How Cells Reproduce

Chapter 9

Impacts, IssuesHenrietta’s Immortal Cells

Henrietta Lacks died of cancer at age 31, but her cells (HeLa cells) are still growing in laboratories

Henrietta Lacks

Cancer cells used in research to identify viral strains, investigate cancer, study the effects of radiation on cells, develop techniques for polio vaccine.

Mitosis, Meiosis, and the Prokaryotes

Eukaryotic cells• Mitosis copies DNA and divides a nucleus,

producing two identical daughter cells; asexual reproduction (clones); exception identical twins

• Meiosis is a nuclear division that produces haploid gametes for sexual reproduction (two parents involved)

Prokaryotic cells reproduce asexually by prokaryotic fission

A comparision of Mitosis and Meiosis

Mitosis – Greek Mitos = thread

Division of eukaryotic cell or mother cell into two identical cells (daughter cells – receives copies of the original chromosome.

Occurs in somatic cells Diploid chromosomes

(46) – two sets

Meiosis A type of division that

reduces the chromosome number from diploid to haploid

Occurs in germ cells (gametes – egg and sperm)

Haploid chromosomes (23) - a single set

Mitosis Clones One nuclear division Parent = 8 chromosomes

and the daughter cells = 8

Meiosis Variation in traits Two nuclear divisions

(meiosis I and meisosis II Meiosis II resembles

mitosis Parent = 46 and

egg/sperm = 23

Why do cells divide (Mitosis)

Growth Repair of damaged tissue (ex. Healing of a

wound) Replacement of worn out cells (RBC’s) Asexual reproduction (single celled organism

producing a clone or exact copy of itself)

Key Points About Chromosome Structure

Each species has a characteristic number of chromosomes that differ in length and shape• Each consists of one double strand of DNA• After duplication, each consists of two double

strands (sister chromatids) that remain attached to each other at a centromere until late in nuclear division – Study Figure 9.2

Key Points About Chromosome Structure

A chromosome consists of DNA that is wrapped around proteins (histones) and condensed

Each histone and the DNA wrapped around it make up a nucleosome

Animation: The cell cycle

9.2 Introducing the Cell Cycle

Cell cycle• A sequence of three stages (interphase, mitosis,

and cytoplasmic division) through which a cell passes between one cell division and the next

Interphase

Interphase consists of three stages, during which a cell increases in size, doubles the number of cytoplasmic components, and duplicates its DNA• G1: Interval of cell growth and activity; before the

onset of DNA replication• S: Interval of DNA replication, synthesis or

duplication (two DNA double helix)• G2: Interval when the cell prepares for division;

the stage before cytoplasmic division

Interphase and the Life of a Cell

Most cell activities take place during G1

Control mechanisms work at certain points in the cell cycle; some can keep cells in G1 (muscle and nerve cells)

Loss of control may cause cell death or cancer

Mitosis and the Chromosome Number

Mitosis produces two diploid nuclei/cells with the same number and kind of chromosomes as the parent or mother cell ( each new cell receives copies of all the original chromosomes)

Chromosome number• The sum of all chromosomes in a type of cell• Human cells have 46 chromosomes paired in 23

sets - Study Figure 9.5a• Pairs have the same shape and information about

the same traits

9.3 A Closer Look at Mitosis

When a cell undergoes mitosis, the daughter cells have identical chromosomes, and the daughter cell has chromosomes identical to those of the mother cell that produced it.

There are four main stages/sequences of mitosis: prophase, metaphase, anaphase, and telophase

Prophase

Prophase• Chromosomes condense• Microtubules form a bipolar spindle or spindle

apparatus (becomes visible)• Nuclear envelope/membrane breaks up and

disappears• Microtubules attach to the chromosomes

Metaphase and Anaphase

Metaphase (meta = between)• All duplicated chromosomes line up at the spindle

equator (midway)

Anaphase• Microtubules separate the sister chromatids of

each chromosome and pull them to opposite spindle poles

Telophase

Telophase• Two clusters of chromosomes reach the spindle

poles• A new nuclear envelope or membrane reforms

Two new cells are formed, each with the same chromosome number as the parent cell

9.4 Cytoplasmic Division Mechanisms

In most kinds of eukaryotes, the cell cytoplasm divides between late anaphase and the end of telophase, but the mechanism of division differs

Cytokinesis• The process of cytoplasmic division or distribution• Plants cytokinesis leads to cell plate formation• Animals cytokinesis leads to the formation of a

contractile ring (cleavage furrow)

Animation: Cytoplasmic division

9.5 When Control is Lost

Sometimes, controls over cell division are lost• Cancer may be the outcome

Cell Cycle Controls

Checkpoints in the cell cycle (at G1, G2, and M) and allow problems to be corrected before the cycle advances

Proteins produced by checkpoint genes interact to advance, delay, or stop the cell cycle• Kinases – stops the cycle if DNA is damaged

or kills the cell• Growth factors – stimulates a cell to grow and

divide (ex repairs injuries)

Checkpoint Failure and Tumors

When all checkpoint mechanisms fail, a cell loses control over its cell cycle and may form a tumor (abnormal mass of cells) in surrounding tissue

Usually one or more checkpoint gene products are missing in tumor cells• Tumor suppressor gene products inhibit mitosis• Proto oncogene products stimulate mitosis

Neoplasms

Carcinogens – chemicals that cause cancer Neoplasms AKA tumors• Abnormal masses of cells that lack control over

how they grow and divide• Benign neoplasms (such as ordinary skin

moles) stay in one place and are not cancerous; do not pose a threat to surrounding tissues

• Malignant neoplasms are cancerous; cells may break away, spread and invade distant tissues (metastasis)

Benign and Malignant Tumors

Animation: Cancer and metastasis

Skin Cancers

Animation: Mitosis

ABC video: Blood test for lung cancer

Video: Henrietta's immortal cells