Meiosis and Sexual Reproduction Chapter 12 Meiosis and Sexual Reproduction

The diagram can be used to illustrate a process directly involved in? A) tissue repair B) Meiosis C) Recombination D) sexual reproduction

A sperm cell of an alligator has 16 chromosomes A sperm cell of an alligator has 16 chromosomes. What is the total number of chromosomes normally present in a stomach cell of this alligator? A) 8 B) 16 C) 32 D) 48

A) meiotic cell division B) mitotic cell division C) fertilization The diagram represents a single-celled organism, such as an ameba, undergoing the changes shown. This could best be described as? A) meiotic cell division B) mitotic cell division C) fertilization D) recombination

12.1 Why Sex? In asexual reproduction, a single individual gives rise to offspring that are identical to itself and others In sexual reproduction, two individuals mix their genetic material

Introducing Alleles Somatic (body) cells of sexually- reproducing eukaryotes contain pairs of homologous chromosomes: One from the mother and one from the father Homologous chromosomes: Carry genes (one from the mother and one from the father) of the same characteristics Different forms of the same gene are called alleles

Introducing Alleles Paired genes on homologous chromosomes may vary in DNA sequences as alleles Arise by mutation Are the basis of differences in shared traits Offspring of sexual reproducers inherit new combinations of parental alleles Results in new combinations of traits

Introducing Alleles Genes occur in pairs on homologous chromosomes. The members of each pair of genes may be identical, or they may differ slightly, as alleles. Figure 12.2 Genes on chromosomes. Different forms of a gene are called alleles. A B

On the Advantages of Sex Sex mixes up the genes of two parents, so offspring have unique combinations of traits Diversity offers a better chance of surviving environmental change than clones

12.2 Why Is Meiosis Necessary for Sexual Reproduction? Asexual reproduction produces clones Sexual reproduction mixes up alleles from two parents Involves the fusion of gametes, mature reproductive cells Meiosis: a nuclear division mechanism that occurs in reproductive cells of eukaryotes

Meiosis Halves the Chromosome Number Gametes are specialized cells that are the basis of sexual reproduction Derive from germ cells: immature reproductive cells All gametes are haploid (n), but they differ in other details Gamete formation differs among plants and animals

testis ovary anther ovary Figure 12.4 {Animated} Examples of reproductive organs in A animals and B plants. ovary B

Meiosis Halves the Chromosome Number The two consecutive nuclear divisions are called meiosis I and meiosis II.

Meiosis Halves the Chromosome Number In males, the diploid germ cell develops into sperm In females, a diploid germ cell becomes an egg

Meiosis Halves the Chromosome Number Occurs in the gametes – mature reproductive cells Gametes are haploid (n) Their chromosome number is half of the diploid (2n) number Meiosis is a nuclear division

Meiosis Halves the Chromosome Number Nuclear division that halves the chromosome number in the reproductive cells Ensures that offspring have the same number of chromosomes as the parents

Meiosis Halves the Chromosome Number DNA replication occurs prior to meiosis: The nucleus is diploid (2n) with two sets of chromosomes, one from each parent During meiosis, chromosomes of a diploid nucleus become distributed into four haploid nuclei Meiosis halves diploid (2n) chromosome to the haploid (n) number for forthcoming gametes

Meiosis Halves the Chromosome Number 1 2 Figure 12.5 How meiosis halves the chromosome number. 1 Chromosomes are duplicated before meiosis begins. During meiosis I, each chromosome in the nucleus pairs with its homologous partner. The nucleus contains two of each chromosome, so it is diploid (2n). 2 Homologous partners separate and are packaged into two new nuclei. Each new nucleus contains one of each chromosome, so it is haploid (n). The chromosomes are still duplicated. 3 Sister chromatids separate in meiosis II and are packaged into four new nuclei. Each new nucleus contains one of each chromosome, so it is haploid (n). The chromosomes are now unduplicated. 3

Fertilization Restores the Chromosome Number Haploid gametes form by meiosis The diploid chromosome number is restored at fertilization Two haploid gametes fuse a zygote is formed A zygote is a cell formed by the fusion of two gametes The first cell of a new individual

Haploid gametes (n  23) Egg cell n n Sperm cell MEIOSIS FERTILIZATION Multicellular diploid adults (2n  46) Diploid zygote (2n  46) Figure 8.12 The human life cycle 2n MITOSIS and development Key Haploid (n) Diploid (2n) Figure 8.12

Fertilization Restores the Chromosome Number If meiosis did not precede fertilization, the chromosome number would double with every generation Chromosome number changes can have drastic consequences in animals An individual’s set of chromosomes is like a fine-tuned blueprint that must be followed exactly to have normal functions

Figure 8.13-3 Homologous chromosomes separate. Chromosomes duplicate. Sister chromatids separate. Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Figure 8.13 How meiosis halves chromosome number. (Step 3) INTERPHASE BEFORE MEIOSIS MEIOSIS I MEIOSIS II Figure 8.13-3

(before chromosome duplication) MITOSIS MEIOSIS Prophase Prophase I MEIOSIS I Chromosome duplication Chromosome duplication Duplicated chromosome (two sister chromatids) Parent cell (before chromosome duplication) 2n  4 Homologous chromosomes come together in pairs. Site of crossing over between homologous (nonsister) chromatids Metaphase Metaphase I Chromosomes align at the middle of the cell. Homologous pairs align at the middle of the cell. Anaphase Telophase Anaphase I Telophase I Chromosome with two sister chromatids Sister chromatids separate during anaphase. Figure 8.15 Comparing mitosis and meiosis Homologous chromosomes separate during anaphase I; sister chromatids remain together. Haploid n  2 2n 2n Daughter cells of meiosis I Daughter cells of mitosis MEIOSIS II Sister chromatids separate during anaphase II. n n n n Daughter cells of meiosis II Figure 8.15

12.4 How Meiosis Introduces Variations in Traits Two events in meiosis introduce novel combinations of alleles into gametes: Crossing over in prophase I Segregation of chromosomes into gametes Along with fertilization, these events contribute to the variation in combinations of traits among the offspring of sexually reproducing species

Crossing Over in Prophase I Chromatids of homologous chromosomes condense and align along their length and exchange segments Introduces novel combinations of traits among offspring

Figure 8.18-5 Prophase I Duplicated pair of of meiosis homologous chromosomes Homologous chromatids exchange corresponding segments. Chiasma, site of crossing over Metaphase I Spindle microtubule Sister chromatids remain joined at their centromeres. Metaphase II Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 5) Gametes Recombinant chromosomes combine genetic information from different parents. Recombinant chromosomes Figure 8.18-5

Crossing Over During meiosis, chromosomes cross over each other This allows for genetic shuffling and increases genetic variability Offspring have a unique combination of genes inherited from both parents

Chromosome Segregation When homologous chromosomes separate in meiosis I, one of each chromosome pair goes to each of the two new nuclei For each chromosome pair, the maternal or paternal version is equally likely to end up in either nucleus

Independent Assortment of Chromosomes or Chromosome Segregation Metaphase of meiosis I Metaphase of meiosis II Figure 8.16 Results of alternative arrangements of chromosomes at metaphase of meiosis I. (Step 3) Gametes Combination a Combination b Combination c Combination d

Chromosome Segregation Human gametes have 23 pairs of homologous chromosomes Each time a human germ cell undergoes meiosis the four gametes that form end up with one of 8,388,608 (or 223) possible combinations of homologous chromosomes Crossing over increases these combinations If each gamete represents one of 8,388,608 different chromosome combinations, at fertilization, humans would have 8,388,608 × 8,388,608, or more than 70 trillion, different possible chromosome combinations.

Chromosome Segregation The chance that the maternal or paternal version of any chromosome will end up in a particular nucleus is 50 percent Due to the way the spindle segregates the homologous chromosome during meiosis I In prophase I, chromosomes are attached to spindle poles Each homologous partner becomes attached to opposite spindle poles

12.6 Application: How To Survive 80 Million Years Without Sex In nature, there are a few all-female species of fishes, reptiles, and birds but not mammals Females have been reproducing themselves for 80 million years through cloning themselves Asexual reproduction is a poor long-term strategy because it lacks crossing over that leads to genetic diversity

How To Survive 80 Million Years Without Sex Figure 12.11 A bdelloid rotifer. All of these tiny animals are female.

NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II Pair of homologous chromosomes fails to separate. Meiosis II Nondisjunction: Pair of sister chromatids fails to separate. Gametes Figure 8.20 Two types of nondisjunction. (Step 3) Number of chromosomes n  1 n  1 n – 1 n – 1 n  1 n – 1 n n Abnormal gametes Abnormal gametes Normal gametes

Abnormal egg cell with extra chromosome n  1 Normal sperm cell Figure 8.21 Fertilization after nondisjunction in the mother. Normal sperm cell Abnormal zygote with extra chromosome 2n  1 n (normal)

LM Figure 8.22 Trisomy 21 and Down syndrome. Chromosome 21

Infants with Down syndrome 90 80 70 60 Infants with Down syndrome (per 1,000 births) 50 40 30 20 Figure 8.23 Maternal age and Down syndrome. 10 20 25 30 35 40 45 50 Age of mother

Why crossing over is important… A male bird with large wings and weak muscles mated with a female bird with small wings and strong muscles. Describe all the possible characteristics that could occur in their offspring. Would any combination(s) be advantageous? Would any combination(s) be disadvantageous? How could these variations affect the survival of the organisms?

10/8 Protein Synthesis 9 10/10 Genes 10 10/15 Mitosis 11 10/17 Meiosis 12 10/22 Genotype & Phenotype 13 10/24 Biotechnology 14 10/29 Exam #2 10/31 Begin Evolution