Chapter 13 Meiosis and Sexual Life Cycles Overview I. Cell Types II. Meiosis I. Meiosis I II. Meiosis II III. Genetic Variation IV. Reproduction Overview: Variations on a Theme Figure 13.1 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 Offspring acquire genes from parents by inheriting In a literal sense, children do not inherit particular physical traits from their parents It is genes that are actually inherited Inheritance of Genes Genes are the units of heredity, and are made up of segments of DNA Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs) Each gene has a specific location called a locus on a certain chromosome Most DNA is packaged into 1
Comparison of Asexual and Sexual Reproduction Figure 13.2 In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes 0.5 mm 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 Bud (a) Hydra Parent (b) Redwoods Reproduction Fertilization Asexual reproduction: Parent cell divides into two daughter cells (similar to mitosis). The end result is a two daughter cells identical to parent cell Fertilization is the union between the sperm and the egg. Sexual reproduction: The union of two gametes (sex cells) to form a single zygote Eggs and Sperm are gametes Fertilized egg is zygote Zygote is different from gametes Cell Types Mitosis occurs in all the body s cells except the cells that are responsible for reproduction Gametes: are the cells that are responsible for reproduction All the rest of the body s cells are somatic cells 11 2
Gametes = reproductive cells Sperm and eggs are reproductive cells gametes Remember that we have 23 pairs of = 46 The cells that divide to produce gametes undergo meiosis If gametes (sperm and egg) combined with all these then the offspring will have 92 How do gametes overcome this problem? Before the gametes come together they need to reduce their number of in half. So instead of 23 pairs (46 ) they need to have 23 total. The answer to their problem is meiosis halving their number of Sets of Chromosomes in Human Cells Human somatic cells (any cell other than a gamete) have 23 pairs of A karyotype is an ordered display of the pairs of from a cell The two in each pair are called homologous, or homologs Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters Figure 13.3 APPLICATION TECHNIQUE Pair of homologous duplicated Centromere 5 m Sister chromatids Metaphase chromosome 3
Sex Chromosomes in Human Cells The sex, which determine the sex of the individual, are called X and Y Human females have a homologous pair of X (XX) Human males have one X and one Y chromosome The remaining 22 pairs of are called autosomes Sets of Chromosomes in Human Cells Each pair of homologous includes one chromosome from each parent The 46 in a human somatic cell are two sets of 23: one from the mother and one from the father A diploid cell (2n) has two sets of For humans, the diploid number is 46 (2n = 46) Terminology Diploid = Cells that contain two sets of. In humans, cells that have 46 or 23 pairs; all somatic cells are diploid (2n) Haploid = Cells that have one set of. In humans, cells that have 23 ; gametes are haploid (1n) Polyploidy = three sets of ; rare in animals, common in plants Meiosis is when a diploid cell divides to produce haploid reproductive cells Meiosis First the (DNA) are duplicated during Interphase Then there are two cell divisions Remember that mitosis had chromosome (DNA) duplication followed by one cell division DNA Replication DNA synthesis has occurred during interphase, each chromosome is replicated Remember that there are pairs, each chromosome has two chromatids just after DNA replication Each replicated chromosome consists of two identical sister chromatids 4
Figure 13.4 2n 6 Key Sister chromatids of one duplicated chromosome Maternal set of (n 3) Paternal set of (n 3) Centromere Meiosis The DNA has already replicated during interphase the have become duplicated In Meiosis the chromosome homologous pairs separate and the cell divides = 1 st cell division Then the chromatids separate and cell divide = 2 cd cell division Two nonsister chromatids in a homologous pair Pair of homologous (one from each set) The figures are going to show only one pair of but there are 23 pairs at the start Figure 13.7-1 Interphase Figure 13.7-2 Interphase Pair of homologous in diploid parent cell Pair of homologous in diploid parent cell Duplicated pair of homologous Chromosomes duplicate Duplicated pair of homologous Chromosomes duplicate Sister chromatids Diploid cell with duplicated Meiosis I Sister chromatids Diploid cell with duplicated 1 Homologous separate Haploid cells with duplicated Figure 13.7-3 Interphase Meiosis I Meiosis II Pair of homologous in diploid parent cell Duplicated pair of homologous Sister chromatids 1 Chromosomes duplicate Homologous separate Haploid cells with duplicated 2 Sister chromatids separate Diploid cell with duplicated Remember Meiosis happens to form gametes the reproductive cells (sperm and eggs) The cells that produce the gametes start out diploid before meiosis, and will end up haploid There are two stages of Meiosis: Meiosis I and II Each Stage of Meiosis has Prophase, Metaphase, Anaphase, and Telophase Haploid cells with unduplicated 5
Meiosis Overview 1. DNA already replicated are duplicated (two chromatids), the cell is diploid (2n). This happened in Interphase. 2. Meiosis 1: homologous separate and the cell divides resulting in two haploid cells (1n) 3. Meiosis 2: The chromatids separate and then the cell divides resulting in four haploid cells (1n) BioFlix: Meiosis Homologous Chromosomes in Prophase I During Prophase I the homologous are attracted to each other and become associated with each other forming a tetrad. The process of homologous pairing up during prophase I is called synapsis. A tetrad contains two, both are duplicated so there are four chromatids. Crossing Over Prophase I: Duplicated homologous condense and intertwine this produces genetic variation Crossing over: genetic material is exchanged between the homologous The sites of crossing over are called chiasmata (singular, chiasma) Crossing Over Prophase I 35 36 6
Metaphase I Independent Assortment During Metaphase I homologous pairs of line up the at the center of the cell (the equator) The tetrads arrange themselves randomly this also gives genetic variation = independent assortment (alignment) 37 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 Independent Assortment Anaphase I 42 7
Anaphase I Telophase I In anaphase I, pairs of homologous 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 44 Telophase I and Cytokinesis In the beginning of telophase I, each half of the cell has a haploid set of ; each chromosome is duplicated, still consisting of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells End of Telophase I - Cytokinesis We now have two haploid cells (1n) which means there are 23 total in each cell The are still in the duplicated form two chromatids Note: not all species have cytokinesis after telophase I Interphase Interphase between Meiosis I and II is brief. The S phase does not take place Preparation for Meiosis II: centrosome replicates Meiosis II Prophase II: The 23 are already condensed. The Nuclear membrane dissolves. Metaphase II: Chromosomes line up at the equator Anaphase II: Chromatids separate Telophase II and cytokinesis: Cells separate Now there are four haploid cells: each has 23 (not in the duplicated state) 8
Figure 13.8b Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Prophase II During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated. Sister chromatids separate Haploid daughter cells forming 50 Metaphase II Anaphase II 51 52 Telophase II At the end of Meiosis I how many cells are there? 1. One 2. Two 3. Three 4. Four 25% 25% 25% 25% 53 One Two Three Four 9
At the end of Meiosis I are these cells haploid or diploid? At the end of Meiosis I, how many are there in each cell? 1. Haploid 2. Diploid 1. 23 2. 46 Haploid Diploid 23 46 At the end of Meiosis I, are the chomosomes in the duplicated state? At the end of Meiosis II how many cells are there? 1. Yes 2. No 1. One 2. Two 3. Three 4. Four 25% 25% 25% 25% Yes No One Two Three Four At the end of Meiosis II are these cells haploid or diploid? At the end of Meiosis II, how many are there in each cell? 1. Haploid 2. Diploid 1. 23 2. 46 Haploid Diploid 23 46 10
At the end of Meiosis II, are the in the duplicated state? Three events are unique to meiosis, and all three occur in meiosis l 1. Yes 2. No Synapsis and crossing over in prophase I: Homologous physically connect and exchange genetic information At the metaphase plate, there are paired homologous (tetrads), instead of individual replicated At anaphase I, it is homologous, instead of sister chromatids, that separate Yes No Genetic diversity through meiosis Independent Assortment There are three places in this process that contribute to the genetic diversity of the offspring. Prophase I: The pairs of crossing over. Metaphase I: The way the line up on the equator is random = independent assortment 64 Figure 13.11-1 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis The number of combinations possible when assort independently into gametes is 2 n, where n is the haploid number For humans (n = 23), there are more than 8 million (2 23 ) possible combinations of 11
Figure 13.11-2 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Figure 13.11-3 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Chiasma Centromere Centromere TEM TEM Anaphase I Figure 13.11-4 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Figure 13.11a Chiasma Chiasma Centromere TEM Anaphase I Centromere Anaphase II TEM Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations Animation: Genetic Variation Rightclick slide / select Play 12
Meoisis and Gender The gametes now contain 23, haploid, and are not in the duplicated form One of these will be a sex chromosome Eggs will contain a X chromosome Sperms will contain either a X or a Y chromosome X and Y are non-homologous Spermatogenesis In the male testes sperm are produced. One cell produces 4 sperm. Each sperm has 23, they are not in the duplicated form The sperm can have either an X or a Y sex chromosome The sperm have a small head and a long tail = flagellum for locomotion 13
The sperm need to contain the genetic material and deliver it to the egg. The heads contain the and lots of mitochondria to power the flagella About 400 million sperm are produced each day Egg Formation cont All of the cells that produce the eggs are made before the female mother is even born. So when a girl is born, her ovaries contain all the cells that produce her eggs Each month one of these cells will leave the ovary and go on to mature and produce the egg and polar bodies Egg Formation The ovaries in females produce eggs One cell will produce one egg and three non-functioning polar bodies The one egg gets most of the cytoplasm, leaving the other three cell not able to survive The one egg has 23, with a X sex chromosome The one egg is large enough to support the embryo Fertilization Fertilization is the union between the sperm and the egg. Results in a diploid zygote. 14
Review of Mitosis vs Meiosis Mitosis and Meiosis both start with a diploid cell (46, 23 pairs) Before both Mitosis and Meiosis the DNA replicates during interphase, forming duplicated, each containing two chromatids Mitosis occurs in somatic cells (cells other than those that produce the gametes), Meiosis produces gametes Mitosis During Mitosis: The chromatids are separated to produce two cells, each with 46, 23 pairs of non duplicated These cells are diploid (2n) cells There is no exchanging of genetic material Meiosis Two stages Meiosis I: the pairs of line up and the are separated, resulting in 2 cells, each with 23, in the duplicated state = haploid cells Meiosis II: The chromatids are separated producing two haploid cells that contain 23 non duplicated. One original cell produces four haploid cells Important concepts Know all the vocabulary presented in the lecture Know which cells undergo mitosis vs meiosis How is genetic diversity introduced into meiosis? What events contribute to genetic diversity and when (what stage of meiosis) do these events take place How is the gender of the offspring determined. 89 15
Important Concepts How many functioning sperm are produced from one spermatocyte. What sex can a sperm have? How many functioning eggs are produced from one oocyte? What sex do eggs have? Important Concepts for Lab Exam For Meiosis: Know each stage, the order of the stages, and what happens in each stage. Know what the end result is of meiosis I and II Know what state the cell and the are in at the beginning and end of mitosis, meiosis I and at the end of meiosis II. For example: Are the cells haploid or diploid? Are the duplicated, or not duplicated? How many are there in the cell? Are they in pairs? 16