CHAPTER 8
The Cellular Basis of
Reproduction and Inheritance
Overview:
Reproduction: asexual & sexual
prokaryotes
eukaryotes
Mitosis
Meiosis
Abnormalities
• The life cycle of a multicellular
organism includes
– development
– Reproduction: the birth of new organisms
• Cell division is at the heart of the reproduction of
cells and organisms; cell division plays a role in
replacement of lost or damaged cells
• Organisms can reproduce sexually or asexually
• Just in the past second,
millions of your cells have
divided in two
• Some organisms make exact copies of
themselves, asexual reproduction
• Some organisms can also reproduce asexually
– This sea star is regenerating a lost arm
– Regeneration results from repeated cell divisions
The Reproduction of Organisms
• In asexual reproduction, single-celled organisms
reproduce by simple cell division
Cells arise only from preexisting cells
• All cells come from cells
• Cellular reproduction is called cell division
– Cell division allows an embryo to develop into an
adult
– It also ensures the continuity of life from one
generation to the next
Passing On the Genes from Cell to Cell
• Before a parent cell divides, it duplicates its
chromosomes
• The two resulting “daughter” cells are
genetically identical
• Sexual reproduction is different
– It requires fertilization of an egg by a sperm
– Production of egg and sperm is called meiosis
Prokaryotes reproduce by binary fission
• Prokaryotic cells divide asexually
– These cells possess a single chromosome, containing
genes
– The chromosome is replicated
– The cell then divides into two cells, a process called
binary fission
THE EUKARYOTIC CELL CYCLE AND
MITOSIS
The large, complex chromosomes of eukaryotes
duplicate with each cell division
• An eukaryotic cell has many more genes than a
prokaryotic cell
– The genes are grouped into
multiple chromosomes,
found in the nucleus
– The chromosomes of this
plant cell are stained
dark purple
• A genome
– Is the complete set of an organism’s genes
– Is located mainly on chromosomes in the cell’s
nucleus
Eukaryotic Chromosomes
• Chromosomes
– Are made of
chromatin, a
combination of
DNA and protein
molecules
– Are not visible in
a cell until cell
division
The Cell Cycle
• Eukaryotic cells that divide undergo an orderly
sequence of events called the cell cycle
• The cell cycle consists of two major phases:
– Interphase, where chromosomes duplicate
and cell parts
are made
– The mitotic
phase, when
cell division
occurs
• Eukaryotic cell division consists of two stages:
– Mitosis
– Cytokinesis
• Mitosis
– Is the division of the
chromosomes
– Is preceded by interphase
Interphase
• In mitosis, the duplicated chromosomes are
distributed into two daughter nuclei
– After the chromosomes coil up, a mitotic spindle
moves them to the middle of the cell
• Mitosis consists of four distinct phases:
• The sister chromatids then separate and move to
opposite poles of the cell
– The process of cytokinesis divides the cell into two
genetically identical cells
• Cytokinesis
– Typically occurs during telophase
– Is the division of the cytoplasm
Cytokinesis differs for plant and animal cells
• In animals, cytokinesis
occurs by cleavage
– This process pinches the
cell apart
• In plants, a
membranous cell
plate splits the cell in
two
Anchorage, cell density, and chemical
growth factors affect cell division
• Most animal cells divide only when stimulated,
and others not at all
• In laboratory cultures, most normal cells divide
only when attached to a surface
– They are anchorage dependent
• Cells continue dividing until they touch one
another
– This is called density-dependent inhibition
• Growth factors are proteins secreted by cells
that stimulate other cells to divide
Growth factors signal the cell cycle control system
• Proteins within the cell control the cell cycle
– Signals affecting critical checkpoints determine
whether the cell will go through a complete cycle and
divide
• The binding of growth factors to specific
receptors on the plasma membrane is usually
necessary for cell division
Cancer Cells: Growing Out of Control
• Normal plant and animal cells have a cell cycle
control system
• When the cell cycle control system malfunctions
– Cells may reproduce at the wrong time or place
– A benign tumor may form
What Is Cancer?
• What is cancer?
– Cancer is caused by a breakdown in control of the
cell cycle
Cancer Treatment
• Cancer treatment
– Radiation therapy disrupts cell division
– Chemotherapy involves drugs that disrupt cell
division
Cancer Prevention and Survival
• Cancer prevention includes changes in lifestyle
–
–
–
–
–
Not smoking
Avoiding exposure to the sun
Eating a high-fiber, low-fat diet
Visiting the doctor regularly
Performing regular self-examinations
MEIOSIS AND CROSSING OVER
Chromosomes are matched in homologous pairs
• Somatic cells of each
species contain a
specific number of
chromosomes
– Human cells have 46,
making up 23 pairs of
homologous
chromosomes
Homologous Chromosomes
• Different organisms of the same species have the
same number and types of chromosomes
• A somatic cell
– Is a typical body cell
– Has 46 chromosomes in a human
• Humans have
– Two different sex chromosomes, X and Y
– 22 pairs of matching chromosomes, called autosomes
Gametes have a single set of chromosomes
• Cells with two sets of chromosomes are said to
be diploid
• Gametes are haploid, with only one set of
chromosomes
• At fertilization, a sperm fuses with an egg,
forming a diploid zygote
– Repeated mitotic divisions lead to the development
of a mature adult
– The adult makes haploid gametes by meiosis
– All of these processes make up the sexual life cycle
of organisms
• Humans are diploid organisms
– Their cells contain two sets of chromosomes
– Their gametes are haploid, having only one set of
chromosomes
• Fertilization
– Is the fusion of sperm and egg
– Creates a zygote, or fertilized egg
Meiosis reduces the chromosome number
from diploid to haploid
• Meiosis, like mitosis, is preceded by
chromosome duplication
– However, in meiosis the cell divides twice to form
four daughter cells
• In the first division, meiosis I, homologous
chromosomes are paired
– While they are paired, they cross over and exchange
genetic information
– The homologous pairs are then separated, and two
daughter cells are produced
• Meiosis II is essentially the same as mitosis
– The sister chromatids of each chromosome separate
– The result is four haploid daughter cells
Review: A comparison of mitosis and
meiosis
• For both processes, chromosomes replicate only
once, during interphase
Independent orientation of chromosomes in
meiosis and random fertilization lead to
varied offspring
• Each chromosome of a homologous pair comes
from a different parent
– Each chromosome thus differs at many points from
the other member of the pair
The Origins of Genetic Variation
• Offspring of sexual reproduction are genetically
different from their parents and from one another
• The large number of possible arrangements of
chromosome pairs at metaphase I of meiosis
leads to many different combinations of
chromosomes in gametes
• Random fertilization also increases variation in
offspring
Random Fertilization
• The human egg cell is fertilized randomly by
one sperm, leading to genetic variety in the
zygote
Homologous chromosomes carry
different versions of genes
• The differences between homologous
chromosomes are based on the fact that they can
carry different versions of a gene at
corresponding loci
Crossing Over
• In crossing over
– Homologous
chromosomes
exchange genetic
information
– Genetic
recombination
occurs
Crossing over further increases genetic
variability
• Crossing over is the exchange of corresponding
segments between two homologous
chromosomes
• Genetic recombination results from crossing
over during prophase I of meiosis
– This increases variation further
ALTERATIONS OF CHROMOSOME
NUMBER AND STRUCTURE
A karyotype is a photographic inventory of an
individual’s chromosomes
• To study human chromosomes microscopically,
researchers stain and display them as a
karyotype
– A karyotype usually shows 22 pairs of autosomes
and one pair of sex chromosomes
• A karyotype is an orderly
arrangement of
chromosomes
Connection: An extra copy of chromosome 21 causes
Down syndrome
• This karyotype shows three number 21
chromosomes
• An extra copy of chromosome 21 causes Down
syndrome
Accidents during meiosis can alter
chromosome number
• Abnormal
chromosome count
is a result of
nondisjunction
– Either
homologous
pairs fail to
separate
during
meiosis I
Connection: Abnormal numbers of sex
chromosomes do not usually affect
survival
• Nondisjunction can also produce gametes with
extra or missing sex chromosomes
– Unusual numbers of sex chromosomes upset the
genetic balance less than an unusual number of
autosomes
Connection: Alterations of chromosome
structure can cause birth defects and cancer
• Chromosome breakage can lead to
rearrangements that can produce genetic
disorders or cancer
– Four types of rearrangement are deletion,
duplication, inversion, and translocation