Mitosis and Meiosis (Chapter 2) PDF

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...

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 Cell Cycle Regulation If mitosis is uncontrolled, the result would be devastating à malignancy Research has identified many mutations that occur at one or another stage in the cell cycle – Cell division cycle (cdc) mutations Many of the genes that are mutated produce kinases (add phosphates to other proteins) – Master control molecules that work along with cyclins Cyclins bind to cyclin-dependent kinases, activating them at appropriate times – These cyclin-dependent kinases then phosphorylate regulatory molecules Research identified at least three cell-cycle checkpoints – More on that when we cover cancer J 32 Cyclins 33 Which statement is true? 1. More complex organisms have a larger haploid number of chromosomes. 2. The genome of an organism is usually defined as the haploid set of chromosomes of that organism. 3. Homologous chromosomes are genetically identical. 34 A cell in the G0 stage… 1. 2. 3. 4. 5. has withdrawn from the cell cycle. is in the process of dividing. is replicating its DNA. has just completed cell division. is preparing for cell division. 35 Question A cell with a haploid number of 7 is undergoing mitosis: 1. How many chromatids are there at the end of metaphase? 2. How many chromosomes are there at the end of metaphase? 3. How many chromatids are there at the end of anaphase? 4. How many chromosomes are there at the end of anaphase? 36 Meiosis Central process of sexual reproduction – For both unicellular and multicellular eukaryotes Reduces the chromosome number from diploid to haploid – Producing spores or gametes Ensures: – Genetic continuity, and – Variety among members of a species Different combinations of maternally and paternally derived chromosomes What about prokaryotes? Do they exist as haploids or diploids? 37 Goals of Meiosis 1. To produce haploid gametes that contain precisely one member of each homologous pair of chromosomes 2. Ensure that during sexual reproduction an enormous amount of genetic variation is produced among members of a species 38 Meiosis Just as in mitosis, the process in meiosis begins with a diploid cell duplicating its genetic material in interphase If we are to achieve haploid cells, then what to do? 39 Meiosis In mitosis, a single division occurs resulting in two identical daughter cells In meiosis, two divisions occur resulting in four separate (and different) gametes 40 Mitosis vs Meiosis 41 Meiosis Two divisions: – Prophase I – Metaphase I – Anaphase I – Telophase I – Prophase II – Metaphase II – Anaphase II – Telophase II 42 Meiosis Meiosis comprises two divisions: Meiosis I (reductional division) and Meiosis II (equational division). Meiosis begins with a diploid cell (DNA duplicated during interphase, chromosomes made up of sister chromatids). Meiosis is a process similar to mitosis, except that homologous chromosomes pair up (synapsis). 43 The First Meiotic Division Prophase I Longest stage in meiosis Chromatin condenses Homologous chromosome pairs undergo synapsis (i.e. tetrad formation) Unlike mitotic prophase, the homologous pairs actually pair up Crossing over between synapsed homologs (i.e. chiasma) Important for forming new combinations of alleles! 44 Prophase I The homologous chromosomes pair up and are initially called bivalents – Number of bivalents is equal to the haploid number As they condense and shorten, each bivalent gives rise to a tetrad consisting of two pairs of sister chromatids 45 Prophase I 46 The Five Substages of Prophase I Leptonema: – Chromosome condensation – Centrosomes begin to migrate Zygonema: – Chromosome continue to condense – Homologous pairs synapse (pair up) – Centrosomes continue to migrate Pachynema: – Crossing over occurs between non-sister chromatids – Kinetochore spindles connect to the kinetochores – Nuclear envelope begins to breaks down Diplonema: – Crossing over is completed – -Non-sister chromatids begin to pull apart, revealing contact points (chiasmata) where crossovers have occurred Diakinesis: – Nuclear envelope fully degraded – Tetrads begin to move towards the middle of the cell 47 Prophase I The two pairs of sister chromatids will have an overlapping region of non-sister chromatids (chiasma). – Exchange of genetic material (paternal/maternal chromosomes) through recombination Nuclear envelope and nucleolus break down, and the two centromeres of the tetrad attach to the spindle fibers. 48 The First Meiotic Division After Prophase I, steps similar to mitosis occur. During Metaphase I, chromosomes have maximally shortened and thickened, chiasma are visible, holding non-sister chromatids together. – Alignment of chromosomes (tetrads) at the metaphase plate is random Half of each tetrad is randomly pulled (disjunction) to opposite poles (Anaphase I). – Karyokinesis occurs here – Creating dyads (half of a tetrad) Telophase I is marked with the reappearance of the nuclear membrane and a short interphase (not a full interphase). – Chromosomes do not replicate, already consist of sister chromatids. 49 The First Meiotic Division 50 Meiosis II Prophase II: Chromosomes are composed of one pair of sister chromatids attached by a common centromere. Metaphase II: Centromere is positioned at the metaphase plate. Anaphase II: Centromeres divide; sister chromatids are pulled to opposite poles. Telophase II: Similar to mitotic telophase. Cytokinesis results in four haploid gametes with equal cytoplasm, containing a combination of maternal and paternal genes. 51 Figure 2-10-2 52 The Second Meiotic Division Ensures that each gamete receives only one chromatid from each original tetrad Follows the same steps as mitotic division: – Centromeres split – No crossing over – Each dyad, composed of two sister chromatids, becomes two monads 53 Mitosis vs Meiosis 54 Spermatogenesis and Oogenesis While meiosis is universal in sexually reproducing organisms, there is a major difference between meiosis in males and in females Spermatogenesis partitions the cytoplasmic volume equally and produces four haploid sperm cells Oogenesis collects the bulk of the cytoplasm in one egg cell and reduces the other haploid products to polar bodies – Extra cytoplasm in the egg contributes to zygote development Zygote: initial cell formed when two gamete cells are joined by means of sexual reproduction 55 Spermatogenesis and Oogenesis 56 Spermatogenesis An undifferentiated germ cell called a spermatogonium enlarges to become a primary spermatocyte. The primary spermatocyte undergoes meiosis I to produce haploid secondary spermatocytes. Secondary spermatocytes then undergo Meiosis II to produce a total of four haploid spermatids that undergo a series of developmental changes, spermiogenesis, and become highly specialized, motile spermatozoa or sperm. 57 Oogenesis An undifferentiated germ cell called an oogonium enlarges to become a primary oocyte. Meiosis I results in two haploid daughter cells wherein one cell (secondary oocyte) receives the bulk of the cytoplasm. The cell with little or almost no cytoplasm is called a polar body, which will ultimately disintegrate 58 Oogenesis The secondary oocyte undergoes meiosis II and produces two haploid cells: an ootid with the bulk of cytoplasm and a second polar body. – The first polar body may or may not divide. – Eventually, all polar bodies disintegrate with only one functional cell remaining. The ootid differentiates into a mature ovum. 59 Oogenesis The two meiotic divisions may occur back-to-back (not in humans) In humans, the first meiotic division begins in the embryonic ovaries But growth is arrested in prophase I – They remain as primary oocytes They resume, one at a time, during ovulation after reaching sexual maturity Now the secondary oocyte’s growth is arrested at metaphase II Once fertilization occurs, meiosis II can be completed! 60 Meiosis allows... 1. the generation of genetic variation among offspring. 2. the generation of new combinations of alleles on the same chromosome. 3. the transmission of equivalent genetic information from generation to generation. 4. the genetic contribution of two individual parents to each offspring. 61