Overview: Variations on a Theme Living organisms are distinguished by their ability to reproduce their own kind 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 Genetics is the scientific study of heredity and variation

It is genes that are actually inherited-the plans for the traits Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes In a literal sense, children do not inherit particular physical traits from their parents It is genes that are actually inherited-the plans for the traits In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Meiosis is a specialized type of cell division involved in sexual reproduction Weisman

Sets of Chromosomes in Human Cells Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes (n = 23, 2n =46) A karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes, or homologs Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters

Human 46XY Karyotype 5 µm Pair of homologous replicated chromosomes Centromere Figure 13.3 Preparing a karyotype Sister chromatids Metaphase chromosome

The sex chromosomes differ in different sexes sexes-in humans they are called X and Y Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome-they are only partly homologous The 22 pairs of chromosomes that do not determine sex are called autosomes

Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father A 2n cell is called diploid and an n cell haploid

Cartoon of a diploid cell with homologous chromosome sets Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Figure 13.4 Describing chromosomes Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

Concept 13.2: In sexual life cycles meiosis alternates with fertilization A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism Most familiar organism have a life cycle that involves sexual reproduction Contribution of genetic material from two individuals of opposite mating types

Concept 13.2: In sexual life cycles meiosis alternates with fertilization Sexual life cycles rely on meiosis-specialized form of cell division Meiosis of germ line cells gives rise to specialized reproductive cells called gametes (the sperm and the egg in humans) Fertilization is the union of gametes that are produced by meiosis

Concept 13.2: In sexual life cycles meiosis alternates with fertilization The fertilized egg is called a zygote and has one set of chromosomes from each parent The zygote produces somatic cells by mitosis and develops into an adult

Sexual life cycles are frequently observed The alternation of meiosis and fertilization is common to all organisms that reproduce sexually The three main types of sexual life cycles (gametic, sporic, zygotic) differ in the timing of meiosis and fertilization

In animals, meiosis produces gametes, which undergo no further cell division before fertilization Germ line cells including gametes are the only haploid cells in animals Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism Sometimes called gametic cycle

Key Haploid (n) Diploid (2n) Gametes n n n MEIOSIS FERTILIZATION Figure 13.6a Three types of sexual life cycles—animals Zygote 2n 2n Diploid multicellular organism Mitosis (a) Animals

Plants and some algae exhibit an alternation of generations This life cycle includes both a diploid and haploid multicellular stage The diploid organism, called the sporophyte, makes haploid spores by meiosis

Each spore grows by mitosis into a haploid organism called a gametophyte A gametophyte makes haploid gametes by mitosis Fertilization of gametes results in a diploid sporophyte Sometimes called sporic life cycle

(b) Plants and some algae Key Haploid (n) Haploid multi- cellular organism (gametophyte) Diploid (2n) Mitosis Mitosis n n n n n Spores Gametes Figure 13.6b Three types of sexual life cycles—plants and some algae MEIOSIS FERTILIZATION 2n 2n Zygote Diploid multicellular organism (sporophyte) Mitosis (b) Plants and some algae

In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage The zygote produces haploid cells by meiosis Each haploid cell grows by mitosis into a haploid multicellular organism Sometimes called zygotic cycle The haploid adult produces gametes by mitosis

Haploid unicellular or multicellular organism Diploid (2n) Key Haploid (n) Haploid unicellular or multicellular organism Diploid (2n) Mitosis Mitosis n n n n Figure 13.6c Three types of sexual life cycles—most fungi and some protists Gametes n MEIOSIS FERTILIZATION 2n Zygote (c) Most fungi and some protists

Summary-3 life cycles Gametic-haploid phase is…. Sporic-haploid phase is…… Zygotic-haploid phase is……..

Why bother with complex life cycles and strategies??? Because they provide an enormous evolutionary advantage to species that use them: genetic diversity.

Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis, meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II One replication followed by the two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell

The Stages of Meiosis In the first cell division (meiosis I), homologous chromosomes (each with two sister chromatids) separate from each other and go to daughter cells In the second cell division (meiosis II), sister chromatids separate Each division has prophase, metaphase anaphase and telophase. The interphase between Meiosis I and II is usually very short or even nonexistent.

The single centrosome replicates, forming two centrosomes 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 So far just like interphase of mitosis For the Cell Biology Video Meiosis I in Sperm Formation, go to Animation and Video Files.

Chromosomes begin to condense 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 not like mitosis

Each pair of chromosomes forms a tetrad, a group of four chromatids joined together In crossing over or reecombination , nonsister chromatids exchange DNA segments Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred Crossing over allows exchange of genetic information between homologous chromosomes

Tetrad This is not like mitosis

Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one homologous chromosome facing each pole Microtubules attach homologous chromosomes to opposite spindle poles The alignment of homologous chromosomes is random. There is a 50:50 chance they will align in any one direction This is not like mitosis

Cartoon Version Of Prophase I And Metaphase I 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-synapsis is over 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 This is not like mitosis

Telophase I and Cytokinesis 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 usually occurs simultaneously, forming two haploid daughter cells

Cytokinesis usually takes place but daughter cells might not receive an equal share of cytoplasm There is no S phase between Meiosis I and II

Cartoon version of Anaphase I and Telophase I 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

Prophase II In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate 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

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

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

Cartoon Version of Anaphase II and Telophase II 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

Three events are unique to meiosis (compared to mitosis), 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- but maternal and paternal homologs assort independently! – At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

Summary-3 Unique Events in Meiosis Synapsis/Crossing Over-Prophase I Independent Assortment of Homologs-Metaphase I Separation of Homologs-Anaphase I

Concept 13.4: 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 known as alleles Reshuffling of maternal and paternal alleles during sexual reproduction helps to increase genetic diversity

Three mechanisms contribute to the increase in genetic diversity in sexual reproduction: Crossing over (Prophase I) Independent assortment of chromosomes (Metaphase and Anaphase I) Random fertilization (following meiosis) THE ABOVE LIST IS IMPORTANT

Crossing Over Crossing over produces recombinant chromosomes, which include genes inherited from each parent Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

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

Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs

(independent assortment) Fig. 13-11-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I (independent assortment) Figure 13.11 The independent assortment of homologous chromosomes in meiosis Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 No crossover shown!

This makes possible many combinations The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes at each meiosis.

Random Fertilization At fertilization, one haploid sperm cell penetrates the haploid unfertilized egg (ovum) to generate a diploid zygote. A random event-any sperm can fuse with any ovum (humans not a good example here) Random fertilization adds to genetic variation because it produces new combinations of chromosomes in the zygote.

Sperm Cells In males, the products of meiosis develop into mature sperm cells There are many of them No two are likely to have identical genetic information due to crossing over and independent assortment

Independent assortment Egg Cells A very different developmental pathway In humans-only one product of female meiosis goes on to become an egg cell Large cell with huge cytoplasm Genes reassorted due To crossing over and Independent assortment

The fusion of two human gametes (each with about 8 million possible chromosome combinations from independent assortment) produces a zygote with any of about 64 trillion diploid combinations There are 64 trillion possible chromosome outcomes of each human fertilization!

Independent assortment of chromosomes SUMMARY Three mechanisms contribute to genetic variation in sexual reproduction: Crossing over Independent assortment of chromosomes Random fertilization THE ABOVE SUMMARY IS IMPORTANT