Mitosis and Cell Division • All cells go through a cell cycle composed of cell division portion (mitosis) and "resting period" (interphase).
Comparing Mitosis and Meiosis • Observational activity with the person next to you… look at the two processes…..( pages 249 and 273) • What do these two processes seem to have in common? • Be prepared to share your results with the class!
Some questions to ponder • 1. How does the number of daughter cells produced from mitosis and meiosis differ? • 2. How does the ploidy of the daughter cells produced from mitosis and meiosis differ? • 3. Do the daughter cells produced from mitosis contain identical genetic complements? • 4. Do any of the daughter cells produced from meiosis contain identical genetic complements?
5. When do the homologous chromosomes separate during mitosis? • 6. When do the homologous chromosomes separate during meiosis? • 7. When do sister chromatids separate during mitosis? • 8. When do sister chromatids separate during meiosis?
Vocabulary section 1 • gene, • homologous chromosomes, • haploid, • fertilization, • diploid, • meiosis, • crossing over
gene • A gene is a discrete linear sequence of DNA which corresponds to a trait that can be passed on by that sequence of DNA. In more general terms, one can think of genes as the smallest unit of heredity.
Homologous chromosomes • Example - an organism is 2n = 4. • This means the organism has a total of 4 chromosomes • Chromosomes 1 & 2 are homologous chromosomes • Chromosomes 3 & 4 are homologous chromosomes • Chromosomes 1 & 3 came from the mother • Chromosomes 2 & 4 came from the father
Haploid or n • A cell with half the number of chromosomes as the normal number is called haploid. This is represented by the letter “n”. • Gametes or sex cells are haploid so that when they fuse to form a zygote the correct number or chromosomes is restored!
Diploid or 2n • Diploid cells have the total number of chromosomes in them. These are somatic cells or body cells. If these were to fuse, the number of chromosomes would be double the correct number! • This is why the process of meiosis is needed!
2 sets of 23 make up the 46 in a human. • 1 set from mother 23 • 1 set from father 23 • 46
A quick glance at any karyotype will tell you one of the most important facts about chromosomes: • They come in pairs. • The members of a pair are the same size and shape, and they have the same banding patterns. • In other words, each person actually possesses two copies of chromosome 1, two copies of chromosome 2, and so on. • Human cells contain 23 pairs of chromosomes.
Vocabulary section 1: • gene, • homologous chromosomes, • haploid, • fertilization, • diploid, • meiosis, • crossing over
Meiosis • Meiosis Is a Special Type of Cell Division That Occurs in Sexually Reproducing Organisms • Why is this different? • Normal body cells have a complete set of chromosomes • _ Meiosis reduces the number of chromosomes by half to form reproductive cells.
If normal body cells from mom and dad fused to form a baby, the fertilized egg would have twice as many chromosomes as it should. • Meiosis is sometimes called "reduction division" because it reduces the number of chromosomes to half the normal number • so that when fusion of sperm and egg occurs, the offspring will have the correct number.
Meiosis reduces the chromosome number by half, enabling sexual recombination to occur. • Meiosis of diploid cells produces haploid daughter cells, which may function as gametes. • Gametes undergo fertilization, restoring the diploid number of chromosomes in the zygote
Meiosis is NOT sexual reproduction • It does produce cells used in sexual reproduction! • Gametes!
Sexual or Asexual? • Is there an advantage to one or the other as far as helping a population survive?
The Role of Sexual Reproduction in Evolution Sexual reproduction in a population should decline in frequency relative to asexual reproduction because... • Asexual reproduction—No males are needed, all individuals can produce offspring. • Sexual reproduction—Only females can produce offspring, therefore fewer are produced. • Sexual reproduction may exist because it provides genetic variability that reduces susceptibility of a population to pathogen attack.
purpose • the purpose of meiosis is to produce gametes; the sperm and eggs that have ½ the genetic information of a somatic cell. • When the reproductive cells unite in fertilization, the normal diploid number is restored.
Meiosis • Interphase I: Identical to Interphase in mitosis.
Prophase I: Chromosomes condense and become visible Homologous chromosomes come together in pairs to form tetrads in a process called synapsis Chromatids exchange segments in a process called crossing over Rearranges genetic information and increases diversity This is Different from mitosis!!! • Nuclear envelope breaks down and disappears Mitotic spindle forms
Homologous chromosomes contain the matching alleles donated from mother and father. This is also when meiotic recombination, also know as "crossing over" occurs. This process allows for a genetic shuffling of the characteristics of the two parents, creating an almost infinite variety of possible combinations. See the close-up diagram below.
This is also when meiotic recombination, also know as "crossing over" occurs. • This process allows for a genetic shuffling of the characteristics of the two parents, creating an almost infinite variety of possible combinations.
Metaphase I: • Instead of all chromosomes pairing up single file along the midline of the cell as in mitosis, homologous chromosome pairs line up next to each other. • Chromosome tetrads align at the cell midline • Mitotic spindle fibers attach to centromeres • This is Different • from mitosis!!!
Anaphase I • Instead of chromatids splitting at the centromere, • homologous chromosome pairs (now shuffled by crossing over) move along the spindle fibers to opposite poles. • Anaphase I: • Homologous chromosomes separate to opposite poles of the cell • Sister chromatids stay together!!!
Telophase I • The cell pinches and divides, making 2 cells • Are the cells still diploid?
Telophase I: • Chromosomes arrive at the poles • Cytokinesis results in the formation of two haploid cells with duplicated chromosomes
Summary of Meiosis 1 • During meiosis I, homologous chromosomes separate. • Crossing-over during prophase I results in the exchange of genetic material between homologous chromosomes.
Prophase II: • It is visibly obvious that replication has not occurred again.
Metaphase II: • The paired chromosomes line up.
Anaphase II: • The chromatids split at the centromere and migrate along the spindle fibers to opposite poles.
Telophase II: • The cells pinch in the center and divide again. • The final outcome is four cells, • each with half of the genetic material found in the original.