Non-Fatal Injuries • Zipper: 1: 2,600 • Toilet: 1: 6,500 • Alarm Clock: 1: 350,000 • Bed: 1: 400
Fatal Injuries 1 • Snake Bite: 1: 36,000,000 • Dog Bite: 1: 20,000,000
Fatal Injuries 2 • Ebola: 1: 14,000,000 • Appendicitis: 1: 700
Fatal Injuries 3 • Earth Destroyed By Meteor: 1: 20,000 • You Alone Are Killed By Meteor: 1: 150,000,000,000
Unlikely… • 3.2 billion nucleotides in human genome • 97% of this is intron (non-coding), dropping the number of coding base pairs to 210,000 • Two parents = 2^210,000 • 1 chance in 10^63,000 • However, only ~108 billion humans have EVER been born.
Probability and Genetics • Probability: The determination of certain outcomes based upon the number and type of possible outcomes • Genetics: The determination of heritable outcomes due to the passage of DNA
If the central dogma dictates that DNA is copied in its entirety, and mitosis ensures that each daughter cell gets the same information as the previous, why do organisms need to be different?
Genetic Variety • Genetic variety ensures that populations/species continue to survive despite changes in their ecosystems • Populations that do not change genetically must have environments that rarely change
Sources of Genetic Variety • Mutations • Point Mutations (Single Nucleotide Polymorphisms) • Frameshift/Nonsense Mutations • Chromosomal Mutations
7 Billion Individuals v. 3 Million SNP’s How Is Such Variety Possible If 99.9% of our Genes are identical?
Sources of Genetic Variety v 2.0 • Sexual Reproduction Builds In Genetic Variety
Asexual Reproduction • Def: Reproduction requiring only one parent/genetic source • i.e. binary fission, parthenogenesis, budding • PRO: Quick, Fast, Low Requirements • CON: Low Genetic Variety
Sexual Reproduction • Def: Reproduction requiring two parents/genetic sources • i.e. conjugation, fertilization • PRO: Genetic Variety • CON: Need water, partners and GAMETES
GAMETES • Def: Specialized reproductive cells (i.e. sperm, ovum) • Gametes contain half the genetic content of other cells in the organism (yet still contain all of the genes) • To produce another generation, gametes must be fused together to produce a ZYGOTE
Somatic (body) cells are DIPLOID (two sets of chromosomes) In each diploid cell, there are pairs of homologous chromosomes containing the same number and type of genes but NOT necessarily coding for identical proteins GAMETES are HAPLOID (only one set of chromosomes) Diploid v. Haploid
How does a cell go from diploid to haploid and yet retain all of the genes necessary for the organism?
Meiosis • To develop gametes, a cell must undergo MEIOTIC DIVISION (i.e. MEIOSIS)
Mitosis occurs in somatic (body) cells One diploid cell produces two diploid cells Daughter cell is genetically identical to parent cell Meiotic division only occurs in ovary or testes cells Meiosis contains two sets of steps/divisions One diploid cell produces four haploid cells All resultant cells have half the genetic material as the original cell Mitosis v. Meiosis
Tetrads/Bivalents • NON-SISTER HOMOLOGOUS CHROMOSOMES move next to each other, forming groups of four called TETRADS or BIVALENTS
SYNAPSIS • The non-sister homologous chromosomes, pressed together as tetrads, may “swap” genes between their NON-SISTER HOMOLOGOUS CHROMOSOMES • Creates “hybrid” chromosomes carrying the same traits but different “versions”
Meiotic Metaphase 1 • Unlike meiosis, the centromeres of the sister chromatids are not lined up directly on the equatorial plane • Homologous pairs are arranged side by side on either side of the equator, with one spindle fiber attaching to each centromere. • Because they are not on the equator, there are two ways to place the homologous pairs
Meiotic Anaphase (1) • Homologous pairs separate, but the sister chromatids DO NOT.
Meiotic Telophase & Interkinesis • Same actions as mitotic telophase • However, after cell has divided into two diploid cells, DNA REPLICATION STOPS • This period of time is called INTERKINESIS
Why is the lack of DNA replication during interkinesis essential in the formation of gametes?
Metaphase II • As in mitotic division, the sister chromatids line up above each other with the centromeres directly on top of the equator
Anaphase II • Now that spindle fibers have been attached to each side of the centromere, the retraction of the spindle fibers caused the centromere to break
Telophase II & Cytokinesis • Four cells are created by the second anaphase division • Resultant cells have less cytoplasm but also half the genetic material (HAPLOID))
Oogenesis v. Spermatogenesis • The meiotic division of testes cells produces four viable spermatids • The meiotic division of an ovary cell/ovum is unequal in terms of cytokinesis • Only one of four egg cells is viable
Why does the double division of meiosis actually increase genetic diversity?
Lab: Modeling Meiotic Division • With your lab partner(s), build two sets of homologous chromosomes, • The first pair of homologues should contain 10 genes (i.e. beads), 5 on each side of the centromere. • The second pair of homologues should contain 6 genes, 3 on each side of the centromere • Use two different colors to differentiate between the homologues. • Replicate both homologues, connecting each to its sister chromosome with a magnetic centromere. You should have 8 total chromosomes at this point
Lab: Modeling Meiotic Division Part 2 • Move the homologous chromosomes & their attached sister chromatids through the various steps of meiotic division • Include synapsis of two genes in the first homologous pair and one gene in the second homologous pair as part of Prophase 1 • Your lab group will be checked for understanding. One step of the processes (mitosis AND meiosis) will be selected • Complete the analysis questions #1-10
Modeling Meiosis Analysis Questions • 1) How does meiosis accurately resemble the old adage, “The more things change, the more they stay the same”? Be specific in what is changing and what is continuous • 2) Why is a consistency of chromosome number an important prerequisite of successful sexual reproduction? • 3) Spores are cells produced by meiosis that can independently grow into a new plant. If spores are produced by meiosis, why is the new generation not the product of sexual reproduction?
Modeling Meiosis Analysis Questions cont. • 4) Why is sexual reproduction the most efficient way of bringing genetic variety to a population? • 5) The ovaries and testes of pre-pubescent humans must grow and develop, yet do not produce sperm or eggs. How is cellular division different during these years than post-puberty? • 6) Monera and Protista are both single celled organisms, yet while there are 20,000 different Monera, there are 100,000 different Protista. Why?
Modeling Meiosis Analysis Questions cont. • 7) How does the fact that horses and donkeys have different numbers of chromosomes (48 and 50) explain why mules are sterile? • 8) Why is there no synapsis during mitotic division? • 9) Why is there no synapsis between sister chromatids? • 10) Patau Syndrome is a fatal genetic condition caused by three copies of chromosome 15. Explain how this condition occurs in every cell of the fetus, while neither parent has the condition.