4.2 – Sexual Reproduction
(Taken from Biology 11, McGraw-Hill)
sexual reproduction:
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reproduction that requires two parents and produces genetically
distinct offspring
Sexual reproduction involves the joining of male and female reproductive cells called
gametes (sperm and egg).
The process of combining gametes is called fertilization and the cell that results from
fertilization is called a zygote.
Gametes are haploid meaning
that they contain half the number
of chromosomes as the parent
cell. Each human gamete is
haploid, with n=23.
After fertilization, the zygote cell
is diploid with a total of 2n
chromosomes – n chromosomes
from the female parent and n
from the male parent. The diploid
number in humans is 46.
Meiosis – Producing Haploid Gametes
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Meiosis has two key outcomes:
1) Genetic Reduction: it produces daughter cells with half the number of
chromosomes of the parent cell
2) Genetic Recombination: gametes have different combinations of alleles; offspring
are genetically different from each other and their parents; this increases genetic
variation
Meiosis I
Prophase I
- homologous chromosomes (1 maternal, 1
paternal) pair to make a tetrad (4 chromatids)
- homologous chromosomes contain different
genes (alleles) but code for the same trait
- crossing over occurs – non-sister chromatids
exchange genes
- individual chromosomes can contain some
maternal genes and some paternal genes
Metaphase I
- homologous chromosomes travel to equator of
cell
- have exchanged genetic information
Anaphase I
- each chromosome separates from its
homologue
- move to opposite poles due to spindle fibres
shortening
Telophase I
- cell completes division
- chromosomes are still double stranded
- cytoplasm divides during cytokinesis and results
in 2 haploid daughter cells
Meiosis II
Prophase II
- cells are now haploid
- centrioles in 2 new cells move to poles and new
spindle fibres form
- centromere attaches to spindle fibres
Metaphase II
- haploid number of chromosomes line up at the
equator
Anaphase II
- sister chromatids are pulled apart at the
centromere by spindle fibres
- chromosomes move towards opposite poles
Telophase II
- nuclear membrane and nuclei reform
- cytokinesis occurs, resulting in 4 haploid
daughter cells
Gamete Formation in Animals
Spermatogenesis
- The process of sperm production is called spermatogenesis.
- In most male animals, meiosis takes place in the testes.
- In spermatogenesis, four haploid sperm cells form from one diploid cell.
Oogenesis
- The process of egg production is called oogenesis.
- In most female animals, meiosis takes place in the ovaries.
- Meiosis I occurs for one cell each month beginning at puberty.
- Oogenesis involves an unequal division of cytoplasm. The cell that receives most of
the cytoplasm after the first division continues through meiosis I and II to form an
egg.
- The other, smaller cell formed is called a polar body.
- In oogenesis, one haploid mature egg cell and 3 polar bodies are formed from one
diploid cell.
The Importance of Meiosis for Genetic Variation
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The genetic variation occurs through three mechanisms:
1) Crossing over during prophase I exchanges genes on the chromosomes.
2) During metaphase I, the parental and maternal chromosomes are randomly
segregated to the poles. Depending on how the chromosomes line up, a number
of different combinations of chromosomes may be found in the gametes.
(Independent Assortment)
3) During fertilization, different combinations of chromosomes and genes combine
when gametes unite.
Errors During Meiosis
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There are two types of chromosomal errors that can occur during meiosis: changes in
chromosome structure and changes to chromosome number.
Errors Caused by Changes in Chromosome Structure
- Deletion: a piece of chromosome is deleted
- Duplication: a section of a chromosome appears two or more times in a row
- Inversion: a section of a chromosome is inverted
- Translocation: a segment of one chromosome becomes attached to a different
chromosome
- See Table 4.1, pg 177 for examples of Genetic Disorders
- Scientists have identified duplications, inversions, and translocations in individuals with
autism.
Errors Caused by Changes in Chromosome Number
- non-disjunction: the failure of homologous chromosome pairs or sister chromatids
to separate during meiosis (anaphase I or II)
- non-disjunction produces gametes that have too few or too many chromosomes
- Down syndrome is an example of non-disjunction where individuals are born with an
extra chromosome or an extra piece of chromosome 21.
- monosomy: the loss of a chromosome as a result of non-disjunction
- trisomy: the gain of an extra chromosome as the result of non-disjunction
- The most common trisomies are found in chromosomes 21, 18, 13, and the number of
sex chromosomes.
- See Table 4.2, pg 179 – Chromosomal Abnormalities in Humans
Parental Genetic Testing
- refers to tests performed on a fetus that are based on testing for genetic-based
abnormalities
- Some ethical issues related to prenatal genetic testing include abortions and potential
discrimination against persons with disabilities.
- Prenatal testing involves using non-invasive methods, such as blood tests and
ultrasound and invasive methods, such as amniocentesis and chorionic villus sampling.
HOMEWORK:
pg 181 #1-8, 12-14
Comparison of Mitosis and Meiosis