Meiosis & Sexual Life Cycles

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Meiosis And Sexual Life Cycle
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Meiosis & Sexual Life Cycles Chapter 13

Offspring acquire genes from parents Genes are segments of DNA that program specific traits. Genetic info is transmitted as specific sequences of the four deoxyribonucleotides in DNA Most genes program cells to synthesize specific enzymes and other proteins which combine to form the organisms traits. Almost all of the DNA in a eukaryotic cell is subdivided into chromosomes in the nucleus. Living species have a characteristic number of chromosomes. Chromosomes consist of DNA and various proteins Each chromosome has hundreds or thousands of genes, each at a specific location or locus

Asexual Reproduction Single individual is the sole parent to donate genes to its offspring Single-celled eukaryotes (like ameba) can reproduce asexually by mitotic cell division Some multicellular eukaryotes, like Hydra, can reproduce by budding, producing a mass of cells by mitosis An individual that reproduces asexually gives rise to a clone genetic differences among clones may arise through mutations

Sexual Reproduction Two parents produce offspring that have unique combinations of genes inherited from the two parents. Offspring vary genetically from their siblings and parents. Life cycle= generation-to-generation sequence of stages in the reproductive history of an organism It begins at conception of an organism and continues until the organism produces its own offspring.

Human chromosomes Somatic cells = body cells (all cells other than egg or sperm) have 46 chromosomes Chromosomes can be distinguished by size, position of the centromere, pattern of staining with certain dyes. Homologous chromosomes = pairs of chromosomes that carry genes that control the same inherited characters. Homologous chrom. Have the same length, centromere postion and staining pattern. A karyotype is an image of the 46 chromosomes arranged in pairs in order of size. Sex chromosomes: X and Y. These are not homologous – different genes are present on X and Y Human females have XX Human males have XY Autosomes: non-sex chromosomes 22 pairs

Sexual reproduction results in homologous pairs of chromosomes Humans inherit one chromosome of each homologous pair from each parent Number of chromosomes in a single set is represented by n Gametes (sperm and ovum) have one set of chromosomes → 22 autosomes and an X or a Y Gametes have the haploid number of chromosomes Any cell with two sets of chromosomes is called diploid and has a diploid number of chromosomes represented by 2n. Human haploid number = 23 Human diploid number = 46

The Human Life Cycle Begins when haploid sperm fuses with haploid ovum (syngamy = fusion) the fusion results in fertilization which produces a diploid zygote Mitosis of the zygote produces the somatic cells of the body Gametes are produced by meiosis which halves the chromosome number in these cells.

Organisms display a variety of sexual life cycles Fertilization and meiosis alternate in all sexual life cycles Timing of meiosis and fertilization varies among species Three main types of life cycles: Most animals including humans – gametes are the only haploid cells Plants and some algae have alternation of generations – includes a multicellular haploid (gametophyte) and a multicellular diploid (sporophyte) phase Fungi and some protists: zygote is the only diploid phase.

Meiosis reduces the chromosome number Meiosis is preceded by replication of chromosomes. Two divisions Meiosis I = separates homologous chromosomes Meiosis II = separates sister chromatids Results in 4 haploid daughter cells.

Details of Meiosis I Prophase I – occupies more than 90% of time required for meiosis Chromosomes condense Homologous chromosomes pair up = synapsis = forms a tetrad Crossing over may occur Metaphase I Anaphase I – homologous pairs separate Telophase I & cytokinesis

Details of Meiosis II No chromosome replication occurs between meiosis I and meiosis II Prophase II Metaphase II – sister chromatids are arranged at the metaphase plate Anaphase II – sister chromatids separate Telophase II & cytokinesis

Differences between mitosis & meiosis Chromosome number is reduced from diploid to haploid in meiosis but is conserved in mitosis Mitosis produces daughter cells tha are genetically identical to the parent and to each other Meiosis produces cells that are genetically distinct from the parent cell and from each other

Events unique to meiosis Synapsis occurs during prophase I Homologous pairs line up along the metaphase plate during metaphase I Homologous chromosomes separate during anaphase I

Genetic variation that results from sexual reproduction contributes to evolution Mutations are the original source of genetic diversity Shuffling of genes during meiosis and fertilization produce offspring with their own unique set of traits.

Three mechanisms contribute to genetic variation Independent assortment of chromosomes Random orientation of homologous pairs of chromosomes at the metaphase plate during meiosis I. Each homologous pair segregates independently of the other homologous pairs The number of combinations possible when chromosomes assort independently is 2n For humans where n=23 there are 223 (more than 8 million) possible combinations of chromosomes

Three mechanisms contribute to genetic variation (cont.) 2. Crossing over produces recombinant chromosomes, which combine genes inherited from each parent. Crossing over occurs during synapsis in prophase I Homologous portions of two nonsister chromatids trade places. In humans this occurs an average of one to three times per chromosome pair

Three mechanisms contribute to genetic variation (cont.) 3. Random fertilization adds to genetic variation Any sperm can fuse with any egg The ovum is one of more than 8 million possible chromosome combinations The sperm is one of more than 8 million possible chromosome combinations The resulting zygote could contain any one of more than 70 trillion possible combinations of chromosomes. Crossing over adds even more variation to this.

Genetic variation and evolutionary adaptation A population evolves through the differential reproductive success of its variant members The individuals best suited to the local environment leave the most offspring, transmitting their genes in the process. If the environment changes or a population moves to a new environment, new genetic combinations that work best in the new conditions will produce more offspring.