Presentation on theme: "Meiosis and Sexual Life Cycles"— Presentation transcript:
1Meiosis and Sexual Life Cycles Chapter 13Meiosis and Sexual Life Cycles
2Inheritance/Heredity When traits are passed down from one generation to the next, we say they are inherited.The offspring differ from one generation to the next and we use the term variation to describe these differences.
3GeneticsWhen people study genetics, they are studying the heredity and inherited variations.For example, when we say we have our parent’s nose or eyes, we are actually saying that we have their segments of DNA (genes)
4GenesGenes are regions of our genome that program the specific traits we see (phenotype) such as eye color or nasal appearance.
5So, how does this happen?The exact replication of our genes/DNA in our gametes which produces copies that can be passed from one generation to the next.
6So, how does this happen?Remember, our gametes are the sperm and egg that unite during fertilization and give rise to the new individuals.
7So, how does this happen?The new individuals now have traits of both parents.Remember, the chromosome that is inherited is tightly coiled and contains a few hundred to a few thousand genes.
8The LocusThe locus is the region of the chromosome where a particular gene is located.
9Modes of ReproductionWhen discussing heredity, there are a couple of modes of reproduction that need to be discussed.Sexual and asexual.
10Sexual Reproduction2 parents “mate” and give rise to offspring that have a variety of combinations of genes which are inherited by the new offspring.They are not genetically identical to the parents.
11Asexual Reproduction A single parent gives rise to the offspring. In the process, all of the parent’s genes are passed on to the offspring and a clone is formed.
12Asexual ReproductionA hydra budding is an example of asexual reproduction.Movie
13The Sexual Lifecycle--Humans Somatic cells are the cells other than the sperm and egg.Each one contains 46 c-somes (23 pairs, 1 pair from each parent).Each parent contributes 22 autosomes and 1 sex c-some.
14Homologous Chromosomes Homologous c-somes are c-somes of the same number.Each homologous c-some contains genes that control the same inherited characteristic.For example, if the gene that codes for height is found on one c-some, it is found at the exact same locus on the homologous c-some.
16Homologous Chromosomes--Sort Of… The only two c-somes that don’t really fit the idea of being homologous are the sex c-somes, the X and the Y.The two sets of genes on these c-somes are more different than they are alike.They determine the sex of the individual: XX-female; XY-male
17Chromosome Number-Autosomes When discussing c-some number, we use an “n” to represent this idea.Any cell having both copies of c-somes from the parent are said to be diploid which is 2n.In humans, 2n=46.
18Chromosome Number-Sex Chromosomes The gametes of sexually reproducing organisms contain a single c-some set and are said to be haploid.n=23 (22 autosomes + 1 sex c-some).
20FertilizationWhen 2 haploid cells fuse together, a diploid zygote is formed.The zygote now contains a mix of genes from the parents.It will divide mitotically and give rise to a mature organism.
21Sexual MaturityAs the organism reaches sexual maturity, its gonads will begin producing haploid cells through a process called meiosis.
22MeiosisMeiosis will allow the chromosome number to remain constant throughout the generations.
23Meiosis and Fertilization The process of meiosis and fertilization are common to sexually reproducing organisms.
243 Main Types of Life Cycles in Sexually Reproducing Organisms 1. Animals2. Plants and some algae3. Fungi and some protists
25AnimalsIn animals, meiosis occurs in the production of gametes and the gametes are the only haploid cells.The gametes unit during fertilization.Mitosis gives rise to a diploid, multicellular organism.
26Plants and Some Algae Alternation of Generations A complex reproductive event where a plant gives rise to spores which gives rise to gametophytes.The gametophytes give rise to gametes that fertilize forming a zygote that becomes a new plant.
27Most Fungi and Some Protists The adult is a haploid, multicellular organism.Mitosis produces gametes that fertilize forming a zygote.Meiosis then produces haploid cells which divide mitotically to form another haploid, multicellular organism.
293 Main Types of Life Cycles in Sexually Reproducing Organisms All 3 result in meiosis and mitosis.All 3 accomplish the same thing: chromosome halving and doubling which results in genetic variation among offspring.
30Meiosis Meiosis reduces the ploidy. Provides for genetic variation. Keeps the c-some number constant through the generations.
31MeiosisMeiosis can be broken into two parts: Meiosis I and Meiosis II.Meiosis I separates homologous c-somes.Meiosis II is essentially a mitotic division.
32MeiosisMeiosis halves the number of chromosomes.
40Meiosis II Sister Chromatids Separate Prophase II: Chromosome condense again, the nuclear envelope disappears and the mitotic spindle reforms.Movie
41Meiosis II Sister Chromatids Separate Metaphase II: C-somes line along the metaphase plate.Movie
42Meiosis II Sister Chromatids Separate Anaphase II: Sister chromatids separate and migrate as individual c-somes to the spindle poles.Movie
43Meiosis II Sister Chromatids Separate Telophase II: C-somes arrive at the poles, the spindle breaks down, the nuclear envelope reforms.Cytokinesis: The cytoplasm divides.Movie
44Mitosis and Meiosis, A Comparison The biggest difference is that in mitosis, genetically identical cells are formed and ploidy is conserved.In meiosis, genetically different cells are formed and the ploidy is reduced.
463 Main Events Important to Meiosis 1. Synapsis and Crossing Over2. Tetrad Formation3. Separation of Homologues
471. Synapsis and Crossing Over In prophase I, duplicated homologues line up and become physically attached.Crossing over occurs where genetic information is rearranged between the non-sister chromatids comprising the tetrad.An “X” shaped region appears called the chiasma at the site of crossing over.
492. Tetrad FormationThe paired tetrads align along the metaphase (I) plate rather than the individual replicated c-somes (as in mitosis).
503. Separation of Homologues At anaphase I of meiosis, the duplicated c-somes of each homologous pair move to opposite poles, sister chromatids remain attached (instead of separating as they do in mitosis).
513 Mechanisms that Produce Genetic Variation 1. Independent Assortment2. Crossing Over3. Random Fertilization
521. Independent Assortment Homologous c-somes can be positioned independently at the metaphase plate.Maternal and paternal c-somes sort independently of every other pair.
531. Independent Assortment Independent assortment give a large number of possible daughter cells.The cells are dividing in two and are sorting independently.The number of combinations is 2n.“n” is the haploid number (23 in humans)223 possible combinations (8,000,000+)
542. Crossing Over Gene mixing. Non-sister chromatids trade places. Taking crossing over into account makes the number of possible combinations endless.
553. Random FertilizationThe different combinations of gametes produced during meiosis will randomly fertilize upon mating. This produces the endless amount of gamete combinations.