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Biology 1 All Lectures Summary.docx

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BIOL 1010U
Sylvie Bardin

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The Cell Cycle ‘Omnis cellula e cellula’- every cell from a cell Cell Division 1. Reproduction 2. Growth and Development (From zygote) 3. Tissue Repair 2 Types of cell division (Eukaryotic Cells) - Mitosis (somatic cells) - Meiosis (gametes) - Skin cells divide frequently; nerve and muscle cells do not divide Mitosis evolved from prokaryotic binary fission (Below are eukaryotic examples) - Dinoflagelates’ DNA remains attached to the nuclear membrane and separates when the nucleus elongates. - Diatoms and yeast have their mitotic spindle inside the nucleus. Prokaryotic Cells do not undergo mitosis - Binary Fission  DNA begins to replicate and cell division is initiated  Origin of Replication – DNA attaches to opposite ends of the plasma membrane   When the chromosomes pull apart, a ring of FtsZ protein (tubulin) forms. - This ‘ring’ constricts and the membrane/cell wall infolds – cytokinesis. Chromosome - The form that DNA takes when chromatin is condensed - Can exist as one chromatid (not replicated) or two chromatids (replicated) - When DNA is replicated and has 2 sister chromatids, it is still referred to as one chromosome Interphase - G1 /G2 (4/5-6h) where cell growth, organelle and cytoplasm production occurs - DNA is in the form of chromatin during G1 (unbundled) - DNA is in the form of a chromosome upon condensing - S Phase (10/12h) Replication of DNA (Each daughter cell receives a copy of all 46 chromosomes); dependent on DNA amount - Duplicated chromatin is referred to as a chromatid but they REMAIN uncondensed in this phase - Sister chromatids attach to the centromere (where chromatin are closest, heterochromatin) of the chromosome Mitotic Phase (evolved from binary fission) - DNA condenses at start of mitosis - Sister chromatics separate during mitosis - Look at the state of the mitotic spindle, location of chromosomes and how condensed they are - Creation of 2 identical daughter cells - Mitosis (division of nucleus) - Cytokinesis (division of cytoplasm) - One copy of each chromosome goes to each daughter cell Genome – Set of genetic data - Prokaryotes have 1 chromosome of a circular molecule of DNA (plasmid) - Eukaryotes have several chromosomes made of long linear DNA - Chromatin is DNA associated with histone proteins. It can be more/less condensed depending on the stage of the cell cycle. Diploid – possesses 2 sets of a given chromosome (homologous); one maternal, one paternal - Humans have 46 chromosomes (23 pairs of homologous chromosomes – 2 sets; 2n) Haploid –Meiosis yields non-identical cells with 23 chromosomes (1 set; n) – Gametes produced from germ cells in the gonads - ‘n’ indicates how many different types of chromosomes are present - The coefficient in front of n indicates the number of each type of chromosome present - 2n will have 2 chromosomes of each type - Fertilization yields a zygote and returns the number of chromosomes to 46 (diploid) Mitotic Division Mitotic spindle – constructed of tubulin proteins that forms microtubules (kinetochore tubules attach to chromosomes and nonkinetochores do not) Kinetochores – protein structured regions on the chromosome where some microtubules attach Centrosomes - Organizes microtubules in animals - Replicates during G2 of interphase - Consists of 2 cylinder structures called centrioles (animal cell only) Prophase - DNA prep into chromosomes, mitotic spindle begins - Tubulins assemble into spindle microtubules - Centrosomes move apart due to nonkinetochore microtubule lengthening in the overlapping area  Nucleoli disappears and the DNA condenses  Cohesion proteins joins sister chromatin Prometaphase – Tubules attach to chromosomes - Nuclear envelope disappears - Kinetochore microtubules attach at kinetochores Metaphase – Chromosomes line up at the plate - Kinetochore microtubules positions chromosomes at metaphase plate Anaphase - Cell elongates, sister chromatids split - Cohesion proteins break down, sister chromatids separate - Kinetochore microtubules shorten to pull apart chromatids (depolymerization) - Non-kinetochores lengthen via polymerization to elongate the cell Telophase – Mitotic spindle disassembled - Mitotic spindle disassembly HOW? - 2 Nuclear envolopes and nucleoli form - Chromosomes de-condense Cytokinesis – Cell splits into two - Began in Telophase - Animals have cleavage furrow – contracting ring of microfilaments (actin and myosin) - Plants have Cell Plate – accumulation of vesicles from Golgi Cell Cycle Control System - Differences in cell cycle are explained by regulation at the molecular level Checkpoints at G1, G2, M (built in stop signals) - control point where stop and go ahead signals can regulate the cycle G1 Checkpoint - Sufficient nutrients - Growth factors - Adequate cell size - Undamaged DNA - 3 other CDKs and several cyclins G2 Checkpoint - Undamaged DNA - Successful DNA replication - Activated MPF is present M Checkpoint - All chromosomes attached to spindle G1 Checkpoint: Restriction Point - Most important in animal cells. - Usually if the G1 Checkpoint passes, the S, G2, and M phase complete as well. - If there is a stop, cells exit the cycle and enter G0 (non-dividing state – e.g. neurons) - If S phase or M phase cells fuse with stopped G1 cells, the G1 stopped cell immediately begins mitosis. - If an S cell fuses with a stopped G1 cell, the G1 cell begins to synthesize a second copy of DNA. - If a M cell fuses with a stopped G1 cell, the G1 cell skips the S phase and directly enters mitosis (without a duplicated set of DNA). - Therefore, the molecules that allow for progression to further phases of the life of a cell are present in the cytoplasm during the S or G2 or M phase. Internal Regulation of Cell Cycle Cell cycle is driven by maturation promoting factors (MPF) in the cytoplasm. MPF is made up of: - Protein kinase – Enzyme that phosphorylates inactive proteins giving the ‘go ahead’ at G1 or G2 checkpoints - Kinases are present in the cell at a constant concentration but require cyclin in order to be active - Hence, they are deemed cyclin-dependent kinases (CDKs) - [Cyclin] fluctuates during cell cycle - Cyclin is constructed during the S and G2 phases, used during mitosis and degraded in G1 What do MPFs do? - Triggers passage past G2 checkpoint into M phase by phosphorylation of proteins required to start mitosis - Contributes to chromosome condensation, spindle formation during prophase, and nuclear membrane fragmentation during prometaphase - Triggers negative feedback of cyclin during anaphase EXAMPLE of internal regulation occurs at the M checkpoint: - During anaphase, the sister chromatids only separate when every centromere is attached by a kinetochore and each kinetochores opposite end is attached to the spindle - This ensures that daughter cells do not have missing or extra chromosomes Growth factor – protein released by certain cells that stimulates other cells to divide (PDGF in fibroblasts allow them to pass the G1 checkpoint and heal wounds) +50 exist DNA Damage - Protein p53 (tumor suppressor protein) stops cell cycle; if not present, DNA remains damaged (can cause cancer) - Ensure that DNA is repaired before cell cycle continues or apoptosis is induced (programmed cell death) External Regulatory Mechanisms Density-dependent Inhibition (Phenomenon in which crowded cells stop dividing) - Neighbouring cells interact through cell-surface proteins - When in close proximity these proteins send growth inhibiting signals - However, when there is empty space on a horizontal level, division may occur in that direction. Anchorage Dependence - Cells much be attached to a substratum in order to divide - Effect of plasma membrane proteins interacting with the cytoskeleton Cancer - Does not heed the normal signals that regulate cell cycle - They do not listen to density-dependent inhibition or anchorage dependence - Lack of external growth factors does not inhibit their division - If they do stop division it is not at expected checkpoints - Develops after several genes have been damaged - Normal cells have 20-50 divisions only Transformation – The process of a normal cell becoming a cancer cell - Normally these are destroyed but can be evasive enough to survive and develop into a tumor Benign Tumor – A cancerous mass that remains at its origin Malignant Tumor – A cancerous mass that it is invasive enough to disrupt regular organ function (Metastatis is the spread of cancer cells beyond their original site.) - Treatment of cancer cells by high energy radiation freezes the mitotic spindle. _____________________________________________________________________________________ Meiosis and Sexual Life Cycles Heredity - The transmission of traits from one generation to the next Asexual Reproduction – A single individual is the sole parent creating a clone (a group of genetically identically individuals) unless a mutation occurs Sexual Reproduction – Two parents give rise to offspring with unique combinations of genes inherited from each parent Gametes – vehicles for transmission of genes from one generation to the next that form from germ cells in the gonads (ovaries/testes) Variation – The offspring of the parent generation is different among themselves and from the parent generation. An advantage of sexual reproduction. Mutations are original source Genetics – The study of genetic heredity and genetic variation Gene - Units of heredity information - Segments of DNA containing specific sequences of the 4 nucleotides  Cells look at genes like we look at words. There are multiple meanings for a 3 letter – sequence; like how a gene can code for multiple traits etc. Locus – A genes specific location along the length of a chromosome Chromosomes (Duplicated) - Two sister chromatids side by side - Differ by position of centromere - Length and size - Patterns formed when stained - Contains many genes - Mostly in nucleus, except for small amount in mitochondria and chloroplast Karyotype – Ordered display of homologous pairs of chromosomes Homologous Pair (Homologs) (22 Autosomes) - Two chromosomes that have the same length, centromere position, and staining pattern - Each chromosome carries genes controlling the same traits at the same respective locus - However, the alleles may be different (eye color – gene, blue/brown eyes – allele) - One of the pair is maternal, the other is paternal - The occurrence of homologous pairs is a result of sexual reproduction (one homologous pair from each chromosome) Sex Chromosomes - XX (female) - XY (male *decides the sex of the child since he has gametes which can have a x or y chromosome) - Some parts are homologous Sexual life cycles all have the alternation between meiosis and fertilization, but they can occur at different times. Animals - Long phase of diploid mitosis - Short phase of haploid life, only used for fertilization as gametes Plants - Can live as a diploid (sporophyte whose gametes are spores) or haploid organism (gametophyte) - Gametophytes undergo mitosis to create gametes - Mitosis occurs when the plant is a haploid or diploid - Alternation of Generation – Both diploid and haploid stages are multicellular Fungi and Protist - Long phase of haploid mitosis - Short phase of diploid life (only zygote) - *Only diploid zygotes can undergo meiosis and allow for genetic variation Meiosis - Cell division that cuts the chromosome number from diploid to haploid Meiosis I - Separation of homologous chromosomes - Reductional division occurs (2n -> n) - Prophase I - Chromosomes begin condensing and homologs are close together - Each allele is matched up with its counterpart on the partner homolog - They are held together by proteins along their lengths  Synapsis - Precise alignment of the 2 homologous chromosomes that are joined together by a protein complex, forming a tetrad linked by chiasmata - Crossing over (non sister chromatid strands are broken at the same point and reattached to each other) occurs while homologs are in synapsis due to the close association - Points where crossing over has occurred are called chiasmata (usually near the ends of the chromatid); these points keep the pair together - There is cohesion between the homologs as well as between each sister chromatid - Ends mid-prophase Metaphase I - The homologous pairs line up at the metaphase plate - Kinetochores attach to one centromere of each homolog - Shugoshin protects the centromeres of each homologus pair - Chiasmata is degraded Anaphase I - The homologous pair is separated - The proteins that allowed for sister chromatid cohesion between homologous pairs were broken down Telophase I and Cytokinesis - Each cell has a complete haploid set of replicated chromosomes -> 23 chromosomes (2 sister chromatids in each chromosome) - Each chromosome is composed of two sister chromatids which contain non-sister chromatid DNA Meiosis II is the exact same as mitosis (Separation of sister chromatids) – Equational Division n Independent Assortment of Chromosomes (2 possible combinations) - Homologous pairs line up at the metaphase plate without having to have maternal on one side and paternal on the other - There is a random orientation of homologous pairs at the metaphase plate in meiosis I - Each homologous chromosome is positioned independently of the others at metaphase I Crossing Over (Recombinant Chromosomes) - The exchange of corresponding segments of two non sister chromatids - Occurs about 1 – 3 times per chromosome pair Random Fertilization (2xIA) - Any sperm can fuse with any egg Darwin said population evolves because of the reproductive success of its members - Natural selection says that the best suited individuals to their environment will produce more similar offspring - Mutations arise to adapt to new environments - Heritable variation makes evolution possible Mendel and the Gene Idea Heritable - Passed from parent to offspring. Character - Heritable feature that varies among individuals; e.g. hair color. Trait - Variant of a character; e.g. brown, blond, red color. Gene - Portion of DNA that codes for a protein with specific function (character) Alleles - Alternative forms of a gene giving rise to different traits. Arise from mutations Phenotype - The expression of an individual’s physiological and physical appearance. Genotype - The genetic makeup of one or more genes in an organism aka allele composition Homozygous - The alleles are the same for a specific gene. (Pure dominance or pure recessive). Heterozygous - The alleles are different for a specific gene. (One is dominant, the other recessive) The chromosome theory of inheritance formulated in 1903 by Sutton and Boveri linked meiosis and Mendel’s theories. Mendel succeeded because he: – Used an appropriate model organism • Short-lived • Produce large number of offspring • Easy to manipulate experimentally • Many of the observable characters that vary in pea plants are controlled by single genes – Studied the inheritance of a characters with only two distinct traits – Studied the inheritance of a genes that assort independently from each other Advantages of crossing pea plants  They have distinct characters and traits  Plants can self-pollinate therefore they can create homozygous offspring  Hybridization (cross pollination) is achieved by dusting one plant with pollen from another. Monohybrid Cross (3:1 Phenotypic Ratio/ 1:2:1 Genotypic Ratio) - Mendel started with a homozygous p – generation obtained from multiple self-fertilizations, each creating the same color - The hybrid (from 2 parents) offspring are the F1 (first filial) generation obtained from cross – pollination - He was looking at one character only, where the gene was defined by one type of allele only - F2 (second filial) generation was obtained by self-pollination of the F1 hybrids Mendel’s Model 1. Genes do not blend (No intermediate) 2. Each individual has two alleles that account for the variations in inherited characters 3. For each character, a diploid organism inherits one maternal and one paternal allele 4. The trait that is masked is recessive; the trait that is phenotypically showed is dominant 5. The Law of Segregation - The two alleles for each character segregate during gamete formation (meiosis) Testcross - Cross of individual with unknown genotype with a homozygous recessive to determine the genotype. Monohybrid Cross – Cross that tracks the inheritance of a single character Dihybrid Cross - Cross that tracks the inheritance of two characters simultaneously Law of Independent Assortment - Allelic pairs segregate independently from members of another allelic pair - This law applies only to genes on different, non-homologous chromosomes - Genes located near each other on the same chromosome tend to be inherited together Probability - Number of times a particular event occurs divided by the total number of opportunities for the event to occur – For an event that occurs all the time; P = 1 – For an event that does not occur; P = 0 – For an event that occurs sometimes; 0 < P < 1 *When attempting to determine the probability of 2 events happening, multiply the individual probabilities *When attempting to determine the probability of one certain event happening add the individual probabilities of all possible events’ probabilities together • Phenotypic ratio 3:1 (Aa X Aa) - Monohybrid cross F1 xF1 (both heterozygous for one trait) - Dihybrid cross F1 xF1 (both heterozygous for two traits) AND dependent assortment • Phenotypic ratio 9:3:3:1 (AaBb X AaBb) Dihybrid cross F1 xF1 (both heterozygous for two traits) AND independent assortment • Phenotypic ratio 1:1 Testcross with organism heterozygous for one trait (Aa X aa) • Phenotypic ratio 1:1:1:1 Testcross with organism heterozygous for two traits (AaBb X aabb) Mendel’s Model (Not met by all heritable characters) - A character is determined b
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