Lecture 14:
Meiosis: Sexual Reproduction
Sex is Very Costly
- Large amounts of energy required to find a mate and do the mating:
specialized structures and behavior required
- Intimate contact provides route for infection by parasites (AIDS, syphillis,
etc.)
- Genetic costs: in sex you pass on only half your genes to your children
- Males are an expensive luxury- in most species they contribute little
to rearing offspring
But There are Some Advantages
- More genetic diversity: more potential for survival of species when
environmental conditions change
- Shuffling of genes in meiosis (random separation of homologous chromosomes)
- Crossing-over in meiosis (see below)
- Fertilization (genes from 2 individuals brought together)
- DNA back-up and repair
- Asexual organisms don't have back-up copies of genes, sexual organisms
have 2 sets of chromosomes and one can act as a back-up if the other is
damaged
- Sexual mechanisms , especially recombination, are used to repair damaged
DNA- the undamged chromosome acts as a template and eventually both chromosomes
end up with the correct gene
Reproduction Without Sex and Sex Without
Reproduction Both Occur in Nature
- Sex is the transfer of genes from one cell to another and in microorganisms
this often occurs without cell division, so that there is no reproduction
- Bacteria can transfer genetic material through projections called pili
- Many species can reproduce without sex
- Most single-cell organisms (bacteria, protista)
- Some multicellular organisms (i.e., whiptail lizards, hydra, sea anenomes,
aphids)
Terminology of Meiosis
- Review of diploid & haploid
- Haploid = single set of chromosomes
- Diploid = double set of chromosomes (one from each parent)
- Homologous chromosomes: Pairs of chromosomes having the same genes.
Diploid cells have homologous chromosomes (one from each parent).
- Sister chromatids: a chromosome and its identical copy (formed by DNA
duplication), still held together by centromeres.
In Meiosis 2 Cell Divisions Cut the Number
of Chromosomes in Half
- Meiosis is designed for sexual reproduction
- Parents have 2 sets of chromosomes in their somatic cells; they are
diploid
- Sperms and eggs must have only 1 set; they must be haploid
- When a sperm and egg combine in fertilization they create a new individual
with 2 sets of chromosomes (diploid)
- Meiosis creates the sperm and egg cells by reducing the number of chromosomes
to half (1 set)
- Starts with DNA duplication in 1st interphase
- No DNA duplication in 2nd interphase
- Review diagram of cell DNA in meiosis
The First Cell Division Separates Homologous
Chromosome
- In 1st prophase and prometaphase the homologous chromosomes come together
(synapsis) and attach to the spindle together through a protein
complex
- The association of the 2 homologous chromosomes is sometimes called
a tetrad
- In the first meiotic division the centromeres remain attached and the
sister chromatids stay together
- The attachment between the homologous chromosomes breaks down and the
2 homologues are pulled apart
- This gives 2 cells, each with 1 set of duplicated chromosomes (the
cells are considered haploid)
- The stages of cell division are the same as those in mitosis (prophase,
prometaphase, metaphase, anaphase, telophase) except that the prophase/prometaphase
period is more complex due to the association of the homologues
- At the end of the first meiotic division the cell does not go back
into a long interphase
- There is no DNA duplication before the 2nd meiotic division
- In some species the nuclear membrane reforms between divisions; in
other species it does not
The Second Cell Division Separates Sister
Chromatids
- The 2nd meiotic division is like mitosis
- The chromosomes line up at metaphase with sister chromatids attached
to spindle fibers going in the opposite directions
- At anaphase the centromere attachments break and the sister chromatids
are pulled to opposite sides of the cell
- At the end of 2nd meiotic division there will be 4 haploid cells (sperms
or eggs)
Synapsis and Crossing-Over Occur in Meiosis
- In the 1st meiotic division the homologous chromosomes attach to one
another by a protein complex
- This brings chromatids from the 2 homologues into contact with each
other and sometimes they cross
- Cross-overs help to hold homologous chromosomes together
- At the cross-over the 2 chromatids can exchange tips (recombination)
- The exchange is catalyzed by enzymes
- Promotes genetic diversity
- Homologous chromosomes can have more than 1 cross-over
- The cross-over process is unbelievably precise
- Average human chromosome has about 100 million DNA base pairs
- In cross-overs neither chromosome can gain or lose a single base pair
Meiosis Can be Interupted for Long Periods
of Time in Females
- When meiosis is used to produce egg cells the process is often interupted
for long periods (up to a year or more)
- Vertebrate females produce all of their potential egg cells (primary
oocytes) before birth
- These cells have started meiosis, but the process has been arrested
in the first prophase of meiosis
- The homologues are paired, with crossing-over of their chromatids
- In humans the arrested state lasts for about 15 to 50 years
- Chromosomes partly unwind (decondense) in the arrested state and the
potential egg cell matures
- Later in development, under the influence of hormones, some of the
primary oocytes finish the first cell division and start the second
- First cell division is very uneven: the smaller of the 2 cells produced
becomes a "polar body" and dies
- The oocyte (now called a secondary oocyte) arrests division a second
time, usually at the 2nd metaphase of meiosis
- At ovulation the secondary oocyte is released from the ovary
- If it is fertilized it finishes the 2nd meiotic division; otherwise
it dies
- Again a small polar body is produced and is discarded
- The remaining egg nucleus fuses with the sperm nucleus and development
of a new individual begins
- Your text has a diagram of egg development on p. 949
Some Species Alternate Haploid and Diploid
Generations
- Animals are primarily diploid, with haploid sperms and eggs
- Many plants have long-lasting haploid structures, alternating with
diploid structures (alternation of generations)
- Examples:
- In mosses the haploid plant is the dominant structure that you see;
the diploid sporophytes can be seen rising on stalks above the main plant
(figure on p. 534 of text)
- In ferns, conifers and flowering plants the most conspicuous structure
is diploid; the fern has a tiny haploid gametophyte and in conifers and
flowering plants only the pollen, egg and a few cells surrounding the egg
are haploid (see figures on p. 559, 563 and 566 of text)
Genetic Defects Can be Caused by Accidents
in Chromosome Separation
- Sometimes mistakes are made in meiosis and one cell gets an extra chromosome
and the partner cell gets one less
Comparison of Mitosis and Meiosis
- Things common to both meiosis & mitosis:
- Unique events and results of mitosis:
- Produces 2 identical cells
- Does not promote genetic variability
- Used for growth and repair (reproduction in single cell organisms)
- A single cell division
- Separates sister chromatids
- Centromeres divide at anaphase
- Unique events & results of meiosis:
- Produces 4 haploid cells (with half the original number of chromosomes)
- Promotes genetic diversity
- Used for sexual reproduction (produces sperms and eggs)
- Two cell divisions
- First division separates homologous chromosomes
- Second division separates sister chromatids
- Centromeres divide at 2nd anaphase but not at 1st
- No duplication of DNA between 1st and 2nd divisions
- Homologous chromosomes come together (synapsis) in 1st division
- Crossing-over between homologous chromosomes common
Top of Page
Lecture Note Index
/ Homepage/ Next Lecture