Chapter 6
Chromosomes and Cell Division
You do not have to know sections 6:16 -6:18
Chapter Outline
1 There are different types of cell division.
6.1 Immortal cells can spell trouble: cell division in sickness and in health.
TAKE-HOME MESSAGE 6.1: Cell division is an ongoing process in most organisms
and their tissues; disruptions to normal cell division can have serious consequences. In
eukaryotic cells, a protective section of DNA called the telomere, at each end of every
chromosome, plays a role in keeping track of cell division, getting shorter every time the
cell divides. If telomeres become too short, additional cell divisions cause the loss of
essential DNA and cell death. Cells that rebuild their telomeres with each division can
become cancerous.
6.2 Some chromosomes are circular, others are linear.
TAKE-HOME MESSAGE 6.2: In most bacteria and archaea, the genetic information is
carried in a single, circular chromosome, a strand of DNA that is attached at one site to
the cell membrane. Eukaryotes have much more DNA that do bacteria and organize it
into free-floating linear chromosomes within the nucleus.
6.3 Prokaryotes divide by binary fission.
TAKE-HOME MESSAGE 6.3: Bacteria divide by a type of asexual reproduction called
binary fission: First, the circular chromosome duplicates itself, then the cell splits into
two new, genetically identical daughter cells.
6.4 There is a time for everything in the eukaryotic cell cycle.
TAKE-HOME MESSAGE 6.4: Eukaryotic somatic cells alternate in a cycle between
cell division and other cell activities. The cell division portion of the cycle is called the
mitotic phase. The remainder of the cell cycle, called interphase, consists of two gap
phases (during which cell growth and other metabolic activities occur)z` separated by a
DNA synthesis phase during which the genetic material is replicated.
6.5 Cell division is preceded by replication.
TAKE-HOME MESSAGE 6.5: Every time a cell divides, that cell’s DNA must first
duplicate itself so that each of the two new daughter cells has all the genetic material of
the original parent cell. The process of DNA duplication is called replication. Errors in
replication can lead to mutations, which are changes in the DNA sequence.
2 Mitosis replaces worn-out old cells with fresh new
duplicates.
6.6 Most cells are not immortal: Mitosis generates replacements.
TAKE-HOME MESSAGE 6.6: Mitosis enables existing cells to generate new,
genetically identical cells. This makes it possible for organisms to grow and to replace
cells that die.
6.7 Overview: Mitosis leads to duplicate cells.
TAKE-HOME MESSAGE 6.7: Mitosis is the process by which cells duplicate
themselves. Mitosis follows chromosome replication and leads to the production of two
daughter cells from one parent cell.
6.8 The details: Mitosis is a four-step process.
TAKE-HOME MESSAGE 6.8: The ultimate consequence of mitosis and cytokinesis is
the production of two genetically identical cells.
6.9 Cell division out of control means cancer.
TAKE-HOME MESSAGE 6.9: Cancer is unrestrained cell growth and division, which
lead to large masses of cells that may cause serious health problems. Treatment focuses
on killing or slowing down the fast-growing and dividing cells, usually using
chemotherapy and/or radiation.
3 Meiosis generates sperm and eggs and a great deal of
variation.
6.10 Overview: Sexual reproduction requires special cells made by meiosis.
TAKE-HOME MESSAGE 6.10: Meiosis is the process by which reproductive cells are
produced in sexually reproducing organisms. It results in gametes that have only half as
much genetic material as the parent cell and that differ from one another with respect to
the combinations of alleles they carry.
6.11 Sperm and eggs are produced by meiosis: the details, step by step.
TAKE-HOME MESSAGE 6.11: Meiosis occurs only in gamete-producing cells. It is
preceded by DNA replication and consists of two rounds of cellular division, one in
which homologous pairs of sister chromatids separate and a second in which the sister
chromatids separate. The final product of meiosis in a diploid organism is four haploid
gametes.
6.12 Male and female gametes are produced in slightly different ways.
TAKE-HOME MESSAGE 6.12: In species with two sexes—including nearly every
sexually reproducing plant and animal species—females are the sex that produces the
larger gamete, and males produce the smaller gamete. Whether it is male or female
gametes that are being produced, each gamete ends up with just one copy of each
chromosome.
6.13 Crossing over and meiosis are important sources of variation.
TAKE-HOME MESSAGE 6.13: Although it doesn’t create any new traits, crossing
over during the first prophase of meiosis creates gametes with collections of traits that
may never have existed before; this variation is important for evolution.
6.14 What are the costs and benefits of sexual reproduction?
TAKE-HOME MESSAGE 6.14: There are two fundamentally different ways that cells
and organisms can reproduce: (1) mitosis and asexual reproduction via binary fission, and
(2) meiosis and sexual reproduction. Asexual reproduction can be fast and efficient, but
leads to genetically identical offspring that carry all of the genes that their parent carried,
which could be disadvantageous in a changing environment. Sexual reproduction leads to
offspring that are genetically different from one another and from either parent, but it
takes more time and energy and can be risky.
4
There are sex differences in the chromosomes.
6.15 How is sex determined in humans?
TAKE-HOME MESSAGE 6.15: In humans, the sex chromosomes carry information
that directs a growing fetus to develop as either a male (if a Y chromosome is present) or
a female (if no Y chromosome is present). Sex determination depends on the sex
chromosomes inherited from the father.
6.16 The sex of offspring is determined in a variety of ways in non-human species.
TAKE-HOME MESSAGE 6.16: A variety of methods are used for sex determination
across the world of plant and animal species. These include the presence or absence of
sex chromosomes, the number of chromosome sets, and environmental factors such as
incubation temperature.
5
Deviations from the normal chromosome number lead to
problems.
6.17 Down syndrome can be detected before birth: karyotypes reveal an individual’s
entire chromosome set.
TAKE-HOME MESSAGE 6.17: A karyotype is a visual display of a complete set of
chromosomes. It is a useful diagnostic tool because it can be prepared early in a fetus’s
development to assess whether there is an abnormality in the number of chromosomes or
in their structure, such as in the case of Down syndrome. Down syndrome is caused by
having an extra copy of chromosome 21.
6.18 Life is possible with too many or too few sex chromosomes.
TAKE-HOME MESSAGE 6.18: Although it is usually fatal to have one too many or
one too few of the non-sex chromosomes, individuals born with only a single sex
chromosome that is an X, or with an additional X or Y chromosome, usually survive—
though often with physical and/or mental problems.