Sexual Reproductionand Meiosis

Chapter 11

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Overview of Meiosis

Meiosis is a form of cell division that leads to the production of gametes.

gametes: egg cells and sperm cells-contain half the number of chromosomes of

an adult body cellAdult body cells (somatic cells) are diploid,

containing 2 sets of chromosomes.Gametes are haploid, containing only 1 set

of chromosomes.

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Overview of Meiosis

Sexual reproduction includes the fusion of gametes (fertilization) to produce a diploid zygote.

Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages.

Some life cycles include longer diploid phases, some include longer haploid phases.

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Features of Meiosis

Meiosis includes two rounds of division – meiosis I and meiosis II.

During meiosis I, homologous chromosomes (homologues) become closely associated with each other. This is synapsis.

Proteins between the homologues hold them in a synaptonemal complex.

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Features of Meiosis

Crossing over: genetic recombination between non-sister chromatids

-physical exchange of regions of the chromatids

chiasmata: sites of crossing over

The homologues are separated from each other in anaphase I.

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Features of Meiosis

Meiosis involves two successive cell divisions with no replication of genetic material between them.

This results in a reduction of the chromosome number from diploid to haploid.

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The Process of Meiosis

Prophase I:

-chromosomes coil tighter

-nuclear envelope dissolves

-homologues become closely associated in synapsis

-crossing over occurs between non-sister chromatids

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The Process of Meiosis

Metaphase I:-terminal chiasmata hold homologues

together following crossing over-microtubules from opposite poles attach to

each homologue, not each sister chromatid-homologues are aligned at the metaphase

plate side-by-side-the orientation of each pair of homologues

on the spindle is random

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The Process of Meiosis

Anaphase I:

-microtubules of the spindle shorten

-homologues are separated from each other

-sister chromatids remain attached to each other at their centromeres

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The Process of Meiosis

Telophase I:

-nuclear envelopes form around each set of chromosomes

-each new nucleus is now haploid

-sister chromatids are no longer identical because of crossing over

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The Process of Meiosis

Meiosis II resembles a mitotic division:-prophase II: nuclear envelopes dissolve

and spindle apparatus forms-metaphase II: chromosomes align on

metaphase plate-anaphase II: sister chromatids are

separated from each other-telophase II: nuclear envelope re-forms;

cytokinesis follows

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Meiosis vs. Mitosis

Meiosis is characterized by 4 features:

1. Synapsis and crossing over

2. Sister chromatids remain joined at their centromeres throughout meiosis I

3. Kinetochores of sister chromatids attach to the same pole in meiosis I

4. DNA replication is suppressed between meiosis I and meiosis II.

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Meiosis vs. Mitosis

Meiosis produces haploid cells that are not identical to each other.

Genetic differences in these cells arise from:

-crossing over

-random alignment of homologues in metaphase I (independent assortment)

Mitosis produces 2 cells identical to each other.

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Nondisjunction

• Chromosomes fail to separate

• Results in gametes and zygote with an abnormal chromosome number

• Aneuploidy is variations in chromosome number that involve one or more chromosomes

• Most aneuploidy from errors in meiosis

Meiosis: The Creations of Gametes

Meiosis 1

Meiosis 2

Non-Disjunction During Meiosis

Non-disjunction in Meiosis 1 Non-disjunction in Meiosis 2

Trisomy zygoteMonosomy zygote

Aneuploidy

• Effects vary by chromosomal condition

• Many cause early miscarriages

• Leading cause of mental retardation

ID of Chromosomal Abnormalities

Two tests:

• Amniocentesis (> 16 weeks)– Collects amniotic fluid – Fetal cells grown and karyotype produced

• Chorionic villus sampling (CVS) (10–12 weeks)– Rapidly dividing cells– Karyotype within few days

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Removal of about 20 ml of amniotic fluid containing suspended cells that were sloughed off from the fetus

Biochemical analysis of the amniotic fluid after the fetal cells are separated out

Centrifugation

Fetal cells are removed

from the solution

Analysis of fetal cells to determine sex

Cells are grown in an incubator

Karyotype analysis

Amniocentesis Only Used in Certain Conditions

• Risks for miscarriage; typically only done under one of following circumstances:– Mother > 35– History of child with chromosomal

abnormalities– Parent has abnormal chromosomes – Mother carries a X-linked disorder– History of infertility or multiple miscarriages

Chorionic Villus Sampling (CVS)

Karyotype

Other Chromosomal Variations• Haploid: one copy of each chromosome• Diploid: normal two copies of each chromosome• Polyploidy: multiple sets of chromosomes• Aneuploid: A variation in chromosome number,

but not involving all of the chromosomes• Trisomy: three copies of one chromosome • Monosomy: only one copy of a chromosome

• Structural changes: duplication, deletion, inversion, translocation

Duplication

Deletion

Karyotype of Deletion on Chromosome 16

Inversion

Translocation

Translocation Karyotype

Effects of Changes in Chromosomes

• Vary by chromosome and type of variation

• May cause birth defects or fetal death

• Monosomy of any autosome is fatal

• Only a few trisomies result in live births

Patau Syndrome

Trisomy 13: Patau Syndrome (47,+13)

• 1/15,000

• Survival: 1–2 months

• Facial, eye, finger, toe, brain, heart, and nervous system malformations

Trisomy 18: Edwards Syndrome (47,+18)

• 1/11,000, 80% females

• Survival: 2–4 months

• Small, mental disabilities, clenched fists, heart, finger, and foot malformations

• Die from heart failure or pneumonia

Down Syndrome

Trisomy 21: Down Syndrome (47,+21)

• 1/800 (changes with age of mother)

• Survival up to age 50

• Leading cause of childhood mental retardation and heart defects

• Wide, flat skulls; large tongues; physical, mental, development retardation

Maternal Age and Down Syndrome

Aneuploidy and Sex Chromosomes

• More common than in autosomes

• Turner syndrome (45,X): monosomy of X chromosome

• Klinefelter syndrome (47,XXY)

• Jacobs syndrome (47,XYY)

Turner Syndrome

Turner Syndrome (45,X)

• Survival to adulthood

• Female, short, wide-chested, undeveloped ovaries, possible narrowing of aorta

• Normal intelligence

• 1/10,000 female births, 95–99% of 45,X conceptions die before birth

Klinefelter Syndrome

Klinefelter Syndrome (47,XXY)

• Survival to adulthood

• Features do not develop until puberty, usually sterile, may have learning disabilities

• 1/1,000 males

XYY Syndrome

XYY or Jacobs Syndrome (47,XYY)

• Survival to adulthood

• Average height, thin, personality disorders, some form of mental disabilities, and adolescent acne

• Some may have very mild symptoms

• 1/1,000 male births

Ways to Evaluate Risks• Genetic counselors are part of the health

care team

• They assist understanding of: – Risks – Diagnosis– Progression– Possible treatments– Management of disorder– Possible recurrence

Counseling Recommendations \

• Pregnant women or those who are planning pregnancy, after age 35

• Couples with a child with: – Mental retardation– A genetic disorder– A birth defect

Counseling Recommendations

• Couples from certain ethnic groups

• Couples that are closely related

• Individuals with jobs, lifestyles, or medical history that may pose a risk to a pregnancy

• Women who have had two or more miscarriages or babies who died in infancy

Genetic Counseling

• Most see a genetic counselor:– After a prenatal test;– After the birth of a child; or – To determine their risk

• Counselor – Constructs a detailed family history and

pedigree– Shares information that allows an individual or

a couple to make informed decisions

Future of Genetic Counseling

• Human Genome Project (HGP) changed medical care and genetic testing

• Genetic counselor will become more important

• Evaluate reproductive risks and other conditions

• Allow at-risk individuals to make informed choices about lifestyle, children, and medical care