Cellular Reproduction: Cells from Cells
Asexual Reproduction
Although we usually only think about sexual reproduction, many organisms are
capable of asexual reproduction
Asexual reproduction does not involve the fertilization of an egg by sperm
Asexual reproduction is rare among more complex organisms (except plants) and
has never been seen in birds or mammals
Some organisms can reproduce by regenerating from a portion of the organism
In the case of sea stars, some of the central portion must be present
This is what happens when you grow a new plant from a cutting of an existing plant
This asexual reproduction is due to mitosis and leads to offspring that are
genetically identical to the parent
Some organisms can carry out parthenogenesis
Sexual Reproduction
Sexual reproduction does involve the fertilization of an egg by sperm
The production of gametes - egg and sperm - involves meiosis, a special type of cell
division, which only occurs in reproductive organs
The Cell Cycle and Mitosis
DNA
In eukaryotes, the vast majority of genes are located on chromosomes in the cell’s
nucleus
A small number of genes are located on the DNA found in mitochondria and
chloroplasts
Eukaryotic Chromosomes
Each chromosome contains a long DNA molecule bearing many genes
The number of chromosomes in a eukaryotic cell is characteristic of the species of
organism
Chromosomes are made of chromatin, fibers composed of roughly equal amounts of
DNA and protein molecules
The proteins help to
organize the chromatin and
control the activity of its genes
The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding
Histones are proteins used to package DNA in eukaryotes
Nucleosomes consist of DNA wound around histone molecules
DNA replication
During DNA replication, the double helix unwinds and each strand serves as a
template for the production of a new strand
The result is that each duplicated chromosome consists of two copies called sister
chromatids
Each of the resulting two DNA molecules consists of one old strand and one newlysynthesized strand
When the cell divides, the sister chromatids of a duplicated chromosome separate
from each other
Once separated from its sister, each chromatid
is considered a full-fledged chromosome and
is identical to the original chromosome
What does a cell need to do as it divides?
Equally distribute the chromosome copies (mitosis)
Physically divide to form two cells (cytokinesis)
Chromosomes in many species come in matched pairs
One copy is inherited from mom and one from dad
Each copy has the same genes in the same positions
However, there may be different versions of the genes found on each copy
The cell cycle
Cells progress through a series of phases that make up the cell cycle
Each cycle involves growth, DNA replication, and cell division
The cell cycle consists of two main phases
Interphase
Mitotic phase
During interphase, the cell does all of the prep work before actual division
During the mitotic phase, the duplicated chromosome split (mitosis) and the
cell divides (cytokinesis)
Mitotic Phase
Consists of mitosis and cytokinesis
Mitosis consists of four phases
Prophase
Metaphase
Anaphase
Telophase
Although each phase is marked by distinct features, they are parts of a
continuum
Cytokinesis
The chromosomes move because of the attachment, lengthening and shortening, and
then release of spindle fibers
Spindle fibers are microtubules composed of units of the protein tubulin
They lengthen by adding more tubulin; they shorten by removing more tubulin
As mitosis completes, the result is that one exact copy of each of the original
chromosomes will now be present at either end of the cell
Cytokinesis
In animal cells, a cleavage furrow forms as a ring of proteins tightens around the
center of the cell
In plant cells, cell division is complicated by the presence of the cell wall
Cell division must produce a new cell wall between the two daughter cells
A cell plate forms in the middle of the dividing cell
The cell plate expands, eventually forming cell walls for both daughter cells
Cancer
There are several checkpoints within the cell cycle that are designed to prevent
damaged cells from dividing
Cancer occurs when cells grow out of control
This is usually because of problems with the cell cycle control
Among other things, the cancerous cells are no longer subject to normal “contact
inhibition”
Humans are diploid organisms
Our chromosomes come in two matched sets
Chromosomes are present in homologous pairs
These carry the same genes in the same positions
They control the same inherited characteristics
One of each particular pair has come from each parent
Meiosis
Meiosis is a cell division where a special single diploid cell divides to produce four
haploid cells, each with a single set of chromosomes
Meiosis produces gametes
Female gametes are eggs
Male gametes are sperm
The fusion of sperm and egg is fertilization
Fertilization produces a zygote and restores the diploid number of chromosomes
The Process of Meiosis
There are both similarities and differences between mitosis and meiosis
Two of the most important differences are:
Meiosis produces four haploid cells
Requires two divisions (meiosis I and meiosis II)
There is an exchange of genetic material between homologous chromosomes in
meiosis - crossing over
Comparing Mitosis and Meiosis
All the events unique to meiosis occur during meiosis I
Meiosis II is virtually identical to mitosis in that it separates sister chromatids
But unlike mitosis, meiosis II yields daughter cells with a haploid set of
chromosomes
Generation of Genetic Diversity
Different variations of the same gene are called alleles
You can inherit the same alleles from each parent or you can inherit different alleles
from each parent
Crossing over produces new combinations of alleles
It mixes up the chromosomes
It mixes up the combinations of genes on the chromosomes
For any one chromosome type, we are just as likely to contribute our paternal copy
as our maternal copy
It all depends on the chromosome alignment at Metaphase I
Sources of Genetic Diversity
Crossing over
The random alignment of chromosomes at Metaphase I
Even without crossing over, there are 8,388,608 different possible gametes
that can form
The random union of sperm and egg
The original variation at the DNA level is due to mutation leading to different alleles
Generation of Genetic Diversity
So where does that genetic diversity lead?
Biodiversity - diversity of living organisms
The evolution of living organisms requires genetic diversity
What can go wrong?
Sometimes there can be changes in chromosome numbers
Nondisjunction during meiosis can produce gametes with abnormal chromosome
numbers
If these are involved in fertilization, the zygote can have an abnormal number of
chromosomes - aneuploidy
Most cases lead to spontaneous abortion of the embryo
Some erroneous combinations are more readily tolerated than others
Trisomy 21 - Down Syndrome
The most common chromosome number abnormality and serious birth defect in the
U.S.
~1/700 births
Whereas aneuploidy of autosomes is not well tolerated, sex chromosome aneuploidy
is