MEIOSIS CHAPTER 13
CENTRAL DOGMA OF BIOLOGY DNA RNA Protein
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.
IN HUMANS All human body cells have 46 chromosomes (coiled up DNA). Every cell MUST have the same amount of chromosomes. WHY does every cell have the same amount of chromosomes?
SOMATIC CELLS: The 46 chromosomes come in pairs These pairs are called homologous chromosomes
HOMOLOGOUS CHROMOSOMES Homologous Chromosome: Chromosomes that are the SAME (length and traits). Key Point: They are similar, but NOT IDENTICAL
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.
KARYOTYPE A karyotype is an image of the 46 chromosomes arranged in pairs in order of size.
HUMAN CHROMOSOMES 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
MALE OR FEMALE?
MALE OR FEMALE?
MALE OR FEMALE?
CHROMOSOME NUMBER How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis? If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?
DIPLOID CELLS A diploid when the cell has a full set of chromosomes ( ½ from mom and ½ from dad) Somatic cells are DIPLOID cells For Humans: DIPLOD CELL = 46 Chromosomes
HAPLOID CELLS A haploid when the cell has half the set of chromosomes Sex cells are HAPLOID cells For humans a diploid cell has 46 chromosomes. How many chromosomes would a sex cell (haploid) have?
FRUIT FLY CHROMOSOMES Not all organisms have the same number of chromosomes Fruit Fly Example: Somatic Cell has 8 Chromosomes Four chromosomes come from its male parent; Four come from its female parent These two sets of chromosomes are called:
DIPLOID CELLS Diploid: Full set of homologous chromosomes Represented by the symbol 2N Fruit Fly: the diploid number is 8. (2N = 8) Humans: 2N = 46
HAPLOID CELLS Haploid: Half set of chromosomes Gametes (sex cells à sperm, egg) are haploid N = single set of chromosomes in a sperm or egg cell Fruit Fly: Haploid Number is 4 (N = 4) Humans: N=23
GAMETES TO ZYGOTES Haploid cells produced by meiosis are gametes: male animals gametes à sperm plants pollen contain haploid sperm cell female animals gametes à eggs plants - egg cell contain haploid cells Fertilization the fusion of male and female gametes generates new combinations of alleles in a zygote Zygote undergoes cell division by mitosis and eventually forms a new organism
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.
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.
MITOSIS: MAKES Diploid 46 Chromosomes Diploid 46 Chromosomes Diploid 46 Chromosomes Creates.
Meiosis: Makes Diploid 46 Chromosomes Haploid 23 Chromosomes Haploid 23 Chromosomes Haploid 23 Chromosomes Haploid 23 Chromosomes Creates.
CHECK FOR UNDERSTANDING: 1. Unlike mitosis, meiosis results in a) 4 identical, diploid cells b) 2 genetically different haploid cells c) 2 genetically different diploid cells d) 4 genetically different haploid cells
CHECK FOR UNDERSTANDING 1. If a horse has 12 chromosomes in its gametes, how many chromosomes are in it s skin cells? a) 12 b) 24 c) 48 d) 6 2. If there are 10 chromosomes in the parent cell at the start of meiosis, how many chromosomes will be in the daughter cells at the end of meiosis? a) 10 b) 20 c) 5 d) 40
CHECK FOR UNDERSTANDING Question: What is the relationship between the number of chromosomes in the zygote and the number of chromosomes in a gamete?
MITOSIS OR MEIOSIS? Makes gametes Makes somatic cells Creates haploid cells Creates diploid cells Asexual reproduction
MITOSIS OR MEIOSIS? Creates cells with the same number of chromosomes as the original Involved in sexual reproduction Creates cells with ½ the number of chromosomes as the original Creates 4 daughter cells Creates genetically identical offspring
CHECK FOR UNDERSTANDING 1. If human were to have 46 chromosomes in its skin cells, how many chromosomes are in a diploid cell? 2. What about a haploid cell? 3. In Humans, N=?
CHECK FOR UNDERSTANDING In spinach, 2N = 12. What is the haploid number? How many chromosomes are in a spinach leaf? The Red King Crab has 208 chromosomes. What is the diploid number? How many chromosomes are in the sperm cells? The Great White Shark has 82 chromosomes. What is N? How many chromosomes would be in a brain cell?
CHECK FOR UNDERSTANDING Name an example of a cell that is a GAMETE. An organism has 15 chromosomes in its sperm cell. How many chromosomes will be in that organism s skin cell (somatic cell)? Mitosis ends with a (Diploid or Haploid) cell. Meiosis ends with a (Diploid or Haploid) cell.
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 that 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 1. 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 2 n For humans where n=23 there are 2 23 (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.