Biology 2c03 Week 3 notes

Cell Division and Chromosome Heredity  Relevance  Fundamental property of all organisms is reproduction   Asexual: fission, budding, hyphal extension(fungi)(mitosis)   Sexual: meiosis and mating  Chromosomes store genetic information   Important to understand how chromosomes replicate and divide   Threadlike structure of nucleic acids and protein found in nucleus of most living cells  Carry genetic information in from of genes  Four chromosome type based on centromere position  Metacentric:middle  Submetacentric: higher up  Acrocentric: clost to tip  Telocentric: at the fucking top  Can be distinguidhed between 2  Autsome: present in the same copy number in both males and females: number and morphology is species specific  Sex-chromosomes: present in defferent copies in males, versus females; single pair of sex chromosomes o a single sex chromosome  Can be seperated and karyotype through G-banding and painting probes  G-bonding is the Giemsa staining of metaphase chromosomes; heterochromatic of AT-rich and gene poor regions are stained more darkly; vs. euchromatin   Probes cause fluorescence of particular colours on different chromosomes, in order to differentiate them  Chromosome numbers vary within species  Most are more diploid  Most plants are polyploid  In animals 10% species are hybrids  Plants 25% are hybrids,   Seedless fruits are usually 3n  4n reproducing with 2n makes a 3n  3n and 3n can't reproduce together   Some anumals( ceratin fishes and amphibians) are also polyploid\    Cell Divisions (assuming diploidy)  Most body cells are somatic cells(don’t reproduce), usually with chromosome present in pairs and this are diploid(2n)  Somatic cells replicate by mitosis, with each cell producing two identical daughter cells that are genetic replicas of the parental cell  Gametes are haploid(n) reproductive cells, produced from germ-line cells through meiosis  Meiosis produces gametes that are not genetically identical to one another and each has half the number of choromosomes as the original cells  Mitosos divides somatic cells  Controlled to prevent excess or deficiency of cells  Regulated by cell cycle   Cycle of Fna replication and divison  2 principle phases of eu cell cycle:  M(mitosis) phases: short time where cells divide  Interphase: the longer period between M phase  Interphase  During G1: gene expression is generally high.  After G1 a small number of cells enter G0; stop progressing further  DNA is replicated in synthesis   In G2 DNA has been replicated, and cell begins to get ready for mitosis  Mitosis  There is Prophase, prometaphase, metaphase, anaphase, telophase   Chromosomes during mitosis  Prior to, they are diffuse and difficult to visualize  Centromere, speccialize Dna sequences where suster chromatids are joined together, become visible in prophase, centromeres bind protein complexes called kinetochores  In animal cells, two centrosomes appear, whichh migrate to form the opposite poles of the dividing cell  Sister Chromatid Cohesion  The tension created by the pill of kinetochore microtubules is balanced by sister chromatid cohesion  The protein cohesion localies between sister chromatid and holds them together, preventing their premature separation  Cohesin is a 4-subunit protein that coats sister chromatids along their entire length, with the greatest concentration at the centromeres  Anaphase  Sister chromatids separate at anaphase and begin to move toward opposite poles in the cell   In anaphase A, the chromatids is called chromosome disjunction  Also in anaphase A, kinetochore microtubules begin to depolymerize at their plus end, moving individual chromatids towards centrioles  During anaphase B, polar microtubules extend in length  This causes the cell to take on an elongated shaoe  The shape facilitates cytokinesis at the end of telophase, leading to the formation of two daughter cells  Cell Cycle Checkpoints  Common, Genetically controlled signals drive the cell cycle  The checkpoints are monitored by proetin interaction for readiness to progress to the next stage  Mutations altering normal cell cycle control are linkd to various cell growth abnormalities  Cancer is often characterized by out-of-control proliferation of cells that can invade and displace normal cells  G1:if cell size is adequate, nutrient is sufficient, and growth factors are present  S-phase: check if replication is complete, and has been screened to remove base-pair mismatch error  G2: if cell size is adequeat and chromosome replication is complete  Metaphase  If chromosomes are all attached to mitotic spindle  Metaphase Chromosome during Mitosis  3 types of microtubules  Kinetochore microtubules: embed in the kitetochore at the centromere of each chromatid and responsible for chromosome movement  Non-kinetochorre, exted forward the opposite pole of centrosome, and contribute to cell elongation and cell stability  Astral microtubule grow toward the membrane of the cell and contribute to cell stability  Microtubules: tubular poymers of tubulin protein consisting of the alpha bet sub units  50um, 12 and 24 nm   Meiosis  Gametes are produced and used during fertilization  Meiosis vs Mitosis  Meiosis makes different looking gametes  Mitosis replicates somatic cells  Meiosis creates Germ-line cells all non identical  Homologous chromosomes only cross over in meiosis  In Meiosis 1, homologous chromosomes separate from one another in anaphase  Important 3  Homologous chromosome pariing  Crossing over  Segregation of homologous chromosome, which reduces chromosomes to the haploid number  Prophase 1 is segregated into sections  Leptotene  Zygotene  Pachytene  Diplotene  Diakenesis  The resolution of crossing over and synapsis within the meosis 1  How does this seperation support the basis and thoughts stated by Mendel  Why old women have a higher occurances of chromosome dysfunction, trisomy   LEPTO AND ZYGO, HOMOLOGS COME TOGETHER to pair into synapsis  Pachytene is when the crossing over occurs  Diplotene synapsocomplex something complex, the recombination points occur  Crossing over is pretty random, but they are recombination hot spots and cold spots  Sex chromosome is recombination cold spot, almost never recombination  RECOMBINATIO CAN OCCUE AT BOTH sides of the chromosome at once, but it is difficult  Usually occurs one in each hand,   Synaptonemal complex  There are essential elements  Late stage of zygotene, and up to diakenisis  Each chromosone has 2 chromatids, and there are two chromosomes conntected together in the meisosi 1  This is where crossing over happens  Sex chromosomes during meiosis  Some parts of x and y can cross over  Regions which are similar enough can cross over  Ther are called pseudoautosomal regions (PARS)  X chromosomes can cross over  Meiosis and the law of segregation  This shows how meiosis 1 and 2 give 1:1:1:! Ratio that is expected by mendel  In meiosis 1 homologs separate  Mieosis 2 sister chromatids separate  Meiosis and the law of independent assortment  One produces G  Voeghan   Identified that white eyed is x-linked recessive  Morgon's model showed the difference in phenotype according to gender are not anticipated according to menel's law  Based on these reslt morgan proposed that the eye color geen is loacted on the x-chromosome  Morgon's results is evidnece for the chormosome theory fro heredity  Bridge: morgan's under grad student found that the exception, that is caused due to chromosome non-dysjunction, explaining further the chromosome theory of heredity  Bridge's cross  He crossed xxy to the normal xy in order to confirm theory  Chromosomal nondisjunction and the role of chiamata  Nondysjunction causes aneuploidy  Incomplete set of chromosomes  Nullisomy  Loss of a homologous pair  Monosomy  Loss of a single chromosome  Trisomy  Single extra chromosome  Tetrasomy  An extra chromosome pair  Autosomal monosomies occur in lost early in pregnancy, miscarriage  Only 3 autosomal trisomies are seen as live irths  Trisomy 21 down syndrom  13 patau syndrome  18 edwards syndrome  80% trisomies are of maternal origin and most are most are due to a mieosis 1 defect  Frequency of trisomy increases with maternal age   Formation of eggs in humans  In the human female fetus, meiosis begins and the narrests.   Germ cells remain in an arrested prophase 1 until ovulation   Most human chromosomes have 2 chiasmata, one on each arm    Sex-determination most of the time is controlled by chromosomal and genetics  It can be environemental (ESD) or genetic (GSD)  GSD involves the genetic and biological processes that produc the make and female charateristics of a species  Chromosomal sex is the presence of chormosomes characteristic of eadch se and is determined at the momend of fertilization  Phenotypic sec is the internal and eternal morphology of each sex, and results from differences in gene expression  There is a diversity of GSD systems in cellulare organisms,. Below, we illustrate three representative types of sex-determining systems in animals  Sex Determination in Drosophila  Females have 2 x chromosomes and maes have one X chromosome  Thus in flies males have XY(normal), XYY, OR XO, whereas femalase are XX or XXY  The X/A ratio (X/autosome ratio determines gender based on the number of X chromosomes t osets of autosomes  Males have an X/A RATOP PF 0.5 and females have a ratio of   Mammalian Sex Determination  Lacental mammals have the XY sex determination system  Sex determination depends on the presence or absence of a single gene, SRY (sexdetermining region Y protein), found on the Y chromosome  In mammals, males can be XY, XXY, OR XYY, females are xx, xxx, x0  SRY  Early mammalian embryos have clusters of tissue called undiferentiated gonand, where they are female before being converted to male  ZW sex determination  The system is used by birds rptiels ,some fish, bitterflies, nad moths  In this system femlase have two different sex chromosomes, and males have the two sex chromosomes that are the same(zz)  Human Sex-inked Transmission and their traits  X linked recessive inheritance, females are homozygous, and the males are hemizygous  In X-linked dominant traits, females heterozygous and males hemizygous for the dominant allele express the dominant phenotype  Hemizygous males still get sick ,and all their daugters get the disease  X linked recessive    Hemophilia A                                                                                                                  Y-LINKED INHERITANCE  Mammals have fewer than 50 genes on the Y chromosome; those henes likely play roles in male sex dertmoatinon or developemnt  Most o the genes are notced to be dex and development related   Genes on the humand y chromosome fo no thave a copy on the xc= chromosme.  Dosage Compensation  Women have 2 X chromosomes vs men has X chromosomes  How does it balance  The female cells have a bur body, which is the sticking of one of the x-chromosomes to the cell wa  Mechanisms of Dosage Compensation of animals  Random X-chromosome inactivation in placental mammals  The inactivated chromosome would become highly condense and inaccesible by transcription facotrs  In cats coat color, the x chromosome carries the gene  One allele is black color, and the other one is orage  X inactivation in heterozygous females lead to a pattern of orange and black skin  Mechanism of X inactivation  Some genes on the inactive X escape Xinactivation  Random X inactivation requires an X-linked gene called XIST  (X-inactivation-specific-transcript   The gene produces large RNA molecules that spread out and cover the chromosomes to be inactivated  XIST can only act on the chromosome from which it is being transcribed, not the homolog   Why are there only 2 sexes in most organisms    Why are sex ratios tpically 1:1 in most organisms   Temperature plays a role in sex determination  ESD  At low temperature turtles have high % male and opposte  Alligators, at medium temperature that will reults in high percentage of males  but in either extreme temperature, the alligator population would consist of high percentage of females rather than males  Hypothesis for origing of sec  Driven bu the selfish genetic elements  Need for repairing damages  DNA, bringing a new copy as template  Matitenance of sex  Advantage of sex  More effective at purging genes with deterious effects  MoRE EFFECTIVE AT COMBINGIN GENES WITH BENEFICIAL EFFECTS  Generate divers genotypes to ada[t tp deverse ecological niches (abiotic and biotic factors)  Costs of Sex  Genomic burden due to uncontrollabed proliferation of selfish elements an the maintenance of mating and sex-related genes  Breakeing up co-adapted genes and gene complexes  Cost of producing males  Cost of finsding mating partners  Cost of interacting with mating partners