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Chapter 9: Sexual reproduction and meiosis
Life Science 141
Dr Stephen Boatwright
Department of Biodiversity and Conservation Biology
Why sex?
• in asexual reproduction an organism simply copies its DNA and splits the contents of
one cell into two and the offspring are virtually identical
• in sexual reproduction two parents are required
• male contributes the sperm cells, one of which fertilizes the female’s egg cell to begin
the next generation
• earliest form of sexual reproduction is conjunction one bacterial cell uses an
outgrowth called a sex pilus to transfer genetic material to another bacterium
Binary Fission of Prokaryotes
Diploid cells contain two homologous sets of chromosomes
• a sexually reproducing organism consists of diploid cells (2n) which contain two full
sets of chromosomes, one set inherited from each parent
• a karyotype is an organized chart of all the chromosomes in a cell
• of the 23 pairs of chromosome pairs in a human cell, 22 pairs are autosomes or
chromosomes that are the same for both sexes
• remaining pair is made up of the two sex chromosomes which determine whether an
individual is female or male
• females have two X chromosomes whereas males have on X and one Y chromosome
• a homologous pair of chromosomes is a matching pair of chromosomes that look alike
and have the same sequence of genes
• homologous chromosomes, however, are not identical and the two homologs differ in
the combination of alleles or versions of the genes they carry
Homologous Chromosomes
Meiosis is essential in sexual reproduction
• in the case of asexual reproduction genetic material (on the chromosomes) from the
one parent is passed on from one generation to another
• if no mutation, change in the genetic information, occurs it results in genetically
identical copies of the parent cell being formed
• in sexual reproduction two parents contribute to the genetic make-up of the offspring
• gametes are formed through the process of meiosis - each gamete contains one gene
for each characteristic from the parent
• genes are units of information for a specific trait and they are found at specific
locations, called loci (singular = locus), on a chromosome
• the expression of a specific trait is determined by the two units of information received
from the parents; one unit from each parent
• we refer to each unit as an allele – alternate forms of a gene
• when two gametes combine (fertilization) the resulting zygote again contains two
alleles for each characteristic
• due to slight changes in the molecular structure of each gene (mutations), differences
(variation) are observed in the offspring
The Human Life Cycle
Overview of Meiosis
The process of meiosis
• in the case of sexual reproduction the chromosome number of each gamete must be
half (haploid) that of the parent cell (diploid) in order for the zygote to be diploid again
• if this did not happen then each subsequent generation would have had double the
chromosome number of the immediate parents
• diploid cells normally receive one set of genes from one parent and the other set (for
the same trait) from the other parent; we thus have two sets of chromosomes being of
the same length and shape which are referred to as homologous pairs
• in order to have the reduction of the chromosome number, the process of meiosis
involves two nuclear divisions (Meiosis I and Meiosis II)
• in animals meiosis only occurs in primary and secondary gametocytes; and in
higher plants where we have an alternation of generations, meiosis takes place only in
the spore-mother cells of the sporophyte generation
• in a nutshell we can say that meiosis involves three steps in terms of chromosome
number, i.e. doubling, halving and halving again
In meiosis, DNA replicates once but the nucleus divides twice
Interphase: Duplication of chromosomes takes place just before prophase I, and each
duplicated chromosome (sister chromatids) remains attached at the centromere
Prophase I: As in prophase of mitosis similar behavior of nuclear membrane,
nucleolus, centriole and spindle. Duplicated chromosomes are visible as condensed
threads, and each duplicated form draws close to its homologue through process of
synapsis. Crossing-over occurs between nonsister chromatids which results in
exchange of genetic material - genetic variability is increased
Metaphase I: Here we have the alignment of chromosomes at the spindle equator,
also known as the metaphase plate. The spindle fibers are fully formed and will be
attached to the kinetochores of the chromosomes. Random alignment of paternal and
maternal chromosomes results in random distribution of paternal and maternal traits in
offspring, i.e. the paternal and maternal chromosomes of the bivalents can be on either
side of the metaphase plate resulting in a mixture of paternal and maternal
chromosomes going to each daughter cell
Anaphase I: The contraction of the spindle fibers leads to the separation of the
homologous chromosome-pairs with attached sister-chromatids/homologues being
pulled towards the poles. Thus, in humans each pole will receive 23 duplicated
chromosomes
In meiosis, DNA replicates once but the nucleus divides twice
Telophase I: Cytokinesis leads to the formation of two daughter cells, each with 23
duplicated chromosomes (46 chromatids)
Interkinesis: A new spindle apparatus is formed
Prophase II: Duplicated chromosomes (sister chromatids) attach to the spindle fibers
Metaphase II: Duplicated chromosomes are arranged at the metaphase plate and this is
assisted by the movement of the spindle fibers
Anaphase II: Contraction of spindle fibers leads to separation of sister chromatids, now
referred to as daughter chromosomes, and their movement to opposite poles, i.e. 23
daughter chromosomes move to each pole
Telophase II: Four daughter nuclei are formed and after cytokinesis with each cell
containing half the number of chromosomes, i.e. 23, of the parent cell in humans
Meiosis I separates maternal from paternal chromosomes, and Meiosis II
separates sister chromatids
Meiosis I & II in Plant Cells
Crossing-over, independent assortment and genetic variation
• crossing over is a process in which two homologous chromosomes exchange genetic
material
• during prophase I the duplicated homologues are aligned next to one another to form a
bivalent – synapsis
• nonsister chromatids (one chromatid from the one duplicated homologue and one
from the other duplicated homologue) overlap with one another at different sites
• point of crossing is referred to as the chiasma (chiasmata, plural) and this is where
genetic material of paternal chromosome is exchanged with that of the maternal one
• when the bivalents separate during anaphase I the two daughter cells will receive
these chromatids containing different genetic make-ups and this will lead to genetic
variation in the offspring
• alignment of chromosomes in metaphase I is a random process and all possible
combinations are equally probable
• we thus find that each daughter cell can receive a mixture of paternal and maternal
homologues further contributing in genetic variation
• the number of possible arrangements is related to the number of chromosomes
according to the formula 2n (where n is the haploid number)
• for two pairs of homologs, each resulting gamete may have any of four (22) unique
chromosome configurations
• in humans with 23 chromosome pairs each gamete contains one of 8,388,608 (223)
possible chromosome combinations, all equally likely
Crossing-over, independent assortment and genetic variation
• in one mating, any women’s 8,388,608 possible egg cells can combine with any of the
8,388,608 possible sperm cells of a partner
• one couple could therefore theoretically create more than 70 trillion (8,388,6082)
genetically unique individuals
• this is an underestimate as it does not take into account the additional variation from
crossing over
• with so much variability the chances of two parents producing genetically identical
offspring seems exceedingly small
• identical twins result from a single fertilization event
• the resulting zygote or embryo splits in half, creating separate, identical babies. They
are therefore called monozygotic
• in contrast non-identical (fraternal) twins occur when two sperm cells fertilize two
separate egg cells and they are called dizygotic
Crossing Over
Independent Assortment
Meiosis Compared to Mitosis
• both are mechanisms that divide a eukaryotic cell’s genetic material
• mitosis – somatic cells; Meiosis – germ cells
• meiosis – genetic variation through crossing-over and independent assortment
Meiosis I Compared to Mitosis
Meiosis II Compared to Mitosis
Errors sometimes occur in meiosis
Polyploidy means extra chromosome sets
Polyploid cells have one or more extra sets of chromosomes
Non-disjunction results in extra or missing chromosomes
Non-disjunction is the failure of chromosomes to separate in meiosis and it causes
gametes to have incorrect chromosome numbers. A sex chromosome abnormality is
typically less severe than an incorrect number of autosomes
Smaller-scale chromosome abnormalities also occur
Chromosomal rearrangements can delete or duplicate genes. An inversion flips gene
order possibly disrupting vital genes. In a translocation two nonhomologs exchange
parts. Some translocations can cause cancer
Haploid nuclei are packaged into gametes
Plants:
Many plant species (e.g. pine trees, roses, etc.) produce spores, which are haploid
cells. These spores are often adapted to resist unfavourable conditions, and under
favourable conditions they germinate to form gametophytes which will form the gametes
Animals:
Gametes formed through process of gametogenesis
Male (spermatogenesis: diploid germ cell → spermatocyte → four haploid daughter
cells → permatids → sperm.
Female (oogenesis): diploid germ cell → oocyte → (after Meiosis I) secondary oocyte
(gets nearly all the cytoplasm) + first polar body (small) → (after Meiosis II) gamete
(one cell that gets all the cytoplasm). Three polar bodies ultimately disintegrates
Gametogenesis in Mammals