Lecture 14: Meiosis + Chromosome Abnormalities PDF

Lecture 14: Meiosis + Chromosome Abnormalitie s  So basically, you are growing because of mitosis, and you make gametes for reproduction with meiosis  Learning Objectives Learn about different gamete types Understand the advantages of sexual reproduction Know th...

Lecture 14: Meiosis + Chromosome Abnormalitie s  So basically, you are growing because of mitosis, and you make gametes for reproduction with meiosis  Learning Objectives Learn about different gamete types Understand the advantages of sexual reproduction Know the difference between mitosis and meiosis Know the stages of meiosis Sexual Reproduction vs Asexual Reproduction Sexual Reproduction: The process that gives rise to offspring that have unique combinations of genes inherited from their biological parent(s). Asexual Reproduction: The process that gives rise to offspring that are genetically identical to the reproducing individuals. Many species can reproduce sexually and asexually Chlamydomonas spp. Volvocine algae Hallman et al. 2010 Adaptive Significance of Sexual Reproduction Disadvantages Advantages Complicated Increases genetic Costly variation and diversity Time and energy to find between individuals and mate within populations Investment into mating strategies Potentially dangerous Visible to predation Disease from mate Not aways successful  Sexual Reproduction = Genetic & Phenotypic Variability 1. Genetic material from two parents combined 2. Independent assortment of chromosomes during meiosi 3. Recombination during meiosis (crossing over)  Gametes Gametes – haploid reproductive cells, they unite during sexual reproduction to form a zygote Gametes are classified based on their size and mobility, which is used to assign mating Zygo types or sexes te:  Gamete Variation Isogamy: Anisogamy: Oogamy: Gametes are Gametes are Gametes are same size different sizes different sizes Immobile egg Mobile sperm May or may not be mobile May or may not be mobile  Meiosis Meiosis: a type of cell division used in sexual reproduction to create gametes by reducing the number of chromosomes in a cell. In meiosis, cell division happens twice! This creates four unique daughter cells with half the number of chromosomes as the parent cell Meiosis Overview  Gametes (sperm or egg)  Meiosis vs. Mitosis Mitosis produces two diploid daughter cells that are genetically identical ex: Each daughter cell receives copies of all chromosomes in starting cell (starting cell has 6 chromosomes, so do daughters) Meiosis produces four haploid daughter cells ex: Each daughter one complete set of chromosomes (starting cell had 6 chromosomes, daughter cells have 3)  An Overview of Meiosis I During meiosis I, the diploid (2n) parent cell produces two haploid (n) daughter cells The homologs in each chromosome pair: Separate via Law of Independent Assortment 2n (metaphase I) May swap chromosome segments via crossing- over (prophase I) n Go to different daughter cells This creates two haploid (n) daughter cells Each cell consists of two sister chromatids that So what's the difference? Mitosis is for growth Meiosis is for reproduction Moo Deng needs your help  Only ~2000 pygmy hippos left Changes in temp and precipitation have led to more extreme droughts Coupled with habitat destruction from people Currently Endangered Man made Moo climate Deng’s change future  Independent Assortment Independent Assortment: Describes how different genes independently separate from each other when reproductive cells are developing Alleles of at least two genes are sorted into gametes independently of one another The allele inherited by one gamete does not affect the allele inherited by the other gametes….the alleles are not “linked together” Happens in metaphase I Opportunities to align in different ways  Terminology Allele: one of 2+ versions of a DNA sequence at a given loci (genomic location) “Alternate version of a gene” Individuals inherit two alleles (one from each parent) Homozygous: has 2 version of the same allele  Ex: (BB or bb) Heterozygous: has different versions of an allele  Ex: (Bb)  Independent Assortment These are the alleles: A, a, B, b Ch1 Ch1 Ch2 Ch2 A Aa a A Aa a Parent cell before meiosi It is diploid (2n) Ch1 Ch1 Ch1 Ch1 Ch2 Ch2 Ch2 Ch2 BB b b BB b b  Independent Assortment These are the alleles: A, a, B, b Ch1 Ch1 Ch2 Ch2 A Aa a A Aa a Parent cell before meiosi It is diploid (2n) Ch1 Ch1 Ch1 Ch1 Ch2 Ch2 Ch2 Ch2 BB b b BB b b  Independent Assortment For a diploid species, the possible number of different random alignment during Metaphase I = 2n Where n = number of chromosomes in a haploid cell The previous slide showed only 2 chromosomes in a haploid cell… 22 = 4 possible alignments Humans have 23 chromosomes per haploid cell 223= 8,388,608 possible alignments!! What does this enhance? Draw the possible ways a diploid cell with 3 chromosomes could independently assort in Metaphase I. (it might help to use two different colors for the maternal and paternal chromosomes and to make the three chromosomes different sizes in your drawing) HAVE THIS READY FOR NEXT CLASS AND WE’LL GO OVER THE ANSWER   Homologous Recombination (Crossing-Over) Crossing Over (Homologous Recombination): exchange of genetic information between non-sister chromatid (i.e., homologs) Produces recombinant chromosomes Happens in prophase I When the homologous are touching each other Increases genetic variation! It does NOT: create new genes, reduce or increase likelihood of mutation It DOES: create new combinations of your parents’ genes Homologous Recombination (Crossing- Over)  Homologous Recombination (Crossing-Over)  Homologous Recombination (Crossing-Over) Where do the chromatid cross over? There are three locations in this example? Can you find them? X Xx x X Xx x X X x x y Y Yy y y y Y Yy Y Y Z Zz z Z Zz z z Z z Z Draw two chromosomes that cross over at two sites…. And draw their 4 subsequent gametes? HAVE THIS READY FOR NEXT CLASS AND WE’LL GO OVER THE ANSWER   An Overview of Meiosis II During meiosis II, the haploid (n) daughter cells split to make four unique haploid (n) daughter cells. 2n The sister chromatids in each cell pair: Separate n No crossing over or independent assortment here! Go to different daughter cells n Creates four unique haploid daughter cells that each consists of one set of unique  How does meiosis increase genetic variability? How does crossing over further increase genetic variability? Variations in Chromosome Banding Bakloushinskaya et al. 201 Stained with Giemsa (a chemical dye that binds to DNA) Hence the name G-bands! There are other banding methods, too (R-banding, C- banding…) Produces alternating light (G-C rich) and dark (A-T This is a MALE karyotype for this species! rich) bands in the All mole voles lack a y chromosome! Likely the genes controlling male- chromosome determination mutated onto a different chromosome and the Y- chromosome was lost. Variations in Chromosome Shape Name based on centromere location during Metaphase Form p p p p Centromere p p p p q q q q q q q q Metacentric Submetacentric Acrocentric Telocentric Short arms of chromosomes are labeled with a p Long arms of chromosomes are labeled with a q Variations in Chromosome Size Chromosome characteristics are useful in karyotyping Karyotype: lab-produced image of an organism’s complete chromosome set Useful in looking for chromosomal abnormalities, species/hybrid identification, and identifying polyploids Paladino et al. 2013 Chromosome Mutations Chromosome Mutations  Changes in Chromosome Number Aneuploidy = Variations in number of individual chromosomes Ex: Trisomy 21 (extra copy of chromosome 21 in humans) Trisomy = 2n+1 chromosomes Monosomy = 2n-1 chromosomes Polyploidy = Variations in number of chromosome sets Diploid = 2n = 2 chromosome sets Ophioglossum reticulatum I have 96 sets of chromosomes! Triploid = 3n = 3 chromosome sets A total of 1440 chromosomes  Karyotyping me is a botanist’s worst nightm Changes in Chromosome Number Normal 2n fruit fly Polyploid 3n fruit fly (it’s a triploid Polyploid 4n fruit fly (it’s a tetraplo 2n fruit fly with Trisomy 2 and Monosomy 3  How do these major chromosome mutations happen? Nondisjunction: An event where chromosomes do not separate properly during meiosis I or meiosis II This produces gamete cells that have too many or too few chromosome numbers or sets The gametes should have three chromosomes each, if meiosis had happened correctly! The gametes on the left have an extra Creating all aneuploidy gametes Creating half aneuploidy gametes  Aneuploidy in Humans About 5-10% of all fertilized human eggs result in an embryo with an abnormality in chromosome number (trisomy or monosomy) Approximately 50% of all spontaneous abortions (fetus failing to thrive within uterus) in humans are 1/1,500 (females) due to alterations in number of chromosome numbers Most are lethal, but some are not*! Trisomy 21 (Down’s Syndrome), 18*, (Edward’s Syndrome), 13** (Patau Syndrome) Sex chromosomes (XXY, XO, XYY, XXX) Life expectancy: rarely survive to year ** Life 1 expectancy: rarely survive to year 10 How does polyploidy happen? Autopolyploidy = within one single species Misfunctioning spindle fibers/microtubules a frequent c  Polyploidy in Humans Polyploidy is found in some critical tissues: liver and placenta But at the organism level – lethal in humans Polyploidy of the fetus is a common cause of 1st trimester pregnancy loss, occurs in about 2-3% of pregnancies and cause ~15% of miscarriages Triploids (69 chromosomes) can be liveborn, but it is 100% lethal and the neonate will die shortly after birth Often caused by fertilization of an egg by two sperm Tetraploids (92 chromosomes) can form and will fail to thrive early in the pregnancy, 100% lethal  Tetraploid Polyploidy in other Tetraploid organisms (NOT FOUND IN MAMMALS) Hexaploid Triploid https://www.eurekalert.org/news-releases/88857 http://web.ewu.edu/ewflora/Asteraceae/Centaurea%20stoebe.htm Tetraploid www.treehugger.com/natural-sciences/11-facts-about-coast-redwoods Tetraploid ps://www.pmq.com/how-to-make-your-yeast-ensure-peak-performance/ Octoploid Hexaploid http://www.agrifarming.in/strawberry-farming/ https://en.wikipedia.org/wiki/Gossypium_hirsutum www.flickr.com/photos/96039972@N00/4190570128