Continuity and varietyLectures by Mark Manteuffel, St. Louis Community College

Chapter 6: Chromosomes and Cell Division

Learning Objectives Understand and be able to compare and

contrast the different types of cell division Understand and be able to explain how

through mitosis worn out old cells are replaced with fresh new duplicates

Understand and be able to explain how sperm and eggs are generated through meiosis

Describe the sex differences in the chromosomes

Discuss the consequences of deviations from the normal chromosome number

6.1 Immortal cells can spell trouble: cell division in sickness and health.

The body naturally replaces dead or damaged cells through cell division (mitosis).

The average human cell can copy itself about 50 times before it dies.

Uncontrolled cell division can lead to cancer.

Discovering a cure for Cancer involves a deep understanding of cell division.

Telomeres

The telomere is like a protective cap at the end of the DNA.

Every time a cell divides, the telomere gets a bit shorter.

Cells that rebuild the telomere with each division can become cancerous

The genetic condition, Hutchinson-Gilford progeria syndrome, results in shorter telomeres, cells die faster, and the individual appears to age very rapidly.

6.2 Some chromosomes are circular, others are linear.

Prokaryotes vs. Eukaryotes

In prokaryotes, genetic information is carried in a single, circular chromosome.

This strand of DNA is attached at one site to the cell membrane.

Eukaryotes have much more DNA. In eukaryotes, genetic information is

organized into linear chromosomes. Eukaryotic chromosomes float freely in the

nucleus.

6.3 Prokaryotes divide by binary fission.

The circular chromosome duplicates itself and the cell splits into two identical new cells.

Some other primitive organisms also divide by fission.

6.4 A time for everything: the cell cycle.

Focus on S-phase (DNA synthesis) and the 4 stages of Mitosis, including cytokinesis.

6.5 Cell division is preceded by replication.

Let’s take a closer look at DNA synthesis in Interphase…

Recall the rules of base pairingA – TC- G

Mutations: Variability in Mitosis

A variety of errors can occur during DNA replication.

Several DNA editing processes occur after replication.

If an error remains, however, the sequences in a replicated DNA molecule (including the genes) can be different from those in the parent molecule.

Otherwise, the new DNA molecule should be identical.

Mitosis has just one purpose:

To enable cells to generate new, genetically identical cells.

There are two different reasons for this need:

1. Growth2. Replacement – What is

dust?

• You began life as a single cell (zygote – a fertilized egg), but there are now more cells in your body than stars in the Milky Way.

• Just in the past second, millions of your cells have divided in two.

Embryological Development

6.7 OverviewMitosis leads to duplicate

cells.Parent cells daughter

cells

6.8 The DetailsMitosis is a four-step

process.

Before Mitosis: The

Chromosomes Replicate

during the S-phase of

Interphase

After Replication: Sister Chromatids

A chromosome and its identical replicated copy, joined at the

centromere.

Cytokinesis occurs at the very end of Telophase. The result is two genetically identical daughter cells (assuming there were no errors in DNA replication or chromatid separation).

Cytokinesis is Cytokinesis is different in plant different in plant cells. In cells. In plant cells, vesicles vesicles containing cell wall containing cell wall material gradually material gradually fuse to form a cell fuse to form a cell plate.plate.

The The cell plate eventually grows eventually grows into a new cell wall into a new cell wall and divides the and divides the cell.cell.

Cell Division is actually a continual process.

Cancer cells have several features that distinguish them from normal cells, including…

6.9 Cell division out of control means cancer.

• Cancer treatment

Cancer Treatment

– Radiation therapy disrupts cell division.– Chemotherapy involves drugs that disrupt

cell division.• Taxol (from a coniferous tree) freezes the

mitotic spindle after it forms and keeps it from functioning. It has fewer side effects and has been effective against some hard-to-treat cancers.

• Vinblastine (from plant in Madagascar) prevents the mitotic spindle in the first place.

• If the mitotic spindle cannot form or function, then…?

Why is the treatment for cancer often considered as bad as the disease?

Chemotherapy and Radiation interferes with other areas of rapid cell division in the body.

1. Bone Marrow produces immune system cells (white blood cells) and platelets.

2. Hair follicles

• Cancer cells are often grown in culture for study. They can continually divide as long as there is a supply of nutrients.

Figure 8.10

• Cancer prevention includes changes in lifestyle.

– Not smoking– Avoiding exposure to

the sun– Eating a high-fiber,

low-fat, plant-based diet

– Visiting the doctor regularly

– Performing regular self-examinations

6.10 Sexual reproduction requires special cells made by meiosis.

Homologous Chromosomes carry the same sequences of genes and control the same inherited characteristics. Recall there may be alternative forms of these genes (alleles).

• Each sperm or egg produced in your reproductive organs carries one of over 8 million possible combinations of parental chromosomes.

This figure is based on the number of chromosomes in the haploid (n) cells of a particular species.

Sperm and egg cells are haploid gametes.

The Origins of Genetic Variation

• For any species, the total number of possible combinations is 2n, where n is the haploid number.

• For humans (n = 23), so there are 223 (8 million) possible combinations of maternal and paternal chromosomes in sperm and egg cells.

• One egg has 8 million possibilities. One sperm has 8 million possibilities. You multiply that together = 64 trillion possible combinations for a fertilized egg. WOW!

Meiosis and Mitosis

• Mitosis = asexual or cell reproduction– Results in genetically identical daughter

cells– Diploid (2n) to diploid (2n)

• Meiosis = sexual reproduction– Results in genetic diversity (gametes that

all differ from each other with respect to the combinations of alleles they carry)

– Diploid (2n) to haploid (n)

6.11 Sperm and egg are produced by meiosis: the details, step-by-step.

Mitosis occurs almost everywhere in an animal’s

body. Meiosis only occurs in one place.

Where?

Meiosis starts with a specialized diploid cell in the gonads.

a homologous pair, or homologues• the maternal and paternal copies of a

chromosome

Cells undergoing meiosis divide twice.

There are two major parts to meiosis:

1. Meiosis I: The homologues are separated, but the sisters stay together.

2. Meiosis II: Each of the two new cells divides again, separating the sister chromatids into two even newer cells

What are the major differences between Mitosis and Meiosis?

Meiosis creates haploid cells and genetic diversity…How?

Meiosis goes through the 4 phases of cell division twice.

• Crossover can occur in Prophase I.

• In Metaphase I, Homologous chromosomes line up as tetrads.

• In Anaphase I, Sister Chromatids stay together.

1. Prophase I

The most complex of all of the phases of meiosis

Crossing over, an important source of variability.

2. Metaphase I

Each pair of homologous chromosomes moves to the equator of the cell. They line up in groups of 4 (tetrads)

3. Anaphase I

Anaphase I is the phase in which the genetic material is halved, the alternative versions of genes are separated.

4. Telophase I and Cytokinesis

The result is 2 haploid cells, each with both copies of the chromosome (sister chromatids).

Meiosis Division 2

Separating the sister chromatids

8. Telophase II

The cytoplasm then divides, the cell membrane pinches the cell into two new daughter cells, and the process comes to a close.

The result is 4 haploid cells, each with a unique set of traits.

Mitosis provides growth, tissue repair, and asexual reproduction. Meiosis yields haploid cells for sexual reproduction.

Cross-over and the separation of alternative gene pairs (homologous chromosomes) contribute to genetic variety in sexual reproduction.

6.13 Crossing over in Meiosis is an important source of variation.

This variation is important for evolution.

Bacteria reproduce asexually, while most plants and animals reproduce sexually. Which is a better method?

It depends on the environment…

Some organisms can reproduce both sexually and asexually.

What type of environment would be favorable for asexual or sexual

reproduction?

Asexual reproduction can be fast and efficient.

But, asexual reproduction leads to genetically identical offspring.

Sexual reproduction leads to offspring that are genetically different from one another and either parent.

But, sexual reproduction takes more time and energy and can be risky.

In humans, the sex chromosomes carry information that directs a growing fetus to develop as a male or a female.

Male if the Y chromosome is present

Female if there is not Y chromosome

6.15 How is sex determined in humans?

6.17 Down syndrome can be detected before birth: karyotypes reveal an individual’s entire chromosome set.

karyotype a display of an individual’s complete set of chromo-somes

Down syndrome is also known as Trisomy 21 and is a result of nondisjunction.

Nondisjunction

the unequal distribution of chromosomes during meiosis

error of cell division that creates a gamete with zero or two copies of a chromosome rather than a single copy

• Nondisjunction Also affects the sex chromosomes.

Abnormal Numbers of Sex Chromosomes

A man with an extra X chromosome (Klinefelter syndrome) will develop feminine qualities.

A female is missing her second X chromosome (Turner syndrome) and will develop masculine qualities.