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Chromosomes • It was established that in most eukaryotes, species have a characteristic number of chromosomes: – Diploid (2n) • Humans have 46 chromosomes – Homologous chromosomes • In the last 10 years of the 19th century, researchers observed the behavior of these chromosomes during cell division...

Chromosomes • It was established that in most eukaryotes, species have a characteristic number of chromosomes: – Diploid (2n) • Humans have 46 chromosomes – Homologous chromosomes • In the last 10 years of the 19th century, researchers observed the behavior of these chromosomes during cell division: mitosis and meiosis 13 Diploid Chromosomes http://4.bp.blogspot.com/0hW1wgwKhc8/UyPxA8LmQPI/AAAAAAAACvM/9gO6EBexEJQ/s1600/HumanChromosomes.jpg 14 Chromosomes: DNA or Proteins? • Work on the Drosophila (fruit fly) showed that certain genes were responsible for eye colour and mutations of these genes are responsible for “differing” traits – Alleles are variants of a gene controlling a trait – Different alleles produce different phenotypes – Set of alleles for a given trait carried by an organism is called the genotype • We know that genes are carried on chromosomes, which are composed of proteins and DNA – But which of the two was responsible for genetic information? • By 1944 it was determined that DNA was responsible and the race for DNA structural elucidation began 15 The Double-Helix • 1953-Watson and Crick used X-ray diffraction to determine DNA structure • Subunits: nucleotides – Adenine-Thymine – Guanine-Cytosine • RNA: – Single-stranded – Sugar is different – Uracil replaces thymine 16 Base-Pairing http://blog.drwile.com/wpcontent/uploads/2011/02/pairing.jpg http://nar.oxfordjournals.org/content/32/1/354/F1.large.jpg 17 DNA Structures http://www.cella.cn/book/12/images/image011.jpg 18 Central Dogma http://www.ncbi.nlm.nih.gov/Class/MLACourse/Modules/MolBioReview/images/central_dogma.gif 19 Proteins • End products of gene expression • Constructs of a combination of 20 amino acids (a.a.) • Imagine a protein made of 10 amino acids and can have any one of 20 a.a. 2010!!!!!!!! • Enzymes, hemoglobin, insulin, etc. etc. • Shape and chemical nature of proteins are determined by their linear sequence of a.a. that make it up 20 Sickle Cell Anemia • Mutation in the amino acid sequence for hemoglobin • A single nucleotide in a gene changes one of 146 amino acids that encode for a molecule in hemoglobin! 21 Mitosis and Meiosis Chapter 2 1 Genetic Material • Deoxyribonucleic acid (DNA) (except for retroviruses) • Organized in units called genes – Products direct the metabolic activities of cells • These genes are organized into chromosomes – Serve to transmit genetic information • To new cells • From an organism to its descendants – This must be extremely precise (consistent) • Two major processes involved in genetic continuity are mitosis and meiosis • Chromosomes are usually visible during cell division – When not dividing, the genetic material unfolds and uncoils into a network mesh called chromatin 2 Cell Structure • Plasma membrane – Outer cell boundary – Phospholipid bilayer – Actively controls the cell environment • Glycocalyx – Covering over the plasma membrane – Composed of • Glycoproteins and polysaccharides – Functions in cell recognition and identity – Cell-surface markers include: AB, Rh, MN antigens, receptor molecules—recognition sites 3 Cell Structure • Nucleus – – – – Eukaryotes Contains the DNA Chromatin and chromosomes Nucleolus, responsible for rRNA synthesis • Nucleoid area – – – – – Prokaryotes Concentrated DNA No nuclear membrane, no nucleolus DNA not extensively associated with proteins DNA does not condense 4 Prokaryotes 5 Cell Structure • Cytoplasm – Remainder of the cell interior (minus nucleus) = cytosol + organelles + cytoskeleton • Cytoskeleton – Composed of interlinked proteins called filaments • Microfilaments: composed of contractile proteins called actin (8 nm in diameter) • Microtubules: composed of a globular protein called tubulin (25 nm in diameter) • Intermediate filaments: depending on the type, they are made of various proteins (8-10 nm) – Maintains shape, facilitates mobility, anchors organelles http://alevelnotes.com/content_images/i26_i23_ch1_cytoskeleton.jpg 6 http://www.phschool.com/science/biology_place/biocoach/cells/images/Cytoskel.gi f 7 Cell structure • Endoplasmic reticulum – Compartmentalizes and increases surface area for biochemical synthesis – Smooth – synthesizes fatty acids and phospholipids – Rough – contains ribosomes and synthesizes proteins 8 Cell Structure • Mitochondria – Found in both plant and animal cells – House the oxidative phases of cellular respiration – Generate ATP (energy-rich molecule) • Chloroplasts – Plant cells and in algae – Site of photosynthesis • Both contain their own DNA and can replicate themselves 9 Cell Structure • Centrioles – Found in animal and some plant cells – They are found in pairs: a mature and one smaller centriole – Found in specialized region called the centrosome – Organize spindle fibers • Spindle fibers – Important for the movement of chromosomes during cell division – Composed of tubulin polymers (microtubules) http://3.bp.blogspot.com/_tUQhsS1XUW8/SxWb05gYH3I/AAAAAAAAAOc/nXtaOdJZ X4A/s320/Biofreaks+-+Meet+the+chickens+-+Centrosomes+scheme.jpg 10 Human Chromosomes 11 Chromosomes • Somatic cells from the same species contain the same number of chromosomes – This is the diploid number (2n) • For humans – Diploid number = 2n = 46 – Haploid number = n = 23 • 22 are autosomes • 1 is a sex chromosome • The genome of a species is the collective genetic information contained in the haploid set of chromosomes 12 Chromosomes • Nearly all chromosomes exist in pairs – These are called homologous chromosomes – Pairs are matched based on length and centromere placement • Condensed region 13 Centromere Positions 14 Centromere Positions • Metacentric: – When the two arms are almost of equal length – Chromosomes 1 and 3 • Submetacentric: – When the centromere is between the middle and the one end – Chromosomes 2,4,5, 6,7…etc • Acrocentric: – The p arm is so short it’s hard to observe, but it’s there – Chromosomes 13, 14, 15, 21, and 22 • Telocentric: – Centromere is located at the terminal end of the chromosomes • Humans do not possess these types of chromosomes • Holocentric: the entire length of the chromosome acts like a centromere (not in Humans) 15 Sister Chromatids http://www.bio.miami.edu/dana/pix/chromosome_and_locus.jpg 16 Chromosomes • Locus – a gene site along the length if a chromosome • Alleles – alternate forms of the same gene within the same species • Biparental inheritance – one member of each homologous pair of chromosomes is derived from each parent – Therefore, each diploid organism contains two copies of each gene • Can you think of an exception to this? http://www.imgt.org/IMGTrepertoire/LocusGenes/chromosomes/ human/chrom7/chrom.png 17 Genotype vs Phenotype https://epfb.files.wordpress.com/2011/09/genotypephenotype.gif 18 Mitosis • Results in the production of two genetically identical daughter cells • Forms the foundation for the development and growth of an organism, unicellular or multicellular (somatic cells) • Wound healing and normal cell replacement • Skin cells, red blood cells, etc. • Complex process with many controls – Out of control mitosis leads to a tumor – Too slow of a process and there would be abnormal development • Cellular division – karyokinesis and then cytokinesis (less complex) 19 Mitosis • Karyokinesis = nuclear division – Results in 2 identical daughter nuclei – Complex and needs accuracy • Cytokinesis = cytoplasmic division – Encloses both new cells within a distinct plasma membrane – Less complex – Organelles can: • Replicate themselves • Form from existing membrane structures • Synthesized de novo • Nucleus of the two daughter cells are not that much smaller; but the overall cell size of each is smaller 20 Cell Cycle • Interphase – G1 – S phase – G2 • Mitosis – – – – – Prophase Prometaphase Metaphase Anaphase Telophase 21 Interphase • Interval between divisions • Normal growth and cell function • Also includes the replication of the DNA of each chromosome (S phase) • Gene transcription and translation occurs throughout the cell cycle, but the rates of these processes are quite high in G1 • G1 and G2 are gap phases during which intensive metabolic activity, cell growth, and cell differentiation occur • Interphase ends after G2 and mitosis (M) begins – G1, S, G2, M, G1, S, G2, M… 22 Interphase • Lengths of S and G2 phases are fairly consistent • Variation occurs in G1 (Humans: 7 hrs) • In G1, cells can withdraw from the cycle and become quiescent: G0 stage (e.g. some bone and eye cells) • Cells in the G0 stage are viable and metabolically active but are non-proliferative – Cells in this stage rarely escape it and return to the cell cycle and await programmed cell death (apoptosis) – Cancer cells manage to avoid or skip the G0 phase • Cytologically, interphase is characterized by the absence of visible chromosomes 23 Mitosis 24 Prophase • • • • Over half of mitosis is spent in this phase Centrioles migrate to opposite ends of the cell Nuclear envelope begins to break down Chromatin fibers condense until chromosomes become visible – Sister chromatids • Bound together at the centromere http://3.bp.blogspot.com/_sLaVkzhdkDI/TSe6wK35XLI/AAAAAAA AAAs/PW1NgZodGO0/s1600/prophase.gif 25 Spindle Fibers • There are three types of spindle fibers that emanate from centrosomes: – Kinetochore microtubules: • Connect to chromosomes – Inter-polar microtubules: • Contribute to the elongation of the cell – Astral microtubules: • Contribute to cell stability 26 Prometaphase/Metaphase • Prometaphase: – Spindle fibers bind to each centromere via the kinetochore: assembly of protein associated with the centromere – Chromosomes start movement • Metaphase – Chromosomes are now 10,000 fold more condensed since the start of prophase – Chromosomes migrate to the equatorial plate (aka metaphase plate) http://www.evh.k12.nf.ca/rbaker/Bio%203201/The%20Cell%20Cycle/metaphase.gif 27 Cohesin, Separase and Shugoshin • Cohesin – Protein complex that holds sister chromatids together • Separase – Enzyme that degrades cohesin • Shugoshin – Protein that protects cohesin from being degraded by separase 28 Anaphase • Shortest stage • Sister chromatids disjoin and migrate to opposite ends of the cell: disjunction – Shugoshin must be degraded – Cohesin complex is then cleaved by separase – Sister chromatids of each chromosome are pulled towards poles of the cell • Once each centromeric region is split in two, each chromatid is referred to as a daughter chromosome • In human cells, there are 46 chromosomes at each pole at the end of anaphase! 29 Telophase • Final stage of mitosis • Main event of this phase is cytokinesis • Differs in animal vs plant cells due to cell plate formation in plants • Two genetically identical cells are formed by the end of the phase • Late in telophase, chromosomes begin to uncoil, the nuclear envelope reappears, and spindle fibers disappear • The cell then enters interphase 30 Figure 2-7 31