Chapter 10Meiosis 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 use the symbol 2n to represent 2 sets of chromosomes.

How many homologous pairs of chromosomes do humans have?____

4. Haploid (n or 1n): When a cell, such as a sex cell (gamete), contains one set of chromosomes, it is said to be haploid.

n= 1 set of chromosomesThe haploid number in a gamete=23n = 23 (half of 46)

Meiosis Organism Haploid Chromosome #

Mosquito 3

Corn 10

Human 23

Horse 32

Calculate the diploid chromosome number for each organism .....

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 = maleXX chromosomes = femaleBecause 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 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, chromosomes 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• Law of Independent Assortment– states that

the alleles of different genes separate independently of one another during meiosis. Meaning that different traits are inherited separately. (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

• The DNA exchange that occurs during crossing-over adds even more recombination (variety) to the independent assortment of chromosomes that occurs later in meiosis.

• There are many genes on a single chromosome each with its own locus, or place, on the chromosome. Two genes on the same chromosome may be close together of far apart.

• Genes located close together tend to be inherited together, which is called genetic linkage.

• This is essential to evolution

Genetic LinkageA linkage map shows the relative locations of

genes on a chromosome (not the actual location). The percentage of cross-overs has been converted into a unit of measurement called map units.

Random Fertilization

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 by 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)

Types of 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 – Advantages: powerful means of quickly

making different combinations of genes among individuals.

– Disadvantages: takes up energy in finding mates

Advantages and Disadvantages