic diversity is important Genetic diversity is important for the for the evolution of populations evolution of populations and species.and species.

FertilizationFertilization• The fusion of a spermsperm and eggegg to form a zygotezygote.• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

FertilizationFertilization

MeiosisMeiosis• Sex cells Sex cells divide to produce gametesgametes (sperm or egg)(sperm or egg).

• GametesGametes have halfhalf the # of chromosomeschromosomes.

• Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).

Male: spermatogenesisMale: spermatogenesis

Female: oogenesisFemale: oogenesis

• MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.

SpermatogenesisSpermatogenesis

2n=46

humansex cell

diploid (2n)

n=46

n=46

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

Meiosis I (four phases)Meiosis I (four phases)

• Cell division Cell division that reduces the chromosomechromosome number by one-half.one-half.

• four phasesfour phases:

a.a. prophase Iprophase I

b.b. metaphase Imetaphase I

c.c. anaphase Ianaphase I

d.d. telophase Itelophase I

Interphase IInterphase I• Similar to mitosismitosis interphase.

• ChromosomesChromosomes replicate (S phase).(S phase).

• Each duplicated chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.

• CentrioleCentriole pairs also replicate.

Interphase IInterphase I• NucleusNucleus and nucleolusnucleolus visible.

nuclear membrane

nucleolus

cell membrane

chromatin

Homologous ChromosomesHomologous Chromosomes

• Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are similar in shape and size.

• Homologous pairs (tetrads) (tetrads) carry genes controlling the same inherited traits.

• Each locuslocus (position of a gene) (position of a gene) is in the same position on homologues.

• Humans have 23 pairs of homologous chromosomes.homologous chromosomes.

a. 22 pairs of autosomesautosomes

b. 01 pair of sex chromosomessex chromosomes

Prophase IProphase I - - SynapsisSynapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

KaryotypeKaryotype• A method of organizing the chromosomes of a A method of organizing the chromosomes of a

cell in relation to number, size, and typecell in relation to number, size, and type..

Homologous ChromosomesHomologous Chromosomes

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Humans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues.

Homologue

Homologue

AutosomesAutosomes(The Autosomes code for most of the offspring’s traits)(The Autosomes code for most of the offspring’s traits)

In Humans the “Autosomes” are sets 1 - 22

21 trisomy – Downs 21 trisomy – Downs SyndromeSyndrome

Can you see the extra 21st chromosome?

Is this person male or female?

Sex ChromosomesThe Sex Chromosomes code for the sex of the offspring.

** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y”

chromosome it will be a male.

XX chromosome - female XY chromosome - male

In Humans the “Sex

Chromosomes” are the 23rd set

Boy or Girl? The Y Chromosome “DecidesBoy or Girl? The Y Chromosome “Decides””

X chromosomeX chromosomeY chromosomeY chromosome

Crossing OverCrossing Over

• Crossing over (variation) Crossing over (variation) may occur between nonsister chromatidschromatids at the chiasmatachiasmata.

• Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.

• Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing crossing overover.

Crossing Over - variation Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Another Way Meiosis Makes Lots of Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-OverDifferent Sex Cells – Crossing-Over

Crossing-over multiplies the already huge number of Crossing-over multiplies the already huge number of different gamete types produced by independent different gamete types produced by independent

assortment.assortment.

Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male

Prophase IProphase I• Longest and most complex phase (90%).Longest and most complex phase (90%).

• ChromosomesChromosomes condense.

• SynapsisSynapsis occurs: homologous chromosomes homologous chromosomes come together to form a tetradtetrad.

• TetradTetrad is two chromosomeschromosomes or four chromatidschromatids (sister and nonsister chromatids).

Prophase IProphase I

centriolesspindle fiber

asterfibers

Metaphase IMetaphase I• Shortest phaseShortest phase

• TetradsTetrads align on the metaphase platemetaphase plate.

• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n

Example:Example: 2n = 42n = 4then then n = 2 n = 2

thusthus 2 22 2 = 4 combinations= 4 combinations

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Question:Question:1.1. In terms of Independent Assortment In terms of Independent Assortment

-how many different combinations -how many different combinations of sperm could a of sperm could a humanhuman malemale produce? produce?

AnswerAnswer• Formula: 2Formula: 2nn

• Human chromosomes:Human chromosomes: 2n = 462n = 46

n = 23n = 23

• 222323 = ~8 million combinations = ~8 million combinations

Anaphase IAnaphase I• Homologous chromosomes Homologous chromosomes separate and

move towards the poles.

• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

Anaphase IAnaphase I

Telophase ITelophase I• Each pole now has haploidhaploid set of

chromosomeschromosomes.

• CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

Prophase IIProphase II• same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II• same as metaphasemetaphase in mitosismitosis

metaphase platemetaphase plate

Anaphase IIAnaphase II• same as anaphaseanaphase in mitosismitosis

• sister chromatids separatesister chromatids separate

https://www.youtube.com/watch?v=zGVBAHAsjJM

Telophase IITelophase II• Same as telophasetelophase in mitosismitosis.

• Nuclei form.

• CytokinesisCytokinesis occurs.

• Remember:Remember: four haploid daughter cells four haploid daughter cells produced.produced.

gametes = sperm or egggametes = sperm or egg

VariationVariation• Important to population as the raw Important to population as the raw

material for natural selection.material for natural selection.

• Question:Question:

2.2. What are the three sexual sources of What are the three sexual sources of genetic variation? genetic variation?

Answer:Answer:1. crossing over (prophase I)1. crossing over (prophase I)

2. independent assortment (metaphase I)2. independent assortment (metaphase I)

3. random fertilization3. random fertilization

Remember: variation is good!

Question:Question:3. A cell containing 20 chromosomes20 chromosomes

(diploid) (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

Answer:Answer:• 10 chromosomes (haploid)10 chromosomes (haploid)

Question:Question:4. A cell containing 40 chromatids40 chromatids at the

beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

Answer:Answer:• 10 chromosomes10 chromosomes

Sources

• www.ursulinehs.org/powerpoint/meiosis.ppt

• www.biology4teachers.com/Cell%20Division/MEIOSIS.ppt

• http://www.iteachbio.com/Life%20Science/LifeFunctionsandTheCell/Meiosis.ppt