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 dividein 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)
INTERPHASE
SInterval of cellgrowth, whenDNA replicationis completed(chromosomesduplicated)
Inte
rpha
se e
nds
for
pare
nt c
ell
Cytoplasmic division
G1Interval of cellgrowth, beforeDNA replication(chromosomesunduplicated)
G2Interval followingDNA replication;cell preparesto divide
Each daughter cellstarts interphase
MITO
SIS
Telophase
Anaphase
Metaphase
Prop
hase
Fig. 7-2, p.96
The Cell Cycle
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:ProphaseMetaphaseAnaphaseTelophase
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
Spindlesspindle microtubules
chromosomes at spindle equator, midway between spindle poles
Stages of Mitosis
Prophase
Metaphase
Anaphase
Telophase
Mitosis step-by-step
Mitosis
Cytoplasmic Division
• Usually occurs between late anaphase and end of telophase
• Two mechanisms
– Cleavage (animals)– Cell plate formation (plants)
animal cell pinching in two
Cytoplasmic division
Cleavage & Cell Plate Formation
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
testes
ovaries
Reproductive organs
Gamete Formation
Chromosome Number
• Total number of chromosomes
in cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome
number
Diploid To Haploid
one chromatid
its sisterchromatid
centromere
one chromosome in the duplicated state
each homologue in the cell pairs with its partner (synapsis) then partners separate
Stepped Art
p.103
two chromosomes (unduplicated)
one chromosome (duplicated)
Meiosis: Two Divisions
• Two consecutive nuclear divisions – Meiosis I
– Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form
p.102
Meiosis step-by-step
Meiosis
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
Random alignment
Animal Life Cycle
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
Sperm formation
Animal Sperm Formation
Egg formation
Animal 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 and metastasis
Cancer
Mitosis
• Functions– Asexual reproduction– Growth, repair
• Occurs insomatic cells
• Produces clones
Mitosis & Meiosis Compared
Meiosis
• Function – Sexual reproduction
• Occurs in germ cells
• Produces variable offspring
Comparing mitosis and meiosis
Results of Mitosis and Meiosis