Chapter 10 Meiosis and Sexual Reproduction

Chromosome Numbers: 1.All sexually reproducing organisms have pairs of chromosomes. 2.Homologous Chromosomes: are pairs (2) of chromosomes that are similar in size, shape, and genetic content Each homologous chromosome comes from one of the two parents. Example: The 46 chromosomes in human somatic cells (body cells) are actually 2 sets of 23 chromosomes—1 set from the mother - -1 set from the father.

3.Diploid (2n):When a cell, such a somatic cell, contains sets of chromosomes, it is said to be diploid. The diploid number in a somatic cell can be written as 2n=46. **Biologists us the symbol 2n to represent 2 sets of chromosomes. How many homologous pairs of chromosomes do humans have?__23___ 4.Haploid (n): When a cell, such as a sex cell (gamete), contains one set of chromosomes, it is said to be haploid. n= 1 set of chromosomes The haploid number in a gamete=23 n = 23 (half of 46)

Chromosomes Fertilization is the joining of gametes (sex cells_ from opposite sexes. A zygote is a fertilized egg cell, the first cell of a new individual. Fertilization Zygote

Chromosomes Determine Your Sex: Of the 23 pairs of chromosomes in human somatic cells, 22 pairs are called autosomes. Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual. The sex chromosomes, one of the 23 pairs of chromosomes in humans, contain genes that determine the sex of an individual. In humans the two sex chromosomes are X and Y chromosomes. XY chromosomes = male XX chromosomes = female Because a female can donate only an X chromosome to her offspring, the sex of an offspring is determined by the male, who can donate either an X or a Y.

Meiosis OrganismDiploid Chromosome # Mosquito 6 Corn 20 Human 46 Horse 64 Calculate the haploid chromosome number for each organism.....

Meiosis Forms Haploid Cells Meiosis – form of cell division that halves the number of each chromosome when forming specialized reproductive cells (gametes or spores) Involves two divisions of the nucleus  Meiosis I and Meiosis II

Meiosis I Prophase I – Homologous chromosomes pair all along their length and then cross- over. Crossing-over occurs when parts of a chromatid on one homologous chromosome are broken and exchanged with the corresponding parts on one of the chromatids of the other h.chromosome.

Metaphase I – the pairs of h.chromosomes line up along the equator as in mitosis Anaphase I – H.chromosomes separate like mitosis. Chromatids do not separate at the centromere…each chromosome is still composed of 2 chromatids! Telophase I – Individual chromosomes gather at each pole forming two new cells. The chromosomes DO NOT replicate between Meiosis I and Meiosis II !

Meiosis II Prophase II – A new spindle forms around each single chromosome Metaphase II – Once again, chroms line up along the equator Anaphase II – The centromeres divide, and the chromatids move to the opposite sides of the cell. Telophase II – A nuclear envelope forms around each set of chromosomes. Result is 4 haploid cells.

Meiosis Contributes to Genetic Variation 3 mechanisms make key contributions to this genetic variation: independent assortment, crossing-over, and random fertilization. Chromosome pairs separate independently Independent assortment – random distribution of homologous chromosomes during meiosis. Gametes with different gene combinations can be produced from one original cell by this mechanism. (each segregates independently. Ex. Human n = 23. 2^23= ~8 million possibilities)

Independent Assortment Metaphase of Meiosis I Metaphase of Meiosis II Gametes with different possible combos

Crossing-over, independent assortment, and random fertilization The DNA exchange that occurs during crossing-over adds even more recombination (variety) to the independent assortment of chromosomes that occurs later in meiosis. This is essential to evolution Random fertilization- which sperm wins the race to the egg???

Male and Female Gamete Formation involves Meiosis Spermatogenesis – process by which sperm are produced (original single cell called germ cell …final product is four undifferentiated sperm cells.) Oogenesis – process b y which gametes are produced in females (original cell called a germ cell …final product is a single egg cell or ovum. The other 3 polar bodies will be reabsorbed)

Sexual Reproduction Reproduction, the process of producing offspring, can be asexual or sexual. Asexual reproduction – single parent passes copies of all its genes to each of its offspring; no fusion of haploid cell. New individual is a clone (genetically identical) Sexual reproduction – two parents each form haploid reproductive cells, which join to form offspring. Offspring has traits of both parents, but are not exactly like each parent.

Types of Asexual Reproduction Fission – separation of a parent into two or more individuals of about equal size (exs. Amoebas, bacteria- binary fission) Fragmentation – reproduction in which the body breaks into several pieces. Budding – new individuals split off from existing ones (ex. Hydra)

Advantages and Disadvantages Asexual is the simplest and most primitive method. – Advantage: In a stable environment, it allows organisms to produce many offspring in a short period of time w/o using energy to produce gametes or find a mate. – Disadvantage: DNA does not vary which could cause adaption problems if the environment changes.

Sexual reproduction – thought to have evolved as a way to originally repair damaged DNA (remember the G2 checkpoint) – Advantages: powerful means of quickly making different combinations of genes among individuals. – Disadvantages: takes up energy in finding mates

Euks have 3 Kinds of Sexual Life Cycles Life cycle – entire span in the life of an organism from one generation to the next. Sexually reproducing organisms follow a basic pattern alternating between diploid and haploid chrom numbers. Haploid life cycle – haploid cells occupy the major portion of the life cycle. The zygote is the only diploid cell, and it undergoes meiosis immediately after its formed creating new haploid cells (ex. Some algae and molds).

Diploid life cycle – (ex. Humans and most other animals) adult individuals are diploid, each individual inheriting chroms from two parents. Gametes join thru fertilization to create a diploid zygote. Zygote then divides by mitosis to create all the other body cells. Alternation of generations – (plants, algae, and some protists) alternate between diploid and haploid. – In the diploid phase, sporophytes undergo meiosis to produce spores (haploid reproductive cell that doesn’t have to fuse with another cell.

– Spore gives rise to a gametophyte which is the haploid phase that produces gametes by mitosis. These gametes fuse to give rise back to the diploid phase. The 3 types of sexual life cycles differ from each other only in which phases become multicellular. End of Chapter 7 notes.