Cellular ReproductionHow to preserve genetic information.

When and why do cells divide?

• Cells divide when there is a chemical signal to do so.

• Skin cells may divide in response to crowding. Certain cells send out a chemical signal that tells neighboring cells to divide.

• Cells may divide in response to an injury, to mend damaged tissue.

• Growth factors can signal cell division in children to lengthen bones and add other tissues.

Cdk’s are alwayspresent in the cell.

growth

factor

cyclin-dependentkinase

growth factorreceptor

(cytoplasm)

cyclin

(plasmamembrane)

Growth factorbinds to receptorand stimulatescyclin synthesis.

Cyclin activatesCdk; active Cdkstimulates DNAreplication.

Specialized cell membrane proteins signal cell division when growth factors are present.

Normal G1

to S control:

growth factors+

receptor

cyclin synthesis+

Cdk

phosphorylatesRb

Rb

DNA replication

growth factors+

mutated receptoralways “on”

cyclin synthesis+

Cdk

phosphorylatesRb

Rb

uncontrolledDNA replication

growth factors+

receptor

cyclin synthesisalways “on”

+Cdk

phosphorylatesRb

Rb

uncontrolledDNA replication

Mutated growthfactor receptor gene:

Mutatedcyclin gene:

PPP

Cancers begin when something goes wrong with the system controlling cell growth and division.

Binary Fission• Bacteria and other

prokaryotes reproduce by simple binary fission.

• The single ring-shaped chromosome is duplicated, and the cell divides in half.

cell divisionby binaryfission

cell growth andDNA replication

plasmamembrane

The circular DNA double helix is attachedto the plasma membrane at one point.

circularDNA

attachmentsite

cellwall

The DNA replicates and the twoDNA double helices attach to theplasma membrane at nearby points.

The parent cell divides into twodaughter cells.

New plasma membrane is addedbetween the attachment points,pushing them further apart.

The plasma membrane grows inwardat the middle of the cell.

1

2

3

4

5

Mitosis

• One-celled eukaryotic organisms, and individual cells in a multi-cellular organisms, reproduce by mitosis followed by cytokinesis.

The problem

Eukaryotic cells are often diploid:

that is, they have two of each

kind of chromosome.

• Why do eukaryotic cells have TWO of each chromosome?

• If a cell with two of each chromosome divides, how many chromosomes should end up in each of the two daughter cells?

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A cell with 14 chromosomes divides by mitosis. How many chromosomes will each daughter cell

have?

1 2 3 4

25% 25%25%25%1. 282. 143. 74. Each daughter

cell will have a different, unpredictable number.

When is DNA replicated?

1 2 3

33% 33%33%

1. Just before cell division.

2. In preparation for transcription and translation.

3. Any time.

Overview of Mitosis

• After DNA is replicated, it is condensed into chromosomes and identical copies are sorted in the process of mitosis.

• Mitosis assures that the two daughter cells have exactly the same DNA.

Warning: Confusing terminology ahead!

Before cell division, a strand

of DNA is a chromosome.

(Think of it as a one-chromatid chromosome.)

During cell division, two

identical copies of a DNA strand

link together into a two-chromatid

chromosome.

After cell division, the

single strand is a chromosome again. (Again, think of it as a one-chromatid chromosome.)

G0: nondividing

Under certaincircumstances, cellmay return tocell cycle.

G1: cellgrowth anddifferentiation

G2: cellgrowth

S: synthesisof DNA;chromosomesare duplicated

interphase

anaphaseprophasem

etaphase

telophase andcytokinesis

mitotic cell

division

Animated cell cycle at http://cellsalive.com

Prior to Mitosis, DNA is replicated during the S-phase of the cell cycle.

Chromosomes appear late in G2 phase, just prior to mitosis.

If you wanted to count the onion root tip cells in this picture that are in mitosis, what feature would

be in the cell that tells you they are in mitosis?

INTERPHASE

nuclearenvelope

centriolepairs

nucleolus

chromatin

Late Interphase

Can we tell if a cell in Interphase is in G1, S, or G2 of the cell cycle?

DNA (2 nm diameter)

histone proteins

nucleosome: DNA wrappedaround histone proteins(10 nm diameter)

coiled nucleosomes(30 nm diameter)

DNA coils

proteinscaffold

chromosome:coils gathered ontoprotein scaffold(200 nm diameter)

A strand (double helix) of DNA wraps around histone proteins to form chromosomes. This

protects DNA from damage during cell division.

telomeres

genes

centromere

The structure of a condensed chromosome (before pairing).

• Make a prediction: during chromosome formation, which will pair up?

• Homologous chromatids that carry the same genes but different alleles?

• Sister chromatids that carry the same genes and the same exact alleles?

• (Hint: the end goal is to get identical genetic information into the daughter cells.)

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gene 1

different allelessame alleles

gene 2

Homologous chromosomes are those that carry the same genes but may have slightly different

information (such as dominant or recessive versions of a gene). Homologous chromosomes do not pair

together. Chromosomes only pair with their identical sister chromatids.

sister chromatids centromereIdentical (sister) chromatids pair up during

Prophase, and join at a pinched-in point called the centromere.

sisterchromatids

duplicatedchromosome(2 DNA doublehelices)

The chromosome at the end of Prophase consists of two strands of condensed DNA. Each sister

chromatid carries exactly the same information.

Which of these are genetically identical?

1 2 3

33% 33%33%

1. Homologous chromosomes

2. Sister chromatids

3. Neither of these

Which of these will pair up to form chromosomes in

mitosis?

1 2 3

33% 33%33%1. Homologous chromosomes

2. Sister chromatids

3. Chromatids pair at random, so it’s impossible to predict.

MITOSIS: Early Prophase

beginning ofspindle formation

condensingchromosomes

Notice that these cells in prophase have barely visible

chromosomes as DNA begins to condense.

• In prophase, the identical chromatids are paired into chromosomes. The end goal is to get each identical chromatid to opposite ends of the cell. Where should the cell place the chromosomes now to achieve that goal?

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MITOSIS: Late Prophase

pole

kinetochore

pole

As prophase progresses, the chromosomes become more and more visible as they condense.

MITOSIS: Metaphase

spindlemicrotubules

Chromosomes, with their paired identical chromatids, move to the center of the cell.

• In metaphase, the chromosomes are lined up in the middle of the cell. Predict what has to happen next to get the chromatids to opposite poles of the cell.

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MITOSIS: Anaphase"free" spindlefibers

Identical chromatids separate from one another and migrate to opposite poles of the cell.

MITOSIS: Telophasenuclear envelopere-forming

chromosomesextending

Telophase completes Mitosis. Both poles of the cell now have identical DNA, and the cell can

divide in half.

MITOSIS: Cytokinesis

After Mitosis has finished sorting the

chromosomes, cytokinesis takes

place, dividing the cell into two new

cells.

INTERPHASE

Before S phase, the cell was diploid (two

copies of each chromosome).

After cytokinesis, are the cells

diploid or haploid?

• Predict: How does cytokinesis differ between animal and plant cells?

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2 The microfilamentring contracts,pinching in thecell's “waist.”

1 Microfilaments forma ring around thecell's equator.

3 The waistcompletely pinchesoff, forming twodaughter cells.

The process of cytokinesis

1 Carbohydrate-filledvesicles bud off theGolgi apparatus andmove to the equatorof the cell.

3 Completeseparation ofdaughter cells.

2 Vesicles fuse toform a new cellwall (red) andplasma membrane(yellow) betweendaughter cells.

Golgi apparatus

carbohydrate-filled vesicles

plasmamembrane

cell wall

Cytokinesis in plant cells

In which phase do chromosomes condense?

1 2 3 4

25% 25%25%25%

1. Prophase2. Metaphase3. Anaphase4. Telophase

In which phase do sister chromatids separate from one another?

1 2 3 4

25% 25%25%25%

1. Prophase2. Metaphase3. Anaphase4. Telophase

In which phase do chromosomes line up in the middle of the cell?

1 2 3 4

25% 25%25%25%

1. Prophase2. Metaphase3. Anaphase4. Telophase

Meiosis

• Meiosis is cell division that involves the reduction of chromosomes in a cell.

The problem:

• When diploid organisms reproduce sexually, two cells must fuse and share genetic information.

• The end result of sexual reproduction is a new diploid organism that has genetic information from both parents.

• How can two cells from diploid parents fuse to make a diploid offspring?

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meioticcell division

haploidgametes

diploidfertilizedegg

diploidparentalcells

fertilization

2n

2n

2n

n

n

The cells from the parents must be haploid if their offspring is to be diploid.

While diploid cells hold two copies of each chromosome (one from each parent), haploid sex

cells hold one copy of each chromosome.

sisterchromatidshomologous

chromosomes

Meiosis is reduction division. It begins with a diploid cell

and produces haploid cells.

Why does it produce four haploid cells?

G0: nondividing

Under certaincircumstances, cellmay return tocell cycle.

G1: cellgrowth anddifferentiation

G2: cellgrowth

S: synthesisof DNA;chromosomesare duplicated

interphase

mitotic cell

division

Meiosis also involves the

cell cycle, and takes place

after S phase of the cell

cycle. DNA is replicated

before meiosis.

MEIOSIS IHomologous chromosomesmove to opposite poles.

chiasma spindlemicrotubule

recombinedchromosomes

paired homologouschromosomes

Homologous chromosomespair and cross over.

Homologous chromosomesline up in pairs.

(a) Prophase I (b) Metaphase I (c) Anaphase I (d) Telophase I

First half of meiosis: separation of homologous chromosomes.

Prophase I

chiasmaspindlemicrotubule

paired homologouschromosomes

Homologous chromosomespair and cross over.

Notice that four strands — maternal and paternal chromosomes and their identical sister

chromatids — join into a single unit, called a tetrad.

direction of“zipper”formation

protein strandsjoining duplicatedchromosomes

Protein strands “zip” the homologous chromosomestogether.

chiasma

Recombination enzymes snip chromatids apart and reattach the free ends. Chiasmata (the sites of crossing over) form when one end of the paternal chromatid (yellow) attaches to the other end of a maternal chromatid(purple).

Recombinationenzymes bindto the joinedchromosomes.

Recombinationenzymes and proteinzippers leave.chiasmata remain,helping to holdhomologouschromosomestogether.

recombinationenzymes

chiasma

While in tetrads, homologous chromosomes often swap ends, further mixing up genetic information.

Metaphase I

recombinedchromosomes

Homologous chromosomesline up in pairs.

Tetrads line up in the center of the cell.

MITOSIS: Homologouschromosomes are notpaired. Each chromatidhas a functionalkinetochore.

duplicatedchromosomes

Comparing Metaphase of Mitosis with Metaphase I of Meiosis

MEIOSIS I: Homologouschromosomes arepaired. Each pair ofchromatids has a singlefunctional kinetochore.

Anaphase I

Homologous chromosomesmove to opposite poles.

Because homologous chromosomes

separate (instead of identical sister

chromatids), each pole of the cell gets a full set of chromosomes but different genetic

information.

• Stop and think:

• Why is tetrad formation important to reaching the end goal of meiosis?

• How is crossing-over directly related to tetrad formation?

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(e) Prophase II

MEIOSIS II

(f) Metaphase II (g) Anaphase II (h) Telophase II(i) Fourhaploidcells

Meiosis II begins immediately after Meiosis I, with a short rest in between (no interphase in

between). In Meiosis II, sister chromatids separate from one another.

Metaphase II

In both cells, chromosomes line up in Metaphase II

so that sister chromatids can

separate in Anaphase II.

Anaphase II

Telophase II End

The result of meiosis is four haploid cells.

Each has one copy of each chromosome, which may carry

different versions of the same genes. Each gamete (sex cell) can have different genetic

information.

In prophase I of meiosis:

1 2 3 4

25% 25%25%25%1. Sister chromatids pair up.

2. Homologous chromatids pair up.

3. Homologous chromosomes pair up.

4. Nothing pairs up.

In anaphase I of meiosis:

1 2 3

33% 33%33%1. Tetrads split,

homologous chromosomes migrate to opposite poles.

2. All four chromatids migrate to four quarters of the cell.

3. Chromatids are randomly distributed.

The end product of meiosis is:

1 2 3 4

25% 25%25%25%1. Two identical

cells.2. Two non-

identical cells.3. Four identical

cells.4. Four non-

identical cells.

Each cell produced by meiosis is:

1 2 3 4

25% 25%25%25%

1. Diploid2. Haploid3. Tetraploid4. A random ploidy

• How is Meiosis II similar to Mitosis?

• How is it different?

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Recap

• Mitosis divides one diploid cell and produces two diploid daughter cells. It is cell division used for growth and cell replacement.

• Meiosis divides one diploid cell into four haploid cells. It is used in reproduction.