Meiosis

Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? At one level, the answers lie in meiosis.

Variations on a Theme Living organisms are distinguished by their ability to reproduce their own kind Genetics is the scientific study of heredity and variation Heredity is the transmission of traits from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings

In Humans the “Sex Chromosomes” are the 23rd set Sex Chromosomes- The Sex Chromosomes code for the sex of the offspring. **If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male

Sex Chromosomes “Sex Chromosomes” …….the 23rd set This person has 2 “X” chromosomes… and is a female. 23

Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

Inheritance of Genes Genes are the units of heredity, and are made up of segments of DNA Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs) Each gene has a specific location called a locus This is found on a certain section of a chromosome Most DNA is packaged into chromosomes One set of chromosomes is inherited from each parent.

Comparison of Asexual and Sexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis (simple yet fast bacteria do this) A clone is a group of genetically identical individuals from the same parent In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents (this needs meiosis)

Organisms that reproduce Sexually are made up of two different types of cells. Somatic Cells are “body” cells and contain the normal number of chromosomes “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes called the “Haploid” number (the symbol is n)….. Examples would be … Sperm cells and ova.

Gametes The Male Gamete is the Sperm and is produced in the male gonad the Testes. The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the Ovaries.

Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of Two sister chromatids of one replicated chromosome Figure 13.4 Describing chromosomes Centromere Two non-sister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

Meiosis … 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). 2)This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number. 3) In meiosis, one diploid cells produces four haploid cells.

??? Why halve the chromosomes in gametes???? Why do we need meiosis? 1. Meiosis is necessary to halve the number of chromosomes going into the sex cells ??? Why halve the chromosomes in gametes???? At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents.

2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species.

Sperm + Ovum (egg) Zygote During Ovulation the ovum is released from the ovary and transported to an area where fertilization- the joining of the sperm and ovum- can occur…… Fertilization results in the formation of the Zygote. (fertilized egg) fertilization Sperm + Ovum (egg) Zygote

Fertilization n=23 egg 2n=46 zygote The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

Multicellular diploid adults (2n = 46) Key Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS FERTILIZATION Figure 13.5 The human life cycle Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Spermatogenesis n=23 n=23 2n=46 sperm haploid (n) Meiosis II human sex cell diploid (2n) n=23 Meiosis I

Meiosis – mouse testes 1st division Parent cell 2nd division 4 gametes formed

The Stages of Meiosis In the first cell division (meiosis I), homologous chromosomes separate. Meiosis I results in two haploid daughter cells with replicated chromosomes It is called the reductional division. In the second cell division (meiosis II), sister chromatids separate Meiosis II results in four haploid daughter cells with un-replicated chromosomes It is called the equational division.

Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

Before Meiosis… Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids The sister chromatids are genetically identical and joined at the centromere The single centrosome replicates, forming two centrosomes

Microtubule attached to kinetochore Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Centrosome (with centriole pair) Sister chromatids remain attached Centromere Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Cleavage furrow Homologous chromosomes Homologous chromosomes separate Fragments of nuclear envelope Microtubule attached to kinetochore

Prophase I Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense. In synapsis, homologous chromosomes loosely pair up, aligned gene by gene This is going to form the X shape you ultimately see in chromosomes.

In crossing over, nonsister chromatids exchange DNA segments. Each pair of chromosomes forms a tetrad, a group of four chromatids Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

Recombinant chromosomes Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure 13.12 The results of crossing over during meiosis Anaphase I Anaphase II Daughter cells Recombinant chromosomes

Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome

Prophase I Metaphase I Centrosome (with centriole pair) Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

Anaphase I In anaphase I pairs of homologous chromosomes separate. One chromosome moves toward each pole, guided by the spindle apparatus. Sister chromatids remain attached at the centromere and move as one unit toward the pole.

Telophase I In the beginning of telophase I each half of the cell has a haploid set of chromosomes each chromosome still consists of two sister chromatids

Cytokinesis Cytokinesis usually occurs simultaneously It forms two haploid daughter cells In animal cells, a cleavage furrow forms in plant cells, a cell plate forms NO chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

Telophase I and Cytokinesis Anaphase I Sister chromatids remain attached Figure 13.8 The meiotic division of an animal cell Homologous chromosomes separate Cleavage furrow

Division in meiosis II also occurs in four phases: – Prophase II – Metaphase II – Anaphase II – Telophase II and cytokinesis Meiosis II is very similar to mitosis

Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Figure 13.8 The meiotic division of an animal cell Sister chromatids separate Haploid daughter cells forming

Metaphase II In metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles

Prophase II Metaphase II Figure 13.8 The meiotic division of an animal cell

Anaphase II Telophase II In anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing

Cytokinesis Cytokinesis separates the cytoplasm At the end of meiosis: there are four daughter cells each with a haploid set of unreplicated chromosomes Each daughter cell is genetically different from the others and from the parent cell

Telephase II and Cytokinesis Anaphase II Figure 13.8 The meiotic division of an animal cell Sister chromatids separate Haploid daughter cells forming

A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell Both The mechanism for separating sister chromatids is virtually identical in meiosis II & mitosis

Replicated chromosome MITOSIS MEIOSIS MEIOSIS I Parent cell Chiasma Chromosome replication Chromosome replication Prophase Prophase I Homologous chromosome pair 2n = 6 Replicated chromosome Metaphase Metaphase I Figure 13.9 A comparison of mitosis and meiosis in diploid cells Anaphase Telophase Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II

Boy or Girl? The Y Chromosome “Decides”

Three events are unique to meiosis, and all three occur in meiosis l: – Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information – At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes – At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

Genetic variation produced in sexual life cycles contributes to evolution Mutations (changes in an organism’s DNA) are the original source of genetic diversity Mutations create different versions of genes called alleles Reshuffling of alleles during sexual reproduction produces genetic variation

Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Three mechanisms contribute to genetic variation: Independent assortment of chromosomes Crossing over Random fertilization

Figure 13.9 A comparison of mitosis and meiosis in diploid cells SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, metaphase, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Figure 13.9 A comparison of mitosis and meiosis in diploid cells Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure 13.11 The independent assortment of homologous chromosomes in meiosis Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

Meiosis – division error Chromosome pair

Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female?

Question: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: 10 chromosomes

Animations and Videos Meiosis Tutorial Meiosis - 1 Meiosis - 2 Stages of Meiosis Anaphase Separase Spermatogenesis Production of Male and Female Gametes Polar Body Formation

Animations and Videos Random Orientation of Chromosomes During Meiosis Independent Assortment Independent Assortment and Gamete Diversity Alleles That Do Not Assort Independently Comparison of Mitosis and Meiosis Bozeman - Diploid vs. Haploid Cells Activity- Mitosis and Meiosis Bozeman - Mitosis and Meiosis Simulation

Animations and Videos Meiosis - Nondisjunction – 1 Crossing Over Changes in Chromosome Structure The Consequence of Inversion Bozeman - Cell Cycle (Mitosis and Meiosis) X Inactivation Bozeman - X Inactivation

Animations and Videos Density-Dependent Inhibition Cytotoxic T Lymphocytes Attack a Melanoma Cell Metastasis of a Cancer Amniocentesis Animation Chorionic Villus Sampling Karyotype Animation Human Melanoma Cells Dividing Chapter Quiz Questions – 1 Chapter Quiz Questions – 2

Which of the following transmits genes from both parents to child, or from one generation of a family to another? DNA gametes somatic cells mitosis nucleotides Answer: B

Which of the following transmits genes from both parents to child, or from one generation of a family to another? DNA gametes somatic cells mitosis nucleotides 59

Fertilization is to zygote as meiosis is to which of the following? mitosis diploid chromosome replication gamete Answer: E 60

Fertilization is to zygote as meiosis is to which of the following? mitosis diploid chromosome replication gamete 61

Privet shrubs and humans each have a diploid number of 46 chromosomes per cell. Why are the two species so dissimilar? Privet chromosomes undergo only mitosis. Privet chromosomes are shaped differently. Human chromosomes have genes grouped together differently. The two species have appreciably different genes. Privets do not have sex chromosomes. Answer: D 62

Privet shrubs and humans each have a diploid number of 46 chromosomes per cell. Why are the two species so dissimilar? Privet chromosomes undergo only mitosis. Privet chromosomes are shaped differently. Human chromosomes have genes grouped together differently. The two species have appreciably different genes. Privets do not have sex chromosomes. 63

Why is it more practical to prepare karyotypes by viewing somatic diploid cells rather than haploid gametes? Somatic diploid cells do not contain organelles to interfere with karyotyping. Both sets of chromosomes, which are present in somatic diploid cells, need to be examined. DNA in haploid gametes will not stain. The chromosomes are larger in a somatic diploid cell. Haploid gametes do not have sex chromosomes. Answer: B 64

Why is it more practical to prepare karyotypes by viewing somatic diploid cells rather than haploid gametes? Somatic diploid cells do not contain organelles to interfere with karyotyping. Both sets of chromosomes, which are present in somatic diploid cells, need to be examined. DNA in haploid gametes will not stain. The chromosomes are larger in a somatic diploid cell. Haploid gametes do not have sex chromosomes. 65

Diploid cells may undergo either mitosis or meiosis Diploid cells may undergo either mitosis or meiosis. Haploid cells may undergo mitosis (for certain species) but not meiosis because the sister chromatids cannot separate. the synaptonemal complex is too strong. crossing over has occurred. cohesins are no longer present. homologous chromosomes cannot pair. Answer: E 66

Diploid cells may undergo either mitosis or meiosis Diploid cells may undergo either mitosis or meiosis. Haploid cells may undergo mitosis (for certain species) but not meiosis because the sister chromatids cannot separate. the synaptonemal complex is too strong. crossing over has occurred. cohesins are no longer present. homologous chromosomes cannot pair. Answer: E 67

How and at what stage do chromosomes undergo independent assortment? meiosis I pairing of homologs anaphase I separation of homologs meiosis II separation of homologs meiosis I metaphase alignment meiosis I telophase separation Answer: D 68

How and at what stage do chromosomes undergo independent assortment? meiosis I pairing of homologs anaphase I separation of homologs meiosis II separation of homologs meiosis I metaphase alignment meiosis I telophase separation 69

What allows sister chromatids to separate in which phase of meiosis? release of cohesin along sister chromatid arms in anaphase I crossing over of chromatids in prophase I release of cohesin at centromeres in anaphase I release of cohesin at centromeres in anaphase II crossing over of homologs in prophase I Answer: D 70

What allows sister chromatids to separate in which phase of meiosis? release of cohesin along sister chromatid arms in anaphase I crossing over of chromatids in prophase I release of cohesin at centromeres in anaphase I release of cohesin at centromeres in anaphase II crossing over of homologs in prophase I 71

Gametes produced from one meiotic event are genetically identical to each other. each have the same chromosome number. are genetically identical to the cells produced from meiosis I. are genetically identical to the parent cell. each have the same mutations. Answer: B 72

Gametes produced from one meiotic event are genetically identical to each other. each have the same chromosome number. are genetically identical to the cells produced from meiosis I. are genetically identical to the parent cell. each have the same mutations. 73

Crossing over begins to occur during anaphase I anaphase II prophase I metaphase II telophase II Answer: C 74

Crossing over begins to occur during anaphase I anaphase II prophase I metaphase II telophase II Answer: C 75

In this cell, what phase is represented? mitotic metaphase meiosis I anaphase meiosis I metaphase meiosis II anaphase meiosis II metaphase Answer: C 76

In this cell, what phase is represented? mitotic metaphase meiosis I anaphase meiosis I metaphase meiosis II anaphase meiosis II metaphase 77

Scientific Skills Exercise When nutrients are low, cells of the budding yeast (Saccharomyces cerevisiae) exit the mitotic cell cycle and enter meiosis. In this exercise, you will track the DNA content of a population of yeast cells as they progress through meiosis. Researchers grew a culture of yeast cells in a nutrient-rich medium and then transferred them to a nutrient-poor medium to induce meiosis. At different times after induction, the DNA content per cell was measured in a sample of the cells, and the average DNA content per cell was recorded in femtograms (fg; 1 femtogram  1 × 10–15 gram). Their data are shown in the table to the right.

Most of the yeast cells in the culture were in G1 of the cell cycle before being moved to the nutrient-poor medium to induce meiosis. How many femtograms of DNA are there in a yeast cell in G1? Estimate this value from the data in the table. 12 fg 24 fg 40 fg 48 fg Answer: B

Most of the yeast cells in the culture were in G1 of the cell cycle before being moved to the nutrient-poor medium to induce meiosis. How many femtograms of DNA are there in a yeast cell in G1? Estimate this value from the data in the table. 12 fg 24 fg 40 fg 48 fg Answer: B

How many femtograms of DNA are present in a cell in G2? 12 fg 24 fg 40 fg 48 fg Answer: D

How many femtograms of DNA are present in a cell in G2? 12 fg 24 fg 40 fg 48 fg Answer: D

How many femtograms of DNA are present in a cell at the end of meiosis I? 12 fg 24 fg 40 fg 48 fg Answer: B

How many femtograms of DNA are present in a cell at the end of meiosis I? 12 fg 24 fg 40 fg 48 fg

How many femtograms of DNA are present in a cell at the end of meiosis II? 12 fg 24 fg 40 fg 48 fg Answer: A

How many femtograms of DNA are present in a cell at the end of meiosis II? 12 fg 24 fg 40 fg 48 fg Answer: A

A graph with labels indicating the different phases of the meiotic cell cycle (MI  meiosis I; MII  meiosis II) is shown to the right, based on the data from the table. Think carefully about the point on the graph where the line at the highest value begins to slope downward, indicated by the red arrow. What specific point of meiosis does this “corner” represent? Answer: C metaphase I prophase II cytokinesis anaphase I

A graph with labels indicating the different phases of the meiotic cell cycle (MI  meiosis I; MII  meiosis II) is shown to the right, based on the data from the table. Think carefully about the point on the graph where the line at the highest value begins to slope downward, indicated by the red arrow. What specific point of meiosis does this “corner” represent? metaphase I prophase II cytokinesis anaphase I

Based on this data, how much DNA is present in a gamete of Saccharomyces cerevisiae? 12 fg 24 fg 48 fg Answer: A

Based on this data, how much DNA is present in a gamete of Saccharomyces cerevisiae? 12 fg 24 fg 48 fg

0.08 Mb (8.0  104 base pairs) 1.2 Mb (1.2  106 base pairs) Given the fact that 1 fg of DNA  9.78  105 base pairs (on average), you can convert the amount of DNA per cell to the length of DNA in numbers of base pairs. Millions of base pairs (Mb) is the standard unit for expressing genome size. Calculate the approximate number of base pairs of DNA in the haploid yeast genome. Answer: C 0.08 Mb (8.0  104 base pairs) 1.2 Mb (1.2  106 base pairs) 12 Mb (12  106 base pairs) 23 Mb (23  106 base pairs)

0.08 Mb (8.0  104 base pairs) 1.2 Mb (1.2  106 base pairs) Given the fact that 1 fg of DNA  9.78  105 base pairs (on average), you can convert the amount of DNA per cell to the length of DNA in numbers of base pairs. Millions of base pairs (Mb) is the standard unit for expressing genome size. Calculate the approximate number of base pairs of DNA in the haploid yeast genome. 0.08 Mb (8.0  104 base pairs) 1.2 Mb (1.2  106 base pairs) 12 Mb (12  106 base pairs) 23 Mb (23  106 base pairs)

Given the fact that 1 fg of DNA  9 Given the fact that 1 fg of DNA  9.78  105 base pairs (on average), you can estimate the rate of DNA synthesis in Saccharomyces cerevisiae. Approximately how many base pairs per minute were synthesized during the S phase of these yeast cells? 0.19 (1.9  10–1) base pair per minute 100,000 (1.0  105) base pairs per minute 200,000 (2.0  105) base pairs per minute 11,000,000 (11.0  106) base pairs per minute Answer: C Because the S phase took place from approximately the 1-hour mark to the 3-hour mark, start by finding the difference between the amount of DNA at 3 hours and the amount at 1 hour: 47.0 fg – 24.0 fg = 23.0 fg. Now calculate the rate of fg synthesized per minute by dividing that amount by the number of minutes in two hours: 23.0 fg/ 120 minutes = 0.192 fg/min. Finally, you need to convert from fg/min to base pairs/min: 0.192 fg/min × (9.78 × 105 base pairs/fg) = 187,800, or approximately 200,000 base pairs/min.

Given the fact that 1 fg of DNA  9 Given the fact that 1 fg of DNA  9.78  105 base pairs (on average), you can estimate the rate of DNA synthesis in Saccharomyces cerevisiae. Approximately how many base pairs per minute were synthesized during the S phase of these yeast cells? 0.19 (1.9  10–1) base pair per minute 100,000 (1.0  105) base pairs per minute 200,000 (2.0  105) base pairs per minute 11,000,000 (11.0  106) base pairs per minute Answer: C Because the S phase took place from approximately the 1-hour mark to the 3-hour mark, start by finding the difference between the amount of DNA at 3 hours and the amount at 1 hour: 47.0 fg – 24.0 fg = 23.0 fg. Now calculate the rate of fg synthesized per minute by dividing that amount by the number of minutes in two hours: 23.0 fg/ 120 minutes = 0.192 fg/min. Finally, you need to convert from fg/min to base pairs/min: 0.192 fg/min × (9.78 × 105 base pairs/fg) = 187,800, or approximately 200,000 base pairs/min.