How Cells Reproduce
Chapter 7
Henrietta’s Immortal Cells
• HeLa cells
• Derived from cervical
cancer that killed
Henrietta Lacks
• First human cells to
grow and divide
in culture
• Used in research
throughout the world
Division Mechanisms
Eukaryotic organisms
– Mitosis
– Meiosis
Prokaryotic organisms
– Prokaryotic fission
Roles of Mitosis
• Multicelled organisms
– Growth
– Cell replacement
• Some protistans, fungi, plants, animals
– Asexual reproduction
Chromosome
• A DNA molecule and attached proteins
• Duplicated in preparation for cell division
one chromosome (unduplicated)
one chromosome (duplicated)
The Cell Cycle
INTERPHASE
G1
Interval of cell
growth, before
DNA replication
(chromosomes
unduplicated)
Each daughter cell
starts interphase
S
Interval of cell
growth, when
DNA replication
is completed
(chromosomes
duplicated)
G2
Interval following
DNA replication;
cell prepares
to divide
Fig. 7-2, p.96
The Cell Cycle
The cell cycle
Control of the Cycle
• Once S begins, the cycle usually
runs through G2 and mitosis
• Cycle has a built-in molecular brake
in G1
• Cancer involves a loss of control
over the cycle, malfunction of
“brakes”
Interphase
• Usually the longest part of cycle
• Cell increases in mass
• Number of cytoplasmic components
doubles
• DNA is duplicated
Mitosis
• Period of nuclear division
• Usually followed by cytoplasmic division
• Four stages:
Prophase
Metaphase
Anaphase
Telophase
Chromosome Number
• Total number of chromosomes in a cell
• Somatic cells
– Chromosome number is diploid (2n)
– Two of each type of chromosome
• Gametes
– Chromosome number is haploid (n)
– One of each chromosome type
Human Chromosome Number
• Diploid chromosome number (n) = 46
• Two sets of 23 chromosomes
– One set from father
– One set from mother
• Mitosis produces cells with 46
chromosomes: two of each type
Human Chromosomes
Maintaining
Chromosome
Number
chromosomes
(unduplicated) in
parent cell at interphase
same chromosomes
(duplicated) in interphase
prior to mitosis
mitosis, cytoplasmic division
chromosome
(unduplicated)
in daughter cell
at interphase
chromosome
(unduplicated)
in daughter cell
at interphase
The Spindle Apparatus
• Consists of two distinct sets of
microtubules
– Each set extends from one of the cell poles
– Two sets overlap at spindle equator
• Moves chromosomes during mitosis
spindle microtubules
Spindles
chromosomes at spindle equator,
midway between spindle poles
Stages of Mitosis
Prophase
Metaphase
Anaphase
Telophase
Mitosis
Mitosis step-by-step
Cytoplasmic Division
• Usually occurs between late anaphase
and end of telophase
• Two mechanisms
– Cleavage (animals)
– Cell plate formation (plants)
animal cell pinching in two
Cleavage & Cell Plate
Formation
Cytoplasmic division
Asexual Reproduction
• Single parent produces offspring
• All offspring are genetically identical to
one another and to parent
Sexual Reproduction
• Involves
– Meiosis
– Gamete production
– Fertilization
• Produces genetic variation among
offspring
Homologous Chromosomes
Carry Different Alleles
• Cell has two of each chromosome
• Chromosome pairs: one from mother,
one from father
• Paternal and maternal chromosomes
carry different alleles
Sexual Reproduction
Shuffles Alleles
• Through sexual reproduction, offspring
inherit new combinations of alleles,
which lead to variations in traits
• Variation in traits is the basis for
evolutionary change
Gamete Formation
• Gametes are sex cells (sperm, eggs)
• Arise from germ cells in reproductive organs
ovaries
testes
Gamete Formation
Reproductive organs
Chromosome Number
• Total number of chromosomes
in cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome
number
Diploid To Haploid
centromere
one chromatid
its sister
chromatid
one chromosome in the duplicated state
each homologue in the cell
pairs with its partner
(synapsis)
then partners separate
two chromosomes
(unduplicated)
one chromosome
(duplicated)
Stepped Art
p.103
Meiosis: Two Divisions
• Two consecutive nuclear divisions
– Meiosis I
– Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form
p.102
Meiosis
Meiosis step-by-step
Sexual Reproduction and
Genetic Variation
• Two functions of meiosis provide
variation in traits:
– crossing over
– random alignment
Crossing Over
• Each chromosome
attaches to its
homologue
• All four chromatids are
closely aligned
(synapsis)
• Nonsister chromatids
exchange segments
Crossing Over
Crossing over
Effects of Crossing Over
• After crossing over, each chromosome
contains both maternal and paternal
segments
• Creates new allele combinations in
offspring
Random Alignment
• Between prophase I and metaphase I,
chromosome pairs align randomly at
metaphase plate
• Initial contact between microtubule and
either maternal or paternal chromosome
is random
Animal Life Cycle
Random alignment
Factors Contributing to Variation
among Offspring
• Crossing over during prophase I
• Random alignment of
chromosomes at metaphase I
• Random combination of gametes at
fertilization
Fertilization
• Male and female gametes unite
and nuclei fuse
• Fusion of two haploid nuclei produces
diploid nucleus in zygote
• Random chance of sperm fertilizing egg
increases variation in offspring
Animal Sperm Formation
Sperm formation
Animal Egg Formation
Egg formation
Cancer Characteristics
• Plasma membrane and cytoplasm
altered
• Cells grow and divide abnormally
• Weakened capacity for adhesion;
cells can move to new tissues
• Lethal unless eradicated
Cancer
Cancer and metastasis
Mitosis & Meiosis Compared
Mitosis
• Functions
– Asexual reproduction
– Growth, repair
• Occurs in
somatic cells
• Produces clones
Meiosis
• Function
– Sexual reproduction
• Occurs in germ cells
• Produces variable
offspring
Results of Mitosis and Meiosis
Comparing mitosis and meiosis