The continuity of life depends upon the process of reproduction.

Parents produce a new generation (either single-celled or multicellular) of organisms like themselves.

Cell cycles create a bridge between generations.

Through cell division, a parent cell provides each of its daughter cells with the hereditary instructions (DNA) and cytoplasm required to function on its own.

In eukaryotes, the nucleus divides first (along with the genetic material).

Then the cytoplasm divides.

There are two processes through which eukaryotic cells can divide: • Mitosis

• Meiosis

Prokaryotes divide through binary fission, which is asexual.

• This is human chromosome #1• Each half of this chromosome represents sister chromatids.

• They are joined in the middle by a centromere.

• Genes are areas on each chromosome that contain genetic codes for specific hereditary traits

• Chromosomes are made of DNA• DNA is made of

Deoxyribose sugarPhosphate groupNitrogenous base

Nucleotide

• Within the nuclear envelope is chromatin…which includes DNA, and proteins.

Chromosomes in cells differ in length, shape, and which portion of the hereditary instructions they carry.Cells of a given type all contain the same number of chromosomes.Diploid cells (2n) contain two of each kind of chromosome.

One from mom, and one from dad.

Haploid cells (n) contain only one of each chromosome.

Gametes

Somatic

During mitosis the nucleus of the cell divides into two equal nuclei, each with the same number of chromosomes as the parent cell.

This chromosome number is maintained from one generation to the next.

Mitosis is responsible for:• growth of tissue

• repair of tissue• cell replacements in multicellular eukaryotes

Mitosis is the basis of asexual reproduction in many single-celled eukaryotes, as well as prokaryotes.

•In interphase, the cell carries out its functions. • Just before the cell is to divide again, several things happen:

• increases in mass

• doubles the amount of cytoplasm

• duplicates each of its chromosomes, and centrosomes (each w/2 centrioles)

G1

S

G2

Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules. (The pair of centrioles together make up the centrosome)

Mitosis has four continuous stages:

Early• Chromosomes are visible threads.• All chromatids twist and fold• The nucleolus disappears. • Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. • Some fibers cross the cell to form the mitotic spindle.

Late(aka prometaphase)

• Nuclear envelope breaks down

• remnants of nuclear envelope form vesicles•Microtubules from spindle at each pole push apart.

• Microtubules attach to one of two sister chromatids of each chromosome pair

Centromere

Centriole

All chromosomes have become aligned at the spindle equatorAt this stage of mitosis, the chromosomes are most tightly condensed.

The sister chromatids of each chromosome pair separate from each other and move to opposite spindle poles. The chromatids are pulled by the microtubules, which are being disassembled

Once these sister chromatids are separated, we recognize them as chromosomes

As soon as the two clusters of chromosomes get to the poles of the cells, telophase gets under way.The chromosomes return to threadlike forms.Vesicles derived from the breakdown of the original nuclear envelope fuse to form patches of membrane around the chromosomes.Once the nuclear membranes are synthesized, and the chromosomes are separated from the cytoplasm, mitosis is complete.

The cytoplasm usually divides at some time between late anaphase, and the end of telophase.

Cytokinesis differs greatly depending upon the organism.

In animals

In animal cells, a cleavage furrow forms

In plant cells, a cell plate forms because of the cell wall (which isn’t in animal cells)

In Plants

Good health, and survival itself depends absolutely on the proper timing and completion of the cell cycle’s events.

Genetic disorders may arise from mistakes in the duplication or distribution of even one chromosome.

When normal control is lost of the cell cycle, and cell division goes unchecked, tissues, and ultimately the organism can be destroyed.

Meiosis is similar in many ways to mitosisMeiosis is similar in many ways to mitosisHowever, in many ways it is different However, in many ways it is different too…too…

•Meiosis results in the Meiosis results in the formation of haploid (n) cells.formation of haploid (n) cells.

–In Humans, these are the Ova In Humans, these are the Ova (egg) and sperm.(egg) and sperm.–Ova are produced in the ovaries in Ova are produced in the ovaries in femalesfemales

–Sperm are produced in the testes Sperm are produced in the testes of males.of males.

–Involves 2 cell divisionsInvolves 2 cell divisions

–Results in 4 cells with 1/2 the Results in 4 cells with 1/2 the normal genetic information, not normal genetic information, not identical daughter cellsidentical daughter cells

• Process called oogenesis

•Process called spermatogenesis

•Meiosis occurs in 2 phases: Meiosis occurs in 2 phases: Meiosis 1 & Meiosis & Meiosis 22

• Prior to the first division, the amount of DNA Prior to the first division, the amount of DNA doublesdoubles

What stage of the mitotic cell cycle would this “doubling” take place in?

What stage of interphase would this doubling take place?

the largest differences between mitosis and meiosis occur in prophase I.

It is estimated that prophase I accounts for some 85 - 95 percent of the total time for meiosis.

• Chromosomes condense and attach to the nuclear envelope

• Synapsis occurs when a pairing of homologous chromosomes occurs and a tetrad is formed. Each tetrad is composed of four chromatids.

• Crossing over may occur, and chiasmata become apparent.

• Each chromosome gets attached to the spindle in transition to metaphase.

Chiasma

All chromosomes are now positioned at the spindle’s equator.The orientation of the tetrads is random, with either parental homologue on a side.

This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

As there are 46 chromosomes in human cells, that means that 23 chromosomes end up on either side.

Because of the exchange during crossing over, these chromosomes are different than they were

Each chromosome is separated from its homologue, and they are moved to opposite poles of the spindle. Chiasmata separate.

Chromosomes, each with two chromatids, move to separate poles.

This is the point when many chromosomal abnormalities occur, due to incomplete separation.

(Nondisjunction)

•The cytoplasm of the germ cell divides.

• Each of the daughter cells is now haploid, but each chromosome still has two chromatids.

• Nuclear envelopes may reform, or the cell may quickly start meiosis II.

Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis and meiosis I.

Microtubules have moved one member of the centriole pair to the opposite pole of the spindle in each daughter cell.

Microtubules attach to the chromosomes, and they are driven toward the spindle’s equator.

Chromosomes are positioned at the spindle equator, midway between the two poles.

The kinetochores of the sister chromatids point toward opposite poles. The kinetochores assemble on the centromere, and are areas responsible for joining the chromosome to the microtubule from the spindle fiber.

The attachment between the two chromatids of each chromosome breaks.

Each of the former sisters, is now a separate chromosome in its own right.

(Similar to anaphase of mitosis)

Distinct nuclei form at the opposite poles and cytokinesis occurs. At the end of meiosis II, there are four daughter cells each with one half the number of chromosomes of the original parent cell.

In humans, primary oocytes complete interphase and prophase I where they are frozen and remain this way until the female reaches puberty. 

If a woman is fertile on average for 40 years, then 40 X 12 = 580 eggs in her lifetime.  Thus, 400,000 eggs is plenty! 

A female is born with about 2 million primary oocytes.  By the time she reaches puberty, about 400,000 are left (most of them die through the years).  This is still way more than enough!

Of the four haploid cells that form by way of meiosis and cytoplasmic divisions, one or all may develop into gametes and function in sexual reproduction.

Asymmetrical cytokinesis leads to the production of polar bodies during oogenesis. To conserve nutrients, the majority of cytoplasm is segregated into the secondary oocyte during meiosis I , when the secondary oocyte is formed.The remaining daughter cells generated from the meiotic events contain relatively little cytoplasm and are referred to as polar bodies. Eventually, the polar bodies degenerate.

DiploidDiploid

Haploid, but

duplicated

Haploid

Spermatogonia (precursors to sperm)divide by mitosis to produce more spermatogonia or spermatocytes.

With 22 pairs of autosomes and an average of two crossovers between each pair, there is huge variation among sperm.

Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete.

Then the spermatids form sperm, losing most of their cytoplasm in the process.

It is estimated that from 10–20% of all human fertilized eggs contain chromosome abnormalities, and these are the most common cause of pregnancy failure (35% of the cases). These chromosome abnormalities:

• arise from errors in meiosis, usually meiosis I; • occur more often (90%) during egg formation than during sperm formation;

• become more frequent as a woman ages. •Aneuploidy — the gain or loss of whole chromosomes — is the most common

chromosome abnormality. It is caused by nondisjunction, the failure of chromosomes to correctly separate:

• homologues during anaphase of meiosis I • sister chromatids during anaphase of meiosis II

•Zygotes missing one chromosome have monosomy, such as females with a single X chromosome).

•Three of the same chromosome ("trisomy") is also lethal except for chromosomes 13, 18, and 21 (trisomy 21 is the cause of Down syndrome).

•Three or more X chromosomes are viable because all but one of them are inactivated.

Turner’s SyndromeEdward Syndrome

Trisomy 18Klinefelter’s Syndrome