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How Cells Reproduce
Chapter 9
Impacts, Issues
Henrietta’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