Meiosis and Sexual Life Cycles AP Biology Chapter 13

Hereditary Similarity and Variation Living organisms have the ability to reproduce their own kind. Heredity is the transmission of traits from one generation to the next.

Along with inherited similarity, offspring differ somewhat in appearance from parents and siblings. Genetics is the study of heredity and hereditary variation.

Inheritance of Genes Parents give offspring coded information in the form of hereditary units called genes The tens of thousands of genes we inherit constitute our genome. Genes are segments of DNA.

The transmission of hereditary traits has its basis in the precise replication of DNA. DNA produces copies of genes that can be passed from parents to offspring. Gametes are the reproductive cells that transmit genes from one generation to the next.

The DNA in a eukaryotic cell is subdivided into chromosomes Except for tiny amounts in chloroplasts and mitochondria Every living species has a characteristic number of chromosomes Ex. Humans have 46

A gene’s specific location along the length of a chromosome is called the gene’s locus (pl. loci).

Asexual vs. Sexual Organisms that reproduce asexually produce offspring that are exact copies of themselves. In asexual reproduction a single individual is the sole parent and passes copies of all it genes to offspring. A hydra is a multicelluar organism that can reproduce asexually through mitotic division. It gives rise to a clone.

In sexual reproduction 2 parents give rise to offspring that have unique combinations of genes The offspring are genetically different from the parents and their siblings.

Life Cycle The generation-to-generation sequence of stages in the reproductive history of an organism From conception to production of offspring.

Chromosomes in Human Cells In humans somatic cells (any cell other than a gamete) contain 46 chromosomes A karyotype is a display of condensed chromosomes arranged in pairs. The two homologous chromosomes have the same length, centromere position, and staining pattern. Sex chromosomes determine an individual’s sex. The other chromosomes are the autosomes.

Homologous Pairs Inherit one chromosome of each pair from each parent The number of chromosomes in a single set is represented by n. A cell with 2 sets is called diploid (somatic cells) Ex. for humans 2n = 46 A cell with 1 set is called haploid (gametes) Ex. For humans n= 23.

The human life cycle begins when a sperm fuses with an egg- fertilization 46 + 92 46

The Human Life Cycle In each generation the doubling of the # of chromosome sets that results from fertilization is offset by the halving of the # of chromosome sets that results from meiosis. The all cells of the human body are produced by mitosis, except the gametes, which develop in the gonads , they are produced by meiosis.

Variety of Sexual Life Cycles Three main types of life cycles

Humans, and most animals Gametes are the only haploid cells. Meiosis occurs during the production of gametes only. The diploid zygote divides by mitosis –producing a multicellular organism.

Plants and some algae Alternation of generations. Diploid and haploid multicellular stages The haploid gametophyte makes gametes by mitosis. Fertilization among haploid gametes results in a diploid zygote Zygote develops into a sporophyte which produces spores thru meiosis. Spores divide by mitosis to produce a gametophyte.

Fungi and some protists After gametes fuse and form a diploid zygote, meiosis occurs without a diploid offspring developing. Meiosis produces haploid cells (not gametes), that divide by mitosis and give rise to a haploid multicellular adult organism. The haploid organism carries out mitosis, producing cells that develop into gametes. The only diploid stage is a single cell zygote.

How Meiosis Reduces Chromosome Number The chromosomes replicate in interphase The diploid cell divides twice Producing 4 haploid daughter cells.

Mitosis vs. Meiosis Property Mitosis Meiosis DNA Replication Occurs during Interphase before mitosis begins Occurs during interphase before meiosis I begins. Number of divisions One Two Crossing over of homologous chromosomes Does not occur Occurs during prophase I, forming tetrads # of daughter cells 2 4 Role in animal body Enables multicellular adult to arise from zygote; produces cells for growth and tissue repair Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes.

3 events unique to meiosis Synapsis and Crossing over Homologous chromosomes line up and connect to form tetrads Each tetrad contains a chiasma (chiasmata-pl.) where crossing over occurs

Tetrads on the metaphase plate Metaphase I paired homologous chromosomes are positioned on the plate instead of individual chromosomes, as in mitosis.

Separation of homologues Anaphase I duplicated chromosomes of each homologous pair move toward opposite poles. Sister chromatids remain attached.

Genetic Variation and Evolution Mutations are the original source of genetic diversity Creating different versions of genes Reshuffling of the versions during sexual reproduction produces unique combination of traits.

Independent Assortment of Chromosomes Variation is generated by the random orientation of homologous chromosomes during metaphase I. Each homologous pair is positioned independently of the other pairs 1st division results in each pair sorting its maternal and paternal homologues into daughter cells independently.

The number of possible combinations when chromosomes sort independently during meiosis is 2n, n= haploid # Ex. In humans n = 23; so the number of possible combinations is 223 or about 8 million.

Crossing Over Crossing over produces recombinant chromosomes that carry different genes (DNA) derived from two different parents. Begins in prophase I The DNA molecules of two nonsister chromatids of a homologous pair are broken at the same place and rejoined to each other’s DNA. In humans, an average of 1 to 3 crossover events / chromosome

Random Fertilization Adds to genetic variation. Each male and female gamete represents on of appr. 8 million possible combinations Fertilization will produce a zygote with any of about 64 trillion diploid combinations (8 million x 8 million) With crossing over the possibilities increase.

Evolutionary significance Natural selection results in the accumulation of those genetic variations favored by the environment. As environments change the population may survive if, in each generation, at least some of its members can cope effectively with the new conditions.

Different genetic variations may work better than those that previously prevailed.

Darwin and Mendel Although Darwin realized that heritable variation is what makes evolution possible, he could not explain why offspring resemble – but are not identical- to their parents.

Ironically, Gregor Mendel, a contemporary of Darwin, published a theory of inheritance that helps explain genetic variation, but his discoveries had no impact on biologists until 1900, more than 15 years after Darwin (1809-1882) and Mendel (1822-1884) had died. “Brother Mendel, we grow tired of peas”.