Providing Genetic Variation Meiosis Providing Genetic Variation

Genes, Chromosomes, and Numbers Chromosomes occur in pairs. Body cells have two of each kind of chromosome. Diploid = 2n Organisms have two factors – alleles – for each trait One allele is located on each paired chromosome.

The Need for Meiosis In order to reproduce, organisms must pass on their DNA (chromosomes) to their offspring. What would happen if organisms gave a complete set of chromosomes to their offspring? Ex. Organism has 10 chromosomes. How many chromosomes would its offspring have? How many would its grandchildren have? Great-grandchildren?

Diploid and Haploid Cells Gametes contain one of each kind of chromosome. Haploid=1N or N (n = number of chromosomes) Parent organisms give one allele, for each trait to each of their offspring.

Human Karyotype

Chromosomes Each species of organisms contains a characteristic number of chromosomes. Large range of numbers among species Not related to complexity of organism

Homologous chromosomes Paired chromosomes = homologous chromosomes Have genes for the SAME trait arranged in the SAME order However, they are NOT identical Ex. Even though both have hair color alleles, one allele could be for dark hair, and other other could be for light hair.

Meiosis Two separate divisions – Meiosis I and II Meiosis I starts with one diploid cell (2n) By the end of Meiosis II there are four haploid (n) cells In animals and most plants these are called gametes or sex cells.

SEX CELLS! Male gametes = sperm Female gametes = eggs A fertilized egg is called a zygote. This pattern of reproduction that involves production and fusion of haploid sex cells is called sexual reproduction.

Human Spermatogenesis and Oogenesis

The Phases of Meiosis Consist of 2 stages (Meiosis I and Meiosis II) https://www.youtube.com/watch?v=toWK0fIyFlY

Interphase - Review Cell carries out its usual metabolic activities Replicates its chromosomes

Prophase I Chromosomes come together to form a tetrad Consists of 2 pairs of homologous chromosomes (4 total) Nuclear envelope disappears

Crossing Over Homologous chromosomes pair up & form tetrads May exchange portions of their chromatids Results in an exchange of alleles to produce new combinations

Metaphase I Spindle fibers attach to the chromosomes Chromosome pairs (tetrads) are arranged in the middle or equator of the spindle Homologous chromosomes are lined up together in pairs

Anaphase I Spindle fibers separate homologous pairs and pull them to opposite sides of the cell Centromeres do not split as they do in mitosis This step ensures that each new cell will receive only one chromosome from each homologous pair

Telophase I Spindle fibers break down Chromosome uncoil The cytoplasm divides to yield two new cells Nucleus and nuclear envelope reform Each cell has only half of the genetic information of the original cell

Cytokinesis Cytoplasm splits to form 2 haploid cells with sister chromatid

Prophase II Spindle reforms Chromosomes condense 2 haploid cells

Metaphase II Chromosomes line up on the equator of the cell attached to spindle fibers

Anaphase II Centromere splits Sister chromatids separate and move to opposite sides of the cell

Telophase II Cytoplasm divides Results in four haploid (n) daughter cells

Genetic Recombination Peas have 7 pairs of chromosomes Peas can form up to 128 different kinds of sperm Humans have 23 pairs of chromosomes They can form up to 8 million different kinds of sperm or eggs

Why Meiosis? Genetic Diversity Cells formed by mitosis are identical to each other and the parent cell Meiosis shuffles chromosomes and genetic information

Unique Individuals In addition to recombination, crossing over helps provide an endless number of possible chromosomes. This is called genetic recombination.

Mitosis Meiosis Number of Divisions 1 2 Number of Cells Made 2 Diploid Cells 4 Haploid Cells Cells Made Are Identical to Original Cell Different from Original Cell Purpose Growth and Replacement of Body Cells Asexual Reproduction in Some Organisms Sexual Reproduction