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David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Le cellule e i viventi
The Cell Cycle and
Cell Division
How Do Prokaryotic and Eukaryotic Cells Divide?
The life cycle of an organism is closely linked to cell
division.
Cell division is important in growth and repair of
tissues in multicellular organisms, and in the
reproduction of all organisms.
How Do Prokaryotic and Eukaryotic Cells Divide?
Four events must occur for cell division:
• A reproductive signal initiates cell division
• Replication of DNA
• Segregation: distribution of DNA into two new
cells
• Cytokinesis: separation of cellular material
into the two new cells
How Do Prokaryotic and Eukaryotic Cells Divide?
In prokaryotes, binary fission results in
two new single-celled organisms.
How Is Eukaryotic Cell Division Controlled?
Cell cycle: period from one cell division to the next;
divided into mitosis/cytokinesis and interphase.
Interphase: nucleus is
visible and cell functions,
including DNA replication,
occur; begins after
cytokinesis, ends when
mitosis starts (M phase).
Interphase has three subphases: G1, S, and G2.
Duration of interphase is
highly variable.
How Is Eukaryotic Cell Division Controlled?
The signals act through cyclin-dependent
kinases (Cdk’s).
Protein kinases catalyze
transfer of a phosphate group
from ATP to a protein
(phosphorylation). The shape
and function of the protein
changes.
Cdk’s play important roles in
the cell cycle.
Cyclin-Dependent Kinases Regulate Progress through the Cell Cycle
Cyclin–Cdk’s act at cell cycle checkpoints to
regulate progress.
What Happens during Mitosis?
Eukaryotic DNA molecules are extensively “packed” and
organized by histones—proteins with positive charges
that attract the negative phosphate groups of DNA.
Interactions result in the formation of beadlike units, or
nucleosomes.
DNA molecules are
complexed with
proteins to form
chromatin.
DNA is Packed into a Mitotic Chromosome
After replication, the sister
chromatids are held
together during G2 by
proteins called cohesins.
At mitosis the cohesin is
removed, except at the
centromere region.
What Happens during Mitosis?
Mitosis (M phase) ensures accurate segregation of
chromosomes to daughter cells.
The Phases of Mitosis
What Happens during Mitosis?
The spindle apparatus (or mitotic spindle)
moves sister chromatids apart.
Cytokinesis Differs in Animal and Plant Cells
Cytokinesis: division of the cytoplasm.
In animal cells the
plasma membrane
pinches in between the
nuclei.
In plant cells, vesicles
from the Golgi apparatus
appear along the plane
of cell division. These
fuse to form a new
plasma membrane and a
new cell wall.
What Role Does Cell Division Play in a Sexual Life Cycle?
Asexual reproduction is based on mitotic divisions.
Unicellular organisms can reproduce by fission
Multicellular organisms can also reproduce asexually,
producing clones.
Aspen trees have
shoots that sprout
from the root
system. All the
trees in a stand
may be clones of a
single parent.
What Role Does Cell Division Play in a Sexual Life Cycle?
Sexual reproduction: offspring are not identical
to the parents.
Requires gametes created by meiosis; two parents
each contribute one gamete to an offspring.
Somatic cells — body cells not specialized for
reproduction.
What Role Does Cell Division Play in a Sexual Life Cycle?
Gametes contain only one set of chromosomes — one
homolog of each pair. Chromosome number is haploid
(n).
Fertilization: two haploid gametes (female egg and
male sperm) fuse to form a diploid zygote;
chromosome number = 2n.
Evolution has generated many different versions of the
sexual life cycle.
What Role Does Cell Division Play in a Sexual Life Cycle?
What Happens during Meiosis?
Meiosis consists of two nuclear divisions (Meiosis
I and Meiosis II), but DNA is replicated only once.
• Reduces chromosome number from diploid to
haploid.
• Ensures that each haploid product has a
complete set of chromosomes
• Generates genetic diversity among the products
Meiosis: Generating Haploid Cells
What Happens during Meiosis?
During prophase I and metaphase I, the homologous
chromosomes pair by adhering along their lengths:
synapsis.
The four chromatids of each homologous pair form a
tetrad. At some point, the homologs seem to repel
each other but are held together by cohesins at
regions called chiasmata that form between nonsister chromatids.
Crossing Over Forms Genetically Diverse Chromosomes
Crossing over: exchange of genetic material occurs
between non-sister chromatids at the chiasmata.
Crossing over results in
recombinant chromatids
and increases genetic
variability of the products.
In anaphase I, independent
assortment also allows for
chance combinations.
What Happens during Meiosis?
Meiosis II:
• Not preceded by DNA replication
• Sister chromatids are separated
• Chance assortment of the chromatids contributes
further to the genetic diversity.
• Final products are four haploid daughter cells (n).
Meiosis: Generating Haploid Cells
Nondisjunction Leads to Aneuploidy (Part 1)
There can be errors in meiosis. Nondisjunction
which results in aneuploidy — chromosomes are
lacking or present in excess.
In humans, Down syndrome results from a gamete
with two copies of chromosome 21. After
fertilization, there are three copies (trisomic).
A fertilized egg that did not receive a copy of
chromosome 21 will be monosomic, which is
lethal.
Nondisjunction Leads to Aneuploidy (Part 2)
Mitosis and Meiosis: A Comparison
Cells resulting from mitosis are genetically identical to the parent cell.
Meiosis is nuclear division in cells involved in sexual reproduction.
The cells resulting from meiosis are not identical to the parent cells.
What Happens during Meiosis?
When cells are in metaphase of mitosis, it is possible to
count and characterize the chromosomes.
The karyotype is the number, shapes, and sizes of all
the chromosomes of a cell.
Karyotypes can be used to diagnose abnormalities
such as trisomies by a branch of medicine called
cytogenetics.
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved