Cell Reproduction
Cell Reproduction
• The continuity of life is a result of cell division,
which is one of the most distinctive characteristics
of living organisms.
• In the cases of humans, we develop from a single
cell that was originally the product of the fusion of
two special cells, a sperm cell and an egg cell
during sexual reproduction.
• Other organisms such as bacteria produce by
asexual means, dividing rapidly in suitable
conditions to produce millions of cells.
• Many fungi, liverworts and ferns also reproduce
asexually, resulting in clones of genetically identical
offspring.
Bacteria
• Bacteria divide in a process known as binary
fission.
• Each cell has a single circular chromosome which
attaches to the cell membrane at a specific site.
• The DNA molecule replicates, and as the bacterial
cell grows, the two copies of DNA separate.
• The cell cytoplasm then divides, and the cell
membrane and wall grow to separate the parts into
two daughter cells.
• Unlike eukaryotes, the DNA does not condense
during division.
The Cell Cycle
• The period between the
formation of a new cell and
when it divides to produce
two daughter cells is called
the cell cycle.
• In eukaryotes, actively
growing and dividing somatic
cells (non-sex cells) move
through a series of phases in
the cycle.
• Most of the time the cells are
in interphase, the stage
during which molecules are
synthesized and DNA
replicated.
• Interphase alternates with
cell division or mitosis.
The Cell Cycle
Interphase
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Divided into three stages
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Pre-DNA synthesis (G1)
DNA synthesis (S)
Post DNA synthesis (G2)
Chromosomes are not visible during interphase
Cell Division
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Divided into two stages:
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Mitosis (M) – division of nucleus containing chromosomes
Cytokinesis – division of the cytoplasm
Mitosis occurs after G2
Cytokinesis occurs near the end of M.
In dividing cells, chromosomes are readily observed using a microscope.
Once cell division is complete, cells enter G1.
All cells produced during the mitotic cell cycle are genetically identical, cells
differentiate during G1 and G2.
Mitosis
• Each cell (not including sex cells) contains two of each
chromosome, forming homologous (matching) pairs.
• Human cells somatic cells contain 46 chromosomes (23
matching pairs).
• Mitosis is the process by which chromosomes duplicate, sister
chromatids separate and the nucleus of the cell divides. The
genetic information in the two daughter nuclei is identical to
that of the parent nucleus.
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NB – Interesting exception to remember are nematodes. Nematodes
don’t undergo mitosis for growth and development. Each worm is a
tiny, fully formed adult when hatched. All growth after this stage is
due to an increase in size of individual cells.
Mitosis
• Stages of Mitosis:
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Interphase
Prophase
Metaphase
Anaphase
Telephase
• Although mitosis is divided into different
stages, it is important to realize that it is in
fact a continuous process.
Stages of Mitosis
Interphase
• Chromosomes are
not visible
• Cells are replicating
Prophase
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Chromosomes begin to shorten and
thicken. They become visible under
the microsope.
Chromosomes are double stranded at
this stage because DNA replicated
during S phase, resulting in two
identical copies of the chromosomes.
These two copies called chromatids
are held together by a constricted
region called the centromere.
Late in prophase the nuclear
membrane disappears and a network
of fibres known as the spindle
appears. The spindle extends
between the two poles of the cells.
Metaphase
• Chromosomes
gather at the equator
(central region) of
the spindle.
• The centromere of
each chromosome
attaches to the
spindle fibres.
Anaphase
• Spindle fibres contract,
pulling the centromere of
each chromosome in two
directions.
• Splitting of the centrome
separates the chromatids
into single strands.
• The separated chromatids
are drawn to opposite poles
of the cell.
• Now single, the separated
chromatids are once again
called chromosomes.
Telephase
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This is last stage of mitosis.
Nuclear membranes reform around each
group of chromatids.
Once mitosis is complete, the cytoplasm
divides by the process of cytokinesis. This
separates the two daughter nuclei into
separate cells.
In animals the cell membrane pinches in,
dividing the cell into two daughter cells.
In plants, a new cell wall is laid down
between the daughter cells. The
components of the wall are initially
deposited in the centre of the cell. Growth
of the wall extends until the two daughter
cells are completely separated.
Memory Aids for Mitosis
To remember the order in which steps occur:
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Indian People Make Artistic Teepees
Interphase → Prophase → Metaphase → Anaphase → Telephase
To remember what occurs at each stage:
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P – is for Pairs. We can see pairs of chromosomes under the
microscope
M – is for Middle. Sister chromatids gather in the middle of the
spindle
A – is Apart. Spindle contracts and chromatids are pulled apart
T – is for Tidy up. Cell is tidied up – new nuclear membranes are
formed
Mitosis
Meiosis
• Meiosis is the process of cell division
that produces gametes (germ cells),
the specialized cells that combine in
sexual reproduction.
• Meiosis is a reduction division.
• It involves one replication of DNA and
two nuclear divisions that result in
halving the number of chromosomes
(2n) in a parent diploid nucleus to four
haploid (n) daughter nuclei in gametes.
Meiosis
Meiosis
• During the first meiotic division, homologous
chromosomes align and crossing over occurs.
• Crossing over involves the breaking and re-joining
of chromatids and therefore DNA molecules.
• It takes place between two non-sister chromatids of
a chromosome pair.
• The two chromosomes of a pair are held together at
the site of crossing over by a chiasma. A chiasma
connects the chromatids until it is time for them to
separate. Along the length of a long chromosome
there may be several chiasmata.
• Crossing over produces chromosomes with new
combinations of genetic information. This process
is known as recombination.
Meiosis
• Homologous chromosomes
line up together; chromatids
break where they are
twisted
• Chromatid ends join to
'wrong' pieces
• Homologous chromosomes
move apart
• Separated chromosomes
carry new gene
combinations
Meiosis
Meiosis in Brief – First Meiotic Division
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Prophase I
– Chromosomes shorten and thicken.
– Homologous pairs of chromosomes pair with each other and crossing
over occurs.
– Spindle is formed.
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Metaphase I
– Homologous pairs align at equator of spindle.
– They remain attached at crossing over points (this attachment is
random).
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Anaphase I
– Homologous chromosomes separate.
– As they pull apart points of crossing over are visible as chiasmata.
– One member of each homologous pair migrates to each pole of the cell.
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Telephase I
– nuclear membrane begins to reform.
– Each new nuclei has one member of each homologous pair of
chromosomes (n) but each chromosome still consists of two chromatids.
– Cytokinesis occurs.
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Interkinesis
– Brief phase between divisions.
– No replication occurs.
– Interkinesis absent in some organisms.
Meiosis in Brief – Second Meiotic
Division
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Prophase II
– Nuclear membrane breaks down,
– Centrioles divide again and spindle reforms.
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Metaphase II
– Paired chromatids align at equator of spindle
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Anaphase II
– Centromeres divide and chromatids separate and move to opposite poles
of each cell.
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Telephase II
– Nuclear membrane reforms.
– Spindle breaks down and cytokinesis occurs.
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End product is four gametes each of which is different.
Each gamete contains one member of each homologous pair
Usually in females three of the new nuclei are broken down and only
one survives in the newly formed ova.
Summary of inputs and outputs for
meiosis
Meiosis
Important side note to meiosis
• Mitochondria and chloroplasts have a circular molecule of
DNA within them.
• As mitochondria and chloroplasts are part of the cytoplasm,
they are passed from generation to generation via one parent,
usually the female.
• The inheritance of mtDNA or cpDNA from one parent is called
uniparental inheritance.
• This is important when we consider some genetic disorders.
– Leber’s Hereditary Optic Neuropathy is due to a mutation in
mtDNA and results in mitochondria producing less ATP. As optic
cells need high levels of ATP they are the first to die.
• Very important to realize that the inheritance of faulty
mitochondria is random due to the nature of cytokinesis.
Problems with Meiosis
• Meiosis is usually an exact process, but sometimes
errors occur.
• The resulting chromosomal abnormalities (extra or
missing chromosomes) can have severe effects on
off-spring.
• Non-separation of chromosomes, and fusion of
non-homologous chromosomes can both occur
during meiosis and lead to different problems.
• The inheritance of too many or too few
chromosomes is referred to as aneuploidy, and
can occur in autosomes or in sex chromosomes.
• About 15% of pregnancies end with a spontaneous
abortion. Approximately half of these occur
because the zygote has an abnormal number of
chromosomes.
Problems with Meiosis:
Down Syndrome
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Down syndrome is the result of having three copies of chromosome
21 and is often referred to as trisomy-21.
It can occur by chance (non-familial Down Syndrome) or there may
be a history of the syndrome in the family.
The extra copy of chromosome 21 usually results from an error in
meiosis of one of the parents.
Chromosome 21 undergoes nondisjunction so that cell division
produces gametes with either an extra or missing chromosome 21.
If a gamete with (N + 1) chromosomes unites with a normal gamete
(N) the zygote will be trisomic (2N + 1).
People with mild Down syndrome are often mosaics, some of their
cells carry 3 copies of chromosome 21 and others are normal. The
severity of the syndrome will then depend on which cells are
affected.
Unusual example of
Familial Down Syndrome
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A child with familial Down syndrome had the same number of chromosomes
(46) as individuals of normal chromosome complement, rather than the 47
chromosomes usually observed in Down syndrome
Parents of these children were of normal phenotype, but had chromosome
numbers of 46 and 45.
Closer examination of the chromosomes of the parent with 45 showed that his
cells had only one copy of chromosome 15 and one copy of chromosome 21,
rather than the expected two copies of each.
In addition, this parent had a ‘hybrid’ chromosome (termed 15/21) consisting
of a chromosome 15 and a chromosome 21 joined together (called a
translocation).
The child with Down syndrome was found to have one copy of chromosome
15, two copies of chromosome 21 and one copy of chromosome 15/21.
Therefore the child had effectively two copies of chromosome 15 and three
copies of chromosome 21.
The outcome is the same as when trisomy-21 occurs due to nondisjunction,
the only difference being that the chromosomes are ‘packaged’ in a different
way.
Unusual example of
Familial Down Syndrome
Polyploidy
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Polyploidy refers to having more than two sets of chromosomes (eg
3N, 4N, 6N) in a genome.
Polyploidy may arise due to errors in meiosis – e.g. a gamete may be
diploid instead of haploid. If a diploid sperm fertilizes a haploid egg
the resulting zygote has one extra set of chromosomes and is
therefore triploid (3N).
Polyploid zygotes don’t survive in humans, however, polyploidy can
also arise during mitosis and produce groups of somatic polyploidy
cells – this does not necessarily affect health.
Polyploidy is much more common in plants because many plants can
survive by asexual reproduction. Polyploid plants may be sterile
because of problems with chromosome pairing during meiosis, but
continue to survive by vegetative reproduction.
Examples of polyploidy in plants: some bananas (3N), cultivated
cotton and potatoes (4N), strawberries (8N)
Examples of polyploidy in animals: some insects, earthworms and
tree frogs
How polyploidy occurs
(a) A tetraploid organism (4N)
can be the result of the
chance doubling of
chromosomes.
(b) A tetraploid produces 2N
gametes.
If the tetraploid mates with
a normal diploid (1N
gametes), then the offspring
will be a sterile triploid (3N).
However, two diploid
gametes may occasionally
fuse, producing an offspring
with twice the number of
chromosomes (a tetraploid,
4N).
Genes and Development
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During embryonic development cells differentiate under the control of
genes into special cell types.
A master gene functions to regulate the activity of other genes.
These master genes are referred to as homeotic genes as their
products regulate the activity of other genes during development.
All homeotic genes contain a short sequence of DNA which codes for
a sequence of amino acids that bind to DNA. This binding allows the
protein to regulate transcription of other genes.
Some cells remain as undifferentiated stem cells, which can continue
to replace themselves or differentiate into distinct cell types. These
are referred to as stem cells.
There are two types of stem cells: embryonic stem cells (found in
embryos) and adult stem cells (found in mature tissues such as bone
marrow).
Stem cells provide an ability for growth, repair, replacement and
regeneration.
Stem cells can be potentially used as therapeutic agents, but there
are technical and ethical issues to be considered.
Comparison of
embryonic and adult stem cells
Cell death – Apoptosis
• Programmed cell death, or apoptosis, is a normal part of the
life of cells.
• Cell death occurs when the cell membrane shrinks, DNA
fragments and lysosomes empty their contents into the cell
causing the cellular components to be broken down.
• The dead cell is then consumed by phagocytes.
• Cell death is important for:
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Developmental changes in growing embryos
Ridding tissues of old, infected or damaged cells
Removing immune cells which attack “self”
Removing cells which have sustained DNA damage so that they
do not continue to reproduce and form cancers
• Too much apoptosis can also have serious consequences –
apoptosis is a major factor in Alzheimer’s disease
Apoptosis