4.2: Meiosis

★State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei.

★Define homologous chromosomes.

★Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in 4 haploid cells.

4.2: Meiosis

★Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21)

★State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.

4.2: Meiosis

★State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

★Analyze a human karyotype to determine gender and whether non-disjunction has occurred.

4.2: Meiosis

•Review:

•Mitosis is cell division that produces body cells, all identical to each other, all have 46 chromosomes in humans.

•New:

•Meiosis: cell division that produces gametes.

•Gametes: sex cells.

4.2: Meiosis

•In any organism, each cell that is produced as a result of meiosis has half the number of chromosomes of other cells in the body.

•Mitosis = 46 chromosomes in every cell

•Meiosis = 23 chromosomes in every egg and sperm

•Why? When an egg and sperm meet to make a baby, that makes the first fertilized egg = 46 chromosomes!

4.2: Meiosis

•These chromosome #'s have names!

•Diploid (2n): two copies of every chromosome, in humans our diploid number is 46.

•Haploid (n): one copy of every chromosome, in humans our haploid number is 23

•Mango diploid # = 40, haploid # = 20

•Camel diploid # = 70, haploid # = 35

•How many chromosomes does a camel sperm or egg have?

•35

4.2: Meiosis

•During gamete formation, meiosis reduces the diploid number to the haploid number.

•At the time of fertilization (egg meets sperm), the diploid number is restored.

# Chromosomes

Cell TypeDescription of

condition

46Typical body (somatic) cell

Diploid

23Gamete, egg or sperm cell

Haploid

•Meiosis is called a reduction division because the number of chromosomes has been reduced.

•Living organisms who reproduce sexually have to 1/2 their chromosome number at some stage in the life cycle because the fusion of gametes during fertilization doubles it.

4.2: Meiosis

•Homologous Chromosomes:

•In a diploid human cell, the 46 chromosomes can be grouped into 23 pairs or chromosomes called homologous chromosomes.

•Homologous = similar in shape and size, 2 chromosomes carry the same genes

•With a pair of homologous chromosomes, one coms from the mother, one from the father.

4.2: Meiosis

•Although a pair of homologous chromosomes carry the same genes, they are not completely identical

•Homologous chromosomes have the same genes as each other, in the same sequence, but not necessarily the same alleles of those genes.

•EX: A pair of homologous chromosomes (one from mom/one from dad) have the same genes (code for proteins) but some bases are different causing different traits.

•AGGTTA = brown hair

•AGGTTG = blonde hair

4.2: Meiosis

•Phases of Meiosis:

•Meiosis is the process by which a diploid parent cell produces 4 haploid daughter cells.

•Before meiosis, DNA replication occurs (making a complete DNA copy) during interphase.

•In order to produce 4 cells, the original/parent cell must divide 2 times.

•First division makes 2 cells

•Second division makes 4 cells (final product)

4.2: Meiosis

•Prophase I:

•1. Chromosomes become visible/super coil/condense after interphase

•2. Homologous chromosomes pair up (one from mom, one from dad)

•3. Crossing over occurs (structures crossed over homologous chromosomes formed called tetrads)

•4. Spindle fibers (made from microtubules) form

4.2: Meiosis

•What is the purpose of crossing over?

•The biggest event that makes meiosis different from mitosis is crossing over.

•This is the exchange of genetic material between non-sister chromatids.

•This trading of segments of genes happens when sections of the two homologous chromatids break at the same point, twist around each other, and swap DNA.

•This allows DNA from the maternal chromosome to mix with DNA from the paternal chromosome.

4.2: Meiosis

•Crossing over is the reason why, at the end of meiosis, all 4 sperm or egg are genetically different from one another.

•If your parents have 10 kids, all kids will be different because each egg your mom produces, and sperm your dad produces, are different!

•That means millions and millions of combinations!

4.2: Meiosis

•Metaphase I:

•1. Homologous chromosomes (also can be called bivalents) line up across cell's equator/middle

•2. Nuclear membrane disappears

4.2: Meiosis

•Anaphase I:

•1. Spindle fibers from the poles attach to chromosomes and pull them to opposite poles of the cell.

•2. This is the reduction division because chromosome number is halved from diploid to haploid.

4.2: Meiosis

•Telophase I:

•1. Spindle fibers disintegrate/disappear

•2. Usually, chromosomes uncoil and new nuclear membranes form.

•At the end if meiosis 1, cytokinesis happens; cell splits into 2 separate cells.

•Cells are haploid because they contain only 1 chromosome of each pair (sister chromatids still attached). No Interphase/S phase necessary because we don't want to double chromosome number.

4.2: Meiosis

•Prophase II:

•1. In each of the 2 cells after meiosis I, new spindle fibers start to form.

•2. Chromosomes recoil/condense again

•3. Nuclear envelopes break down again

4.2: Meiosis

•Metaphase II:

•1. Nuclear envelopes completely broken down

•2. Individual chromosomes line up at equator/middle of cell

•3. Spindle fibers travel to opposite poles and attached to the centromere of each chromosome.

4.2: Meiosis

•Anaphase II:

•1. Sister chromatids separate at the centromere

•2. Spindle fibers pull the chromatids to opposite sides of the cell.

4.2: Meiosis

•Telophase II:

•1. Nuclear envelopes re-form

•2. Chromosomes uncoil, soon to be chromatin

•3. A second cytokinesis follows

•4. FINAL PRODUCT: 4 haploid/genetically different/sperm or egg

4.2: Meiosis

•Major differences between mitosis and meiosis:Mitosis Meiosis

Why: to replace and repair body cells

Why: to make gametes (sperm/egg)

Final product: 2 identical body cells

Final product: 4 genetically different sex cells (because of

CROSSING OVER)

1 division 2 divisions

Ends with diploid cells Ends with haploid cells

chromosomes always single/by themselves

During meiosis I, chromosomes in pairs (homologous)

Anaphase: sister chromatids separate

Anaphase I: Homologous chromosomes separate

Anaphase II: Sister chromatids separate

4.2: Meiosis•Homework Questions

•1. How are the cells at the end of meiosis different from the cells at the beginning of meiosis?

•2. If the diploid number of goldfish is 90, what is its haploid number? Which type of cells in the goldfish are diploid, and which ones are haploid?

•3. While examining a cell in prophase I of meiosis, you observe a pair of homologous chromosomes pairing tightly. What is the significance of the places at which the chromosomes are joined and what is this "event" called?

•4. Draw and label the stages of meiosis I and meiosis II. Start with 6 chromosomes.

4.2: Meiosis

•Non-disjunction: failure of chromosomes to separate properly.

• If this occurs in meiosis I, some homologous chromosomes don't separate.

• If this occurs in meiosis II, some sister chromatids don't separate properly.

•Consequence: changes in chromosome number

•Any more or less than 23 in each sperm and egg can result in chromosomal disorders.

4.2: Meiosis

•Down Syndrome: Chromosomal disorder due to non-disjunction

•Non-disjunction happens in the 21st pair of chromosomes.

•Child receives 3 chromosomes instead of 2.

•Total #chromosomes in ALL CELLS = 47.

• If one chromosome pair has 3 chromosomes, this is called trisomy.

•Down Syndrome = Trisomy 21

4.2: Meiosis

•People with Down Syndrome have distinct facial features, are short, have heart defects and mental disability (often in special education programs).

•Down Syndrome most common chromosomal disorder - Approximately 1 out of 800 children born in US have Trisomy 21.

•Older women have a greater chance of having a child with Down Syndrome. WHY?

•The "best" eggs are released first during a woman's lifetime (1 egg every month/menstrual cycle)

•Older women have older eggs, more defects

4.2: Meiosis

•Non-disjunction can happen with other chromosomes, and all of them will have a major impact on a child's development.

•Some chromosomal disorders are so severe that the fetus may not survive beyond a few weeks or months in the womb.

•Some miscarriages (not all) are from non-disjunction.

4.2: Meiosis

•Other non-disjunction disorders:

•Trisomy 18/Edwards Syndrome

•95% die in utero, heart/kidney malfunctions

•Trisomy 13/Patau Syndrome

•Mental/motor challenged, heart/eye defects, polydactyly (extra digits)

•Monosomy X/Turner Syndrome

•One sex chromosome instead of 2

•Looks female, infertile, ovaries don't develop

4.2: Meiosis

•Karyotyping:

•Karyotype: photograph of the chromosomes found in a cell.

•Chromosomes are arranged according to their size and shape.

•When during the cell cycle do you think chromosomes are photographed for a karyotype?

4.2: Meiosis

•Why karyotype?

•From a karyotype, the gender of a person can be determined and chromosome abnormalities can be detected.

•The most useful time to do this is before birth.

•In order to do this, cells from the fetus (inside the mother's womb) but be taken.

4.2: Meiosis

•Once fetal cells have been obtained, they are mixed with chemicals that stimulate them to divide by mitosis.

•What mitosis phase do you think is best to capture a picture of the chromosomes for a karyotype?

•Another chemical is used which stops mitosis in metaphase of mitosis.

4.2: Meiosis•How a karyotype is made:

•1. The cells are stained and prepared on a glass slide to see the chromosomes under a light microscope.

•2. Photomicrograph images are obtained of the chromosomes during mitosis/metaphase

•3. The images are cut out and separated using computer software.

•4. The images of each pair of chromosomes are placed in order by size and the position of the centromere.

4.2: Meiosis

•2 ways to take fetus cells:

•1. Amniocentesis

•A sample of amniotic fluid is removed from the amniotic sac around the fetus (when "her water breaks" right before birth, this is the sac that breaks)

•To do this, a hypodermic needle is inserted through the wall of the mother's abdomen and wall of the uterus.

4.2: Meiosis

•Amniotic fluid is drawn out into a syringe.

•This fluid contains cells form the fetus.

4.2: Meiosis

4.2: Meiosis

•2. Chorionic villus sampling

•Cells are removed from fetal tissues in the placenta called chorionic villi.

•As with amniocentesis, a hypodermic needle, inserted through the mother's abdomen and uterus wall, is used to obtain the cells.

•Different from amniocentesis, because rather than cells taken from fluid, cells are taken from tissues surrounding the baby.

4.2: Meiosis

•Debate the pro's and con's of amniocentesis and chorionic villus sampling.

•Since the extraction of cells from an unborn baby is an invasive and expensive procedure, there is a risk that it may harm the child or even cause a miscarriage. Doctors and parents-to-be must consider which is more important, finding out if the baby has a chromosomal disorder or providing safe conditions for its development without extracting fetal cells.

4.2: Meiosis

•Who should decide whether to keep or to abort the babies which present chromosomal disorders?

•The doctors? Future parents? Both?

•If you found our your future child had a chromosome abnormality that would make him or her very different from other children, what would you do?

•The severity of the chromosomal disorder cannot be determined from the information on the karyotype, does this affect your answer?

4.2: Meiosis

•How to analyze a human karyotype for gender or disorders from non-disjunction:

•Female: XX

•Male: Xy

•Normal karyotype: 22 pairs (44) of autosomes (regular chromosomes) and 2 sex chromosomes