Meiosis and Sexual Life Cycles

Chapter 13



•    Asexual v. Sexual Reproduction

•    Life Cycles

•    Meiosis

•    Genetic Variation Through Sexual Reproduction




•      Parents pass on coded information to their of spring that help determine the physical and behavioral traits their off spring will have.

–   Genes

•      Genes are segments of DNA

•      When genes are turned on, particular proteins are made.  The activity of these proteins on the cellular level determines the traits an individual will have.



The Relationship of a Gene to a Chromosome



Asexual vs. sexual reproduction

•     Cell reproduction passes on genes from cell to cell


•  Asexual reproduction


•  Sexual reproduction




Asexual reproduction


•          Mitosis in eukaryotes (Binary fission in prokaryotes)

•          Uses fewer resources  - more rapid. 

•          Genetically identical daughter cells




Sexual reproduction

•   Requires a special type of cell division – meiosis
•   Produces genetic variation in a population
•   More genetic ‘raw material’!
•   Requires more resources at the cellular and the organismal level.



Gametes & the sexual life cycle

•     Life cycle means the sequence of stages leading from the adults of one generation to the adults of the next


•     Humans are diploid (46=2n) organisms since our somatic cells contain two homologous sets of chromosomes


•     The exceptions to this are the egg and the sperm cells, collectively known as gametes



Figure 13.4  The human life cycle



Gametes & the sexual life cycle

•      Gametes have a single set of chromosomes: 22 autosomes plus a single sex chromosome (X in women and X or Y in Men)

•      The gametes are haploid since they only have one set of chromosomes (23=1n)

•      In humans sexual intercourse brings together the haploid gametes

•      Fertilization produces a diploid Zygote with 2 homologous sets of chromosomes, one set from each parent

•      Producing haploid gametes by meiosis prevents the chromosome number doubling in every generation



Figure 13.5  Three sexual life cycles differing in the timing of meiosis and fertilization (syngamy)



Cell Division - Mitosis

•    When the cell divides the sister chromatids separate and go to separate daughter cells

Cell Division - Mitosis

•      During S phase of the cell cycle, the cell duplicates all of its chromosomes

–   DNA is replicated

–   New protein molecules attach as needed

•      This results in two new copies of chromosomes

•      These copies of the original chromosomes are called sister chromatids which are tightly bound at the centromere

•      The sister chromatids are identical



Meiosis - Sexual Reproduction

•      Offspring are not identical as they inherit a unique combination of genes (genotype) from their parents.


•      This unique combination of genes give rise to a unique set of visible traits (phenotype).


•      A special type of cell division, meiosis, produces egg and sperm that have just one set of genes, and have genetic variability.


•      Sperm and egg fuse to form offspring that are genetically unique from the parents and each other.

Meiosis - Sexual Reproduction

•      A karyotype is a ‘picture’ of the chromosomes in a somatic cell (body cell), as they enter mitosis

•      From these karyotypes we can see that almost every duplicated chromosome has a twin - homologous chromosomes

•      These twinned or homologous chromosomes carry the same sequence of genes that control the same inherited characteristics

•      Unlike sister chromatids produced in mitosis they are not identical but can have different versions of the same gene



A Human Karyotype


Figure 13.3  Preparation of a human karyotype



Meiosis - Sexual Reproduction

•     Humans have 23 pairs of homologous chromosomes (46 chromosomes)

•     Women and men both have 22 autosomes but differ in the 23rd pair known as the sex chromosomes – X & Y

•     Women have XX and Men XY

•     For both autosomes and sex chromosomes we inherit one chromosome from each pair from our mother and one from our father



A Human Karyotype


Figure 13.6  Overview of meiosis: how meiosis reduces chromosome number



Figure 13.7  The stages of meiotic cell division: Meiosis I



Figure 13.7  The stages of meiotic cell division: Meiosis II



Figure 13.8  A comparison of mitosis and meiosis



Figure 13.8  A comparison of mitosis and meiosis: summary


Figure 13.9  The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I


Figure 13.10  The results of crossing over during meiosis



The Origins of Genetic Variation

•     Offspring of sexual reproduction are highly varied and this forms the basis for natural selection

•     There are three process that occur during sexual reproduction the produce genetic variation.

–   Crossing over

–   Independent Assortment.

–   Random Fertilization



Crossing Over


The exchange of corresponding segments between 2 homologous chromosomes adds even more genetic diversity to gametes




Independent Assortment

•   Orientation of the homologous pairs during meiosis I is by chance


•   For any species, possible chromosomal combinations are 2n (e.g. Human gametes 223 or 8 million combinations)



Random Fertilization

•      from independent assortment alone, a human zygote represents one of 64 trillion possible combinations of the parental chromosomes. 

•      So, if humans were self fertilizing, such as occurs in hermaphroditic round worms, there would 64 trillion possible genetic make-ups of offspring.

•      However, in humans, the sperm and egg come from two genetically different individuals, so there is even greater genetic variation.



Compare and contrast sexual vs. asexual reproduction?

•      Which provides more genetic raw material for evolution?

•      Which uses more energy and resources?



The End