MEIOSIS AND SEXUAL LIFE CYCLES
After reading this chapter and attending lecture, the student should be able to:
| Distinguish between asexual and sexual reproduction.
|Diagram the human life cycle and indicate where in the human
body that mitosis and meiosis occur.
|Distinguish between mitotic and meiotic cell division.
|List the phases of meiosis I and meiosis II and describe the
events characteristic of each phase.
|Recognize the phases of meiosis from diagrams or micrographs.
|Describe the process of synapsis during prophase I and
explain how genetic recombination occurs.
|Describe key differences between mitosis and meiosis.
|Explain how the end result of meiosis differs from that of
|Distinguish between mitotic interphase and meiotic
|Explain how independent assortment, crossing over and random
fertilization contribute to genetic variation in sexually reproducing organisms.
|Explain why inheritable variation was crucial to Darwin's
theory of evolution.
- Reproduction is an emergent property associated with life. The fact that organisms
reproduce their own kind is a consequence of heredity.
- Heredity = Continuity of biological traits from one generation to the next.
- Results from the transmission of hereditary units, or genes, from parents to
- Because they share similar genes, offspring more closely resemble their parents or close
relatives than unrelated individuals of the same species.
- Variation = Inherited differences among individuals of the same species.
- Though offspring resemble their parents and siblings, they also diverge somewhat as a
consequence of inherited differences among them.
- The development of genetics in this century has increased our understanding about
the mechanisms of variation and heredity.
- Genetics = The scientific study of heredity and variation.
I. Offspring acquire genes from parents by inheriting chromosomes
- DNA = Type of nucleic acid that is a polymer of four different kinds of nucleotides.
- Genes = Units of hereditary information that are made of DNA and are located on chromosomes.
- Have specific sequences of nucleotides, the monomers of DNA.
- Most genes program cells to synthesize specific proteins; the action of these proteins
produce an organism's inherited traits.
- Inheritance is possible because:
- DNA is precisely replicated producing copies of genes that can be passed along from
parents to offspring.
- Sperm and ova carrying each parent's genes are combined in the nucleus of the fertilized
- The actual transmission of genes from parents to offspring depends on the behavior of chromosomes.
- Chromosomes = Threadlike structures in eukaryotic nuclei that are made of DNA and
- Consist of a single long DNA molecule that is highly folded and coiled along with
- Contain genetic information arranged in a linear sequence.
- Contain hundreds or thousands of genes, each of which is a specific region of the DNA
molecule, or locus.
- Locus = Specific location on a chromosome that contains a gene.
- Each species has a characteristic chromosome number; humans have 46.
II. Like begets like, more or less: a comparison of asexual versus sexual reproduction
|Single individual is the sole parent.
||Two parents give rise to offspring.
|Single parent passes on all its genes to its
||Each parent passes on half its genes, to its
|Offspring are genetically identical to the parent.
||Offspring have a unique combination of genes inherited
from both parents.
|Results in a clone, or genetically identical
individual. Rarely, genetic differences occur as a result of mutation, a change in
||Results in greater genetic variation; offspring vary
genetically from their siblings and parents.
III. Fertilization and meiosis alternate in sexual life cycles: an overview
- The human life cycle follows the same basic pattern found
in all sexually reproducing organisms; meiosis and fertilization result in
alternation between the haploid and diploid condition.
- Life cycle = Sequence of stages in an organism's reproductive history, from conception
to production of its own offspring.
- Somatic cell = Any cell other than a sperm or egg cell.
- Human somatic cells contain 46 chromosomes distinguishable by differences in size,
position of the centromere, and staining or banding pattern.
- Using these criteria, chromosomes from a photomicrograph can be matched into homologous
pairs and arranged in a standard sequence to produce a karyotype.
- Karyotype = A display or photomicrograph of an individual's somatic-cell metaphase
chromosomes that are arranged in a standard sequence. (See Campbell, Methods Box: Preparation
of a Karyotype)
- Human karyotypes are often made with lymphocytes.
- Can be used to screen for chromosomal abnormalities.
- Homologous chromosomes (homologues) = A pair of chromosomes
that have the same size, centromere position and staining pattern.
- With one exception, homologues carry the same genetic loci.
- Homologous autosomes carry the same genetic loci; however, human sex
chromosomes carry different loci even though they pair during prophase of Meiosis I.
- Autosome = A chromosome that is not a sex chromosome.
- Sex chromosome = Dissimilar chromosomes that determine an individual's sex.
- Females have a homologous pair of X chromosomes.
- Males have one X and one Y chromosome.
- Thus, humans have 22 pairs of autosomes and 1 pair of sex chromosomes.
- Chromosomal pairs in the human karyotype are a result of our sexual origins.
- One homologue is inherited from each parent.
- Thus, the 46 somatic-cell chromosomes are actually two sets of 23 chromosomes; one a
maternal set and the other a paternal set.
- Somatic cells in humans and most other animals are diploid.
- Diploid = Condition in which cells contain two sets of chromosomes; abbreviated as 2n.
- Haploid = Condition in which cells contain one set of chromosomes; it is the chromosome
number of gametes and is abbreviated as n.
- Gamete = A haploid reproductive cell.
- Sperm cells and ova are gametes, and they differ from somatic cells in their
chromosome number. Gametes only have one set of chromosomes.
- Human gametes contain a single set of 22 autosomes and one sex chromosome (either an X
or a Y).
- Thus, the haploid number of humans is 23.
- The diploid number is restored when two haploid gametes unite in the process of fertilization. Sexual intercourse allows a haploid sperm
cell from the father to reach and fuse with an ovum from the mother.
- Fertilization = The union of two gametes to form a zygote.
- Zygote = A diploid cell that results from the union of two haploid gametes.
- Contains the maternal and parental haploid sets of chromosomes from the gametes and is
- As humans develop from a zygote to a sexually mature adult, the zygote's genetic
information is passed with precision to all somatic cells by mitosis.
- Gametes are the only cells in the body that are not produced by mitosis.
- Gametes are produced in the ovaries or testes by the process of meiosis.
- Meiosis is a special type of cell division that produces haploid cells and
compensates for the doubling of chromosome number that occurs at fertilization.
- Meiosis in humans produces sperm cells and ova which contain 23 chromosomes.
- When fertilization occurs, the diploid condition (2n=46) is restored in the
IV. The Variety of Sexual Life Cycles
Alternation of meiosis and fertilization is common to all sexually reproducing
organisms; however, the timing of these two events in the life cycle varies among species.
There are three basic patterns of sexual life cycles:
- Animal: In animals, including humans, gametes are the only haploid cells.
- Meiosis occurs during gamete production. The resulting gametes undergo no further cell
division before fertilization.
- Fertilization produces a diploid zygote that divides by mitosis to produce a diploid
- Fungi and Some Protists: In many fungi and some protists, the only diploid stage is
- Meiosis occurs immediately after the zygote forms.
- Resulting haploid cells divide by mitosis to produce a haploid multicellular
- Gametes are produced by mitosis from the already haploid organism.
- Plants and Some Algae: Plants and some species of algae alternate between
multicellular haploid and diploid generations.
- This type of life cycle is called an alternation of generations.
- The multicellular diploid stage is called a sporophyte, or spore-producing plant.
Meiosis in this stage produces haploid cells called spores.
- Haploid spores divide mitotically to generate a multicellular haploid stage called a gametophyte,
or gamete-producing plant.
- Haploid gametophytes produce gametes by mitosis.
- Fertilization produces a diploid zygote which develops into the next sporophyte
V. Meiosis reduces chromosome number from diploid to haploid: a closer look
- Meiosis and sexual reproduction significantly contribute to genetic variation among
- Meiosis includes steps that closely resemble corresponding steps in mitosis.
- Like mitosis, meiosis is preceded by replication of the chromosomes.
- Meiosis differs from mitosis in that this single replication is followed by two
consecutive cell divisions: Meiosis I and Meiosis II.
- These cell divisions produce four daughter cells instead of two as in mitosis.
- The resulting daughter cells have half the number of chromosomes as the original
cell; whereas, daughter cells of mitosis have the same number of chromosomes as the parent
A. The Stages of Meiotic Cell Division
- Interphase: Interphase precedes meiosis.
- Preparatory stage to the actual division.
- Chromosomes replicate as in mitosis.
- Each duplicated chromosome consists of two identical sister chromatids attached at their
- Centriole pairs in animal cells also replicate into two pairs.
Meiosis I: This cell division segregates the two chromosomes of
each homologous pair and reduces the chromosome number by one-half. It includes the
following four phases:
- Prophase I. This is a longer and more complex process than
prophase of mitosis.
- Chromosomes condense.
- Synapsis occurs. During this process, homologous chromosomes come together as pairs.
- Chromosomes condense further until they are distinct structures that can be seen with a
microscope. Since each chromosome has two chromatids, each homologous pair in synapsis
appears as a complex of four chromatids or a tetrad.
- In each tetrad, sister chromatids of the same chromosome are attached at their
centromeres. Nonsister chromatids are linked by X-shaped chiasmata, sites where
homologous strand exchange or crossing-over occurs.
- Chromosomes thicken further and detach from the nuclear envelope.
- As prophase I continues, the cell prepares for nuclear division.
- Centriole pairs move apart and spindle microtubules form between them.
- Nuclear envelope and nucleoli disperse.
- Chromosomes begin moving to the metaphase plate, midway between the two poles of the
- Prophase I typically occupies more than 90% of the time required for meiosis.
- Metaphase I.
- Tetrads are aligned on the metaphase plate.
- Each synaptic pair is aligned so that centromeres of homologues point towards opposite
- Each homologue is thus attached to kinetochore microtubules emerging from the pole it
faces, so that the two homologues are destined to separate in anaphase and move towards
- Anaphase I.
- Homologues separate and are moved towards the poles by the spindle apparatus.
- Sister chromatids remain attached at their centromeres and move as a unit towards the
same pole, while the homologue moves towards the opposite pole.
- This differs from mitosis during which chromosomes line up individually on the metaphase
plate (rather than in pairs) and sister chromatids are moved apart towards opposite poles
of the cell.
- Telophase I and Cytokinesis.
- The spindle apparatus continues to separate homologous chromosome pairs until the
chromosomes reach the poles.
- Each pole now has a haploid set of chromosomes that are each still composed of two
sister chromatids attached at the centromere.
- Usually, cytokinesis occurs simultaneously with Telophase I, forming two haploid
daughter cells. Cleavage furrows form in animal cells, and cell plates form in
- In some species, nuclear membranes and nucleoli reappear, and the cell enters a period
of interkinesis before meiosis II. In other species, the daughter cells immediately
prepare for meiosis II.
- Regardless of whether a cell enters interkinesis, no DNA replication occurs before
Meiosis II: This second meiotic division separates sister
chromatids of each chromosome.
- Prophase II.
- If the cell entered interkinesis, the nuclear envelope and nucleoli disperse.
- Spindle apparatus forms and replicated chromosomes move towards the metaphase II plate.
- Metaphase II.
- Replicated Chromosomes align singly on the metaphase plate.
- Kinetochores of sister chromatids point towards opposite poles.
- Anaphase II.
- Sister chromatids separate.
- Centromeres of sister chromatids separate.
- Sister chromatids of each pair (now individual chromosomes) move toward opposite poles
of the cell.
- Telophase II and Cytokinesis.
- Nuclei form at opposite poles of the cell.
- Cytokinesis occurs producing four haploid daughter cells.
B. A Comparison of Mitosis and Meiosis
- Though the processes of mitosis and meiosis are similar in some ways, there are some key
- Meiosis is a reduction division. Cells produced by mitosis have the same number of
chromosomes as the original cell, whereas cells produced by meiosis have half the number
of chromosomes as the parent cell.
- Meiosis creates genetic variation. Mitosis produces two daughter cells
genetically identical to the parent cell and to each other. Meiosis produces four
daughter cells genetically different from the parent cell and from each other.
- Meiosis is two successive nuclear divisions. Mitosis, on the other hand, is
characterized by just one nuclear division.
| COMPARISON OF MEIOSIS I AND MITOSIS
||Synapsis occurs to form tetrads. Chiasmata
appear as evidence that crossing over has occurred.
||Neither synapsis nor crossing over occurs.
||Homologous pairs (tetrads) align on the metaphase plate.
||Individual chromosomes align on the metaphase plate.
||Meiosis I separates pairs of chromosomes. Centromeres do not
divide and sister chromatids stay together. Sister chromatids of each chromosome move to
the same pole of the cell; only the homologues separate.
||Mitosis separates sister chromatids of individual
Centromeres divide and sister chromatids move to opposite poles of
Meiosis II is virtually identical in mechanism to mitosis, separating sister
chromatids, except that cells in meiosis two are haploid (possess a single set of
V. Sexual life cycles produce genetic variation among offspring
- Meiosis and fertilization are the primary sources of genetic variation in sexually
reproducing organisms. Sexual reproduction provides genetic variation by:
- independent assortment
- crossing over during prophase I of meiosis
- random fusion of gametes during fertilization
A. Independent Assortment of Chromosomes
- At Metaphase I, each homologous pair of chromosomes aligns on the metaphase plate. Each
pair consists of one maternal and one paternal chromosome.
- The orientation of the homologous pair to the poles is random, so there is a fifty-fifty
chance that a particular daughter cell produced by meiosis I will receive the maternal
chromosome of a homologous pair, and a fifty-fifty chance that it will receive the
- Each homologous pair of chromosomes orients independently of the other pairs at
metaphase I; thus, the first meiotic division results in independent assortment of
maternal and paternal chromosomes. (See Campbell, Figure 12.8)
- A gamete produced by meiosis contains just one of all the possible combinations of
maternal and paternal chromosomes.
- Independent assortment = The random distribution of maternal and paternal homologues to
the gametes. (In a more specific sense, assortment refers to the random distribution of
genes located on different chromosomes.)
- Since each homologous pair assorts independently from all the others, the process
produces 2n possible combinations of maternal and paternal chromosomes
in gametes, where n is the haploid number.
- In humans, the possible combinations would be 223, or about eight million.
- Thus, each human gamete contains one of eight million possible assortments of
chromosomes inherited from that person's mother and father.
- Genetic variation results from this reshuffling of chromosomes, because the maternal and
paternal homologues will carry different genetic information at many of their
B. Crossing Over
- Another mechanism that increases genetic variation is the process of crossing over, during which homologous chromosomes exchange genes.
- Crossing over = The exchange of genetic material between homologues; occurs during
prophase of meiosis I. This process:
- Occurs when homologous portions of two nonsister chromatids trade places. During
prophase I, X-shaped chiasmata become visible at places where this homologous
strand exchange occurs.
- Produces chromosomes that contain genes from both parents and
result in different combinations of genes in the gametes.
- In humans, there is an average of two or three crossovers per chromosome pair.
- Synapsis during prophase I is precise, so that homologues align gene by gene. The exact
mechanism of synapsis is still unknown, but involves the formation of the synaptonemal
complex, a protein structure that brings the chromosomes into close association.
C. Random Fertilization
- Random fertilization is another source of genetic variation in offspring.
- In predicting the probability of the occurrence of a random event, that is the result of
the occurrence of separate events, you multiply the probability of the occurrence of the
separate events together.
- In humans, the probability that an egg cell (which is one of eight million different
possibilities) will be fertilized by a sperm cell (which is also one of eight million
different possibilities) is over 64 trillion.
- Thus, any one-child produce by human parents is one genetic possibility in over 64
trillion different possible genetic combinations.
- Genetically you are unique, except if you have an identical twin.
VI. Evolutionary adaptation depends on a population's genetic variation
- Inheritable variation is the basis for Charles Darwin's theory that natural selection is
the mechanism for evolutionary change. Natural selection:
- Increases the frequency of inheritable variations that favor the reproductive success of
some individuals over others.
- Results in adaptation, the accumulation of inheritable variations that are
favored by the environment.
- In the face of environmental change, genetic variation increases the likelihood that
some individuals in a population will have inheritable variations that help them cope with
the new conditions.
- There are two sources of genetic variation:
- Sexual reproduction: independent assortment in meiosis I, crossing over in prophase of
meiosis I, and random fusion of gametes during fertilization.
- Mutation, which is rare structural change in a gene.
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