http://www.scienceclarified.com

Cellular Basis of Reproduction and InheritanceLecture 9, Meiosis

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Much of the text material in the lecture notes is from our textbook, “Essential Biology with Physiology” by Neil A. Campbell, Jane B.

Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship. Other sources were sometimes used, and are noted.

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Outline

• Karyotypes • Sexual lifecycle• Fertilization• In vitro fertilization• Meiosis • Genetic variation• Chromosomal disorders• Sexual differentiation• Polyploid organisms• Words and terms to know• Possible test items

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A Time of Wonder

http://nmhm.washingtondc.museum

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Sexual Reproduction

Last week we discussed asexual reproduction—this week we cover some of the aspects of sexual reproduction.

http://about.biology.com

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Homologous Chromosomes

• Almost all chromosomes have a ‘twin’ that matches in shape, size, and bands.

• The pair are known as homologous since each chromosome carries the same sequence of genes controlling an inherited characteristic.

• The genes for eye color, for example, are found at identical locations in homologous pairs.

• The instructions in each matching gene are different since they are inherited from each parent.

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Pairings

• In both sexes, one chromosome from each homologous pair is inherited from the mother and the father.

• The 46 chromosomes in a human female can be organized into an array of 23 homologous pairs.

• In the human male, one of the 46 pairs does not match—the unmatched pair is the male’s sex chromosomes.

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Karyotype

• A typical body cell in humans, called a somatic cell, has 46 chromosomes• If the cell is opened during mitosis, a micrograph of the chromosomes can

be made.• The individual chromosomes are arranged in an array called a karyotype.

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Unordered Chromosomes

http:www.biotechnologyonline.gov

Micrograph of chromosomes during mitosis.

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Karyotype

An ordered array of chromosomes.

http://www.ucl.ac.uk

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Sex Chromosomes and Autosomes

• This 23rd pair—the sex chromosomes—determines the genetic sex of the human.

• Genetic females usually, but not always, have two X chromosomes.• Genetic males usually, but not always, have X and Y chromosomes.• The remaining 22 pairs, common to both females and males, are the

autosomes.

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Sexual Life Cycle

• The sexual lifecycle is the sequence of biological stages from the adults of one generation to the adults of the next generation.

• Paired chromosomes—one inherited from each parent—are are found in all species that reproduce sexually.

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Diploid

• Humans, and most animals and plants, are diploid organisms because all body cells contain paired sets of homologous chromosomes.

• The number of pairs is represented by n (in humans, n = 23 ).• The number of chromosomes (46) is the diploid number, 2n.• Exceptions to this rule are egg and sperm cells, known as gametes.

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Haploid

• Gametes, formed by meiosis in ovaries and testes, contain one member of each homologous chromosome pair.

• Gametes are haploid since they contain one-half the number of chromo-somes found in body cells.

• The total number of chromosomes in human gametes (23) is the haploid number, n.

http://zoology.unh.edu

Spermatozoa(sperm)

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Fertilization

• A sperm cell fuses with an egg cell, known as an ovum, through the process of fertilization.

• Each gamete is haploid, and the fertilized egg, known as a zygote, is diploid

• The two sets of homologous chromosomes—one member of each pair is contributed by each parent.

• Mitotic cell division assures all body cells receive a complete copy of the 46 chromosomes.

• Every one of 60+ trillion cells in the human body can be traced to a single zygote.

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Alternating Stages

• The sexual lifecycles over generations involve alternation between diploid and haploid stages.

• The process of meiosis keeps the number of chromosomes from doubling each generation.

http://iep.water.ca

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Path of the Egg

Cilia help move the egg down the fallopian tube once it is released from one of the ovaries.

http://www.ehd.org

Ovary

Uterus

Fallopian tube

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Sperm

http://spermomax.net

Apparently healthy sperm

Abnormal shapes

http://www.cit.astate.edu

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Sperm and Egg

Many sperm are present, but only one can fertilize the egg.

http://nmhm.washingtondc.museum

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Two-Cell Stage

http://www.midwesttiv.com

The first day after fertilization—mitotic cell division has begun.

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Eight-Cell Stage

http://fig.cox.miami.edu

Three-day-old human embryo

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Embryonic Growth

http://library.thinkquest.org

Five-week-old human embryo

Eight-week-old human embryo

http

://n

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In Vitro Fertilization

• In vitro fertilization is a medical technique for fertilizing an egg outside of the woman’s reproductive tract.

• IVF can be used when a couple cannot conceive through sexual inter-course.

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IVF Procedures

• IVF involves stimulating the ovaries with hormones to release ova, harvesting them, and fertilizing them with sperm in a fluid medium.

• The embryo is transferred to the mother once the uterus has been readied hormonally.

• Precision techniques are sometimes used to insert the sperm into the ovum.

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IVF Procedures

http://www.ivi.es

Traditional IVF method is shown.

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IVF Microtechnique

http://www.tylermedicalclinic.com

http:www.stoeltingco.com

A more recent, precision approach to IVF

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Success Rates

• The success rate of IVF is about 33 percent, although it can be lower for older women.

• Multiple eggs, to improve the likelihood of pregnancy, may result in more than one embryo.

• A woman may provide her own eggs or rely on a donor if one is available.

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Ethical Considerations

The use of donor eggs can involve ethical and legal considerations.

http://www.chass.ncsu.edu

Branching tree of decision possibilities—some medical situations are ethically complex.

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Meiosis

• Meiosis—the basis of sexual reproduction—resembles mitosis, but it has two additional features:

– Halving of the number of chromosomes (2n is reduced to n).– Exchange, or crossing-over, of genetic material between the

homologous pairs of chromosomes.

• The gametes undergo two consecutive divisions in meiosis I and II.• Four daughter cells result, each with one-half as many chromosomes

(n) as the starting cell (2n).

Meiosis takes place in the testes and ovaries—mitosis occurs in body cells.

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Importance

• Meiosis I is the basis of sexual reproduction in eukaryotic organisms (animals, plants, and fungi).

• Each offspring inherits a unique combination of genes from the two parents.

• Unlike asexual reproduction, offspring can show substantial genetic variation.

In vitro fertilization

http

://m

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inew

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.org

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Homologous Pairs

• The two chromosomes in a homologous pair are the individual chromo-somes inherited from each of the parents.

• Each homologous pair appears alike under a light microscope, although they have different versions of some of their genes.

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Sister Chromatids

• During the interphase, before meiosis begins, each chromosome in a homologous pair replicates to form sister chromatids of identical genetic content.

• The sister chromatids remain together until the end of meiosis.

• Before crossing-over, sister chromatids are identical and carry the same versions of all their genes.

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Interphase and Meiosis I

http://www.mun.ca

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Interphase

• Meiosis is preceded by interphase when homologous chromosomes are duplicated, just as in mitosis.

• Each duplicated chromosome consists of two identical sister chroma-tids.

http://www.sinauer.com

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Prophase I

• Specialized proteins keep the homologous chromosomes together in pairs as the chromatin condenses.

• The resulting structure of four chromatids is known as a tetrad.• Within each tetrad, the chromatids exchange segments in a process

known as crossing-over.

http://www.uic.edu

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Prophase I

• Crossing-over is key to genetic variation from generation-to-generation, and between siblings.

• The process rearranges genetic information from the two parents, as we will discuss.

• As prophase I continues, a spindle of microtubules form, and the tetrads are moved toward the cell’s equator.

http://www.uic.edu

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Meiosis I (continued)

http

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Metaphase I

• The sister chromatids in the tetrad are attached at their centromeres, or waists.

• The tetrads are aligned on the cell’s equator by the spindle anchored to the opposite poles of the cell.

• The spindle is arranged so that the homologous chromosomes of each tetrad are posed to move to the opposite poles of the cell.

http://www.uic.edu

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Anaphase I

• The microtubules in the spindle move the chromosomes toward the opposite poles of the cell.

• Unlike mitosis, sister chromatids migrate as pairs rather than splitting up.

• The sister chromatids are not separated from each other, but from their homologous partners.

http://www.uic.edu

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Telophase I and Cytokinesis

• The sister chromatids reach the poles as a haploid set—the chromo-somes are still in duplicate form.

• Cytokinesis forms two haploid daughter cells during Telophase I.• Depending on the species, the nuclei may or may not return to an

interphase state.• No further chromosome duplication occurs in the subsequent stages of

meiosis II.

http://www.uic.edu

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Meiosis II

http

://w

ww

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.ca

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Meiosis II

• Meiosis II is similar to mitosis, but it starts with a haploid cell (n) rather than a diploid cell (2n).

• The processes of prophase II, metaphase II, anaphase II, telophase II, and cytokinesis are very similar to what was discussed in the lecture on mitosis.

• Meiosis I results in two haploid daughter cells, and meiosis II doubles the number to four haploid daughter cells.

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Mitosis versus Meiosis

• Mitosis enables growth, tissue repair, and asexual reproduction by the production of daughter cells that are genetically-identical to the parent cell.

• In comparison, meiosis enables sexual reproduction by the production of genetically-unique daughter cells known as zygotes (i.e., eggs and sperm).

• In both mitosis and meiosis I, chromosomes duplicate only once during the interphase.

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Mitosis versus Meiosis (continued)

• Mitosis has one division of the cell nucleus and cytoplasm to produce two diploid daughter cells.

• In comparison meiosis I and II have two divisions of the cell nucleus and cytoplasm to produce four haploid daughter cells.

• All events unique to meiosis (that is, not occurring in mitosis), happen in the meiosis I stage.

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Sources of Genetic Variation

• Independence• Random fertilization• Crossing-over

Each offspring will have substantial genetic variation from her/his parents and all siblings except an identical

(monozygotic) twin.

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Independence

• The orientation of homologous chromosomes during meiosis I is a matter of chance similar to flipping a coin.

• Each pair of chromosomes orients itself independently during metaphase I.

• The total number of unique chromosome combinations in a gamete is 2n where n is the haploid number.

• Since n = 23 in humans, over 223 combinations of pairings are possible.• Each gamete—egg or sperm—is one of over eight million (8 x 106) com-

binations.

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Random Fertilization

• A human egg (8 x 106 possibilities) when fertilized by a sperm (8 x 106 possibilities) will produce one of over 6.4 x 1013 possible combinations.

• The fertilization process adds a high degree of genetic variability to the offspring.

6.4 x 1013 = 64,000,000,000,000 possible combinations.

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Crossing-Over

The processes of crossing-over and recombination provides even more possibilities for genetic variation.

http://regentsprep.org

Gametes

Tetrads

Homologous chromosomes

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Crossing-Over

• In the prophase of meiosis I, the homologous chromosomes position themselves very precisely along their lengths in a gene-by-gene align-ment.

• Crossing-over sites are X-shaped regions in homologous chromosomes.• The exchange of DNA segments at the crossing-over sites contributes to

genetic variation.• Multiple cross-overs can occur in a tetrad, leading to even more genetic

variation.

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Genetic Recombination

• Chromosomes resulting from the process of crossing-over are known as recombinant.

• The genetic recombinations are different from the parent chromosomes.• A single cross-over can affect many genes because most chromosomes

contain thousands of genes.

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Chromosomal Disorders

• Errors during meiosis can result in chromosomal disorders in humans.• Each disorder has a characteristic set of physical and mental signs.• The sum total (constellation) of these signs is known as a syndrome.• Just one or even a few characteristic signs do not necessarily form a

syndrome.

A chromosomal or genetic disorder does not make the person ‘abnormal’ and separate from other people. We are all finding our way through this life, and we each face our own struggles

and challenges.

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Down Syndrome

http://atlasgeneticoncology.org

Characteristic physical features of a young child

with Down syndrome.

Trisomy-21 karyotype

http://www.impaedcard.com

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Down Syndrome

• Down syndrome, also known as trisomy-21, is associated with three rather than two number-21 chromosomes.

• Trisomy-21 is the most frequently-occurring chromosomal disorder in the United States, affecting about one in every 700 female and male births.

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Down Syndrome

• Characteristic features of Down syndrome include:– Fold of skins at the inner corner of the eye– Round face and flattened nose bridge– Small irregular teeth– Short stature– Heart defects– Susceptibility to some diseases– Sexual underdevelopment and sterility– Varying degrees of intellectual impairment

• Many people with Down syndrome are socially adept, able to hold jobs, and live fulfilling lives.

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Down Syndrome (continued)

http

://w

ww

.cds

s.ca http://www.downsyndrome.com

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Maternal Age

The risk of bearing a child with Down syndrome increase with the age of the mother, especially if in her late 30s or 40s.

Because of the risk, older parents may decide on prenatal genetic testing and counseling for Down syndrome and other chromosomal disorders.

http://fig.cox.miami.edu

Age of mother at conception (20 to 47 years)

Inci

denc

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kelih

ood)

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Amniocentesis

http://embryology.med.unsw.edu.au

Amniocentesis can be performed from about 15

weeks to term. http://www.dkimages.com

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Chorionic Villus Sampling

http://www.contentanswers.com

CVS can be performed earlier than amniocentesis starting at 11-to-14 weeks.

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Resources and Support

http://www.icongrouponline.com

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Nondisjunctions

Nondisjunctions can occur during meiosis—they are the chromosomal mechanism for Down syndrome

and other trisomies.

http://www.uic.edu

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• Meiosis is responsible for the production of gametes by the testes and ovaries.

• The spindle of microtubules usually distributes chromosomes to the daughter cells without error.

• On rare occasions, chromosomes may not separate completely during anaphase I of meiosis I.

• The result is an abnormal number of chromosomes such as found in trisomy-21.

Nondisjunctions

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• Nondisjunctions are generally well understood, but the reasons why they happen are not.

• Egg cells may have been arrested in the middle of meiosis for as long as 40 or more years since meiosis begins in the ovaries before a woman is born.

Why Do Nondisjunctions Happen?

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Edward Syndrome

http://www.slh.wisc.edu

Also known as trisomy-18 because of a third number-18 chromosome.

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Edward Syndrome

• Trisomy-21 occurs about one in 3,000 conceptions, and one in 6,000 live female and male births.

• Characteristics features include:– Low birth weight– Small head and other characteristic facial features– Structural heart defects– Feeding and breathing difficulties– Developmental delays

• Only 5-10 percent of babies with Edward Syndrome survive the first year due to heart defects and severe breathing difficulties (known as apnea).

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Edward Syndrome

• The long-term prognosis is not good—major medical interventions are often withheld.

• Trisomy-18 is the result of a nondisjunction occurring during meiosis I. • Other trisomies can occur, but the embryo / fetus usually fails to survive

to term (birth).

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Klinefelter Syndrome

XXY karyotype—the 23rd set has two X and one Y chromosomes.

XXY

http://www.biology.iupui.edu

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Klinefelter Syndrome

• The embryo / fetus differentiates as male.• Male sex organs are present, but the testes may be small and sterile.• Other physical characteristics include light beard growth, breast

enlargement (gynecomastia), and a body shape of narrow shoulders and wide hips.

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Klinefelter Syndrome

http://www.gulfkids.com

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• Men who have the syndrome are usually of normal intelligence, and can lead normal lives.

• The XXY karyotype is the result of a nondisjunction during meiosis I.

Klinefelter Syndrome

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Resources and Support

http://www.icongrouponline.com

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• Related types of chromosomal disorders include the XXYY, XXXY, and XXXXY karyotypes.

• These patterns may result from multiple nondisjunctions in meiosis I.

Related Chromosomal Disorders

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Turner Syndrome

XO karyotype—the 23rd pair has one X chromosome and no other.

http://images1.clinicaltools.com

XO

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Turner Syndrome

• The embryo / fetus differentiates as female.• Women with the syndrome are often of short stature, and lack the full

development of their breasts and other secondary sex characteristics.• They are sterile because the female sexual reproductive system does

not mature at adolescence.• Women with Turner syndrome are usually of normal intelligence, and

can lead normal lives.• The XO karyotype is the result of a nondisjunction during meiosis I.

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Resources and Support

University of Texas Publicationhttp://www.healthleader.uhouston.edu

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XYY Pattern

XYY karyotype—the 23rd set has one X and two Y chromosomes.

http://www.slh.wisc.edu

XYY

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XYY Pattern

• The embryo / fetus differentiates as male.• Most males who have this karyotype (one out of 1,000 births) are not

aware of it.• Therefore, it may not be accurate or fair to refer to XYY as a syndrome

or disorder.

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XYY Pattern

• Men who have an XYY karyotype are often tall—sometimes much taller than their parents and siblings.

• They can have learning difficulties more than expected in the general population

• XYY was mistakenly associated at one time with antisocial and criminal behaviors.

• However, men with this karyotype can and do lead normal and productive lives.

• The XYY pattern is the result of a nondisjunction during meiosis I.

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Androgen Insensitivity Syndrome

http://montanakids.com

Melodyhttp://www.indiana.edu

AIS has a genetic basis.

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Androgen Insensitivity Syndrome

• A genetic (XY) male with AIS begins to differentiate with female features as an embryo.

• In complete androgen insensitivity, sexual differentiation is in almost all respects as a female.

• The women do not have ovaries and uterus, but in all other respects their bodies are female.

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Female Appearance

http://www.dls.ym.edu

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Biological Basis

• The presence of testosterone and other androgens (types of steroids) normally produce masculine physical traits.

• Cell receptors for testosterone and other androgens are absent from the plasma membranes in AIS.

• The cells have estrogen receptors in their plasma membranes regard-less of whether a person is a genetic female or male.

• The small amounts of estrogen produced by all males is sufficient for female differentiation when the androgen receptors are absent.

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Androgen Receptor

Computer-generated graphichttp://androgendb.mcgill.ca

The receptors are complex structures of protein molecules embedded in the plasma membranes of cells.

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Genetic Basis

http://upload.wikimedia.org

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Implications

• Complete androgen insensitivity is very rare, but the cases have helped increased the understanding of how embryos differentiate as female or male.

• People with this syndrome generally lead normal and productive lives as women. �����

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Sexual Differentiation

Genetics is one component of sexual differentiation—usually, but not always, all five match

1. Genetic2. Hormonal (steroids)3. Reproductive organs4. Secondary sex characteristics5. Gender identity (personal sense of being male or female)

Sexual differentiation as female or male in humans has at least five aspects:

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Transgendered Individuals

Lynn Conwayhttp://www.upload.wikimedia.org

Both individuals have websites to promote awareness.

Loren Cameronhttp://channel.nationalgeographic.com

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When All Aspects Don’t Match

Aspect Androgen

Insensitivity Syndrome

Adrenocortical Syndrome

(fetal onset)

Transgender Male-to-Female (pre-treatment)

Genetic XY XX XY

Hormonal (useful steroid levels)

Female Male Male

Reproductive organs Undeveloped male Undeveloped female Male

Secondary sex characteristics

Female Male Male

Characteristic gender identity

Female Male (depending on

time of onset) Female

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Polyploid

http://www.federalcompress.com

Cotton is a polyploid organism

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Polyploid Organisms

• Errors in meiosis do not always lead to problems, and may actually be instrumental in the continued evolution of many species.

• Some plant species, especially flowering types, are the result of errors in cell division that resulted in extra sets of chromosomes.

• The new species is said to be polyploid—it has more than two homol-ogous chromosomes.

http://botit.botany.wisc.edu

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Polyploid Organisms

• Cotton and wheat are polyploid plants, and contribute substantially to agricultural production.

• Polyploidism in animals is not common—when it does occur it is mostly in fish and amphibians.

• Polyloid organisms and their role in evolution will be discussed in another lecture.

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Words and Terms to Know

• Amniocentesis• Androgen insensitivity• Autosomes• Chorionic villus sampling• Crossing-over• Diploid• Down syndrome• Edward syndrome• Embryo• Fertilization• Gamete• Genetic recombination• Haploid• Homologous chromosomes• In vitro fertilization

•Karyotype •Klinefelter syndrome•Meiosis I •Meiosis II•Nondisjunction•Ovaries•Sex chromosomes•Sexual reproduction•Sister chromatids•Somatic cells•Testes•Turner syndrome•XYY pattern•Zygote

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Possible Test Items

1. What is the sexual life cycle in eukaryotic organisms and what are its key elements?

2. Present the general outline of meiosis I and II and describe each of their stages.

3. How does meiosis differ from mitosis? What functions does each serve?

4. Describe three factors responsible for the substantial amount of genetic variation in siblings (other than identical twins).

5. Describe the karyotypes and biological and behavioral aspects of three chromosomal disorders in humans.

6. Describe the components of sexual differentiation in humans, and discuss their possible significance for society.