Chapter 2: The Chromosomal Basis of Heredity Lecture Notes PDF

Chapter 2. The Chromosomal Basis of Heredity (Lecture Presentation in GEN 100) Honeylet J. Nicolas, Ph.D. Associate Prof. III  Part 1. The Cell Smallest living unit Most are microscopic 30-40T cells in human body  Discovery of Cells Robert H...

Chapter 2. The Chromosomal Basis of Heredity (Lecture Presentation in GEN 100) Honeylet J. Nicolas, Ph.D. Associate Prof. III  Part 1. The Cell Smallest living unit Most are microscopic 30-40T cells in human body  Discovery of Cells Robert Hooke (mid-1600s) – Observed sliver of cork – Saw “row of empty boxes” – Coined the term cell  Principles of Cell Theory 1. All living things are made of cells 2. Smallest living unit of structure and function of all organisms is the cell 3. All cells arise from preexisting cells (this principle discarded the idea of spontaneous generation)  Cell Theory (1839)Theodor Schwann & Matthias Schleiden “ All living things are made of cells” (50 yrs. later) Rudolf Virchow “All cells come from cells” Cell Size Characteristics of All Cells A surrounding membrane Protoplasm – cell contents in thick fluid Organelles – structures for cell function Control center with DNA  Cell Types Prokaryotic – has no true nucleus, e.g. bacteria Eukaryotic – with true nucleus; plant cells, animal cells, fungi  Prokaryotic Cells First cell type on earth Cell type of Bacteria and Archaea No membrane bound nucleus Nucleoid = region of DNA concentration Organelles not bound by membranes  Eukaryotic Cells Nucleus bound by membrane Include fungi, protists, plant, and animal cells Possess many organelles Protozoan Representative Animal Cell Representative Plant Cell  Organelles Cellular machinery Two general kinds – Derived from membranes – Bacteria-like organelles  Plasma Membrane Contains cell contents Double layer of phospholipids & proteins  Cell Wall Found in plants (cellulose), fungi (chitin), & many protists Surrounds plasma membrane  Cytoplasm Viscous fluid containing organelles components of cytoplasm – Interconnected filaments & fibers – Fluid = cytosol – Organelles (not nucleus) – storage substances  Cytoskeleton Network of filaments & fibers in the cytoplasm from nucleus to plasma membrane Made of 3 fiber types – Microfilaments – Microtubules – Intermediate filaments 3 functions: – mechanical support – anchor organelles – help move substances Cilia & Flagella Provide motility Cilia – Short – Used to move substances outside human cells Flagella – Whip-like extensions – Found on sperm cells Basal bodies like centrioles  Centrioles Pairs of microtubular structures Play a role in cell division  Membranous Organelles Functional components within cytoplasm Bound by membranes  Nucleus Control center of cell Nuclear envelope: double membrane, with pores Contains – Chromosomes – Nucleolus  Nucleolus Most cells have 2 or more Directs synthesis of RNA Forms ribosomes  Endoplasmic Reticulum Helps move substances within cells Network of interconnected membranes Two types – Rough endoplasmic reticulum – Smooth endoplasmic reticulum Rough Endoplasmic Reticulum Ribosomes attached to surface – Manufacture proteins – Not all ribosomes attached to rough ER May modify proteins from ribosomes Smooth Endoplasmic Reticulum No attached ribosomes Has enzymes that help build molecules – Carbohydrates – Lipids  Golgi Apparatus Involved in synthesis of plant cell wall Packaging & shipping station of cell  Lysosomes Contain digestive enzymes Functions – Aid in cell renewal – Break down old cell parts – Digests invaders  Vacuoles Membrane bound storage sacs More common in plants than animals Contents – Water – Food – wastes  Bacteria-Like Organelles Release & store energy Types – Mitochondria (release energy) – Chloroplasts (store energy)  Mitochondria Have their own DNA Bound by double membrane Break down fuel molecules (cellular respiration) – Glucose – Fatty acids Release energy – ATP  Chloroplasts Derived from photosynthetic bacteria Solar energy- capturing organelle Photosynthesis takes place here Makes cellular food – glucose Part 2. Chromosome Structure Literally means “colored bodies;” they are contained in the nucleus and are visible under the microscope as dark- staining, rod-like or rounded bodies. They were first reported as threadlike bodies and called chromatins by Walther Flemming, and were renamed as chromosomes by Heinrich Wilhelm Gottfried von Waldeyer- 33 Hartz.  Chromosome Structure Each unit of chromosomes consists of two rod-like structures (chromatids) joined together at a constricted point (centromere). DNA and chromosome proteins are called chromatin. 34  CHROMOSOMES Based on the deductive reasoning by Walter Stanborough Sutton, the chromosomes were determined to be carriers of the hereditary material. Chromosomes occur in pairs in the body cells except sperm cells and ovum. The number of chromosomes in each cell is constant for individual species, but it differs among species. SPECIES CHROMOSOME NO. SPECIES CHROMOSOME (2n) NO. (2n) Cattle 60 River buffalo 50 Horse 64 Swamp buffalo 48 Goat 60 Chicken, quail 78 Sheep 54 Duck, goose 80 Pig 38 Turkey 82 35 Rabbit 44 Ostrich, pigeon 80  Ploidy The chromosomes in the nuclei of somatic cells are usually present in pairs, Diploid. Humans have 23 pairs of chromosomes The germ cells, or gametes, are Haploid and contain only one set of chromosomes The haploid gametes unite in fertilization to produce the diploid state of somatic cell. Thus each pair has one chromosome derived from the maternal parent and the other from the paternal parent 36  Part 2. Chromosome Structure Eukaryotic chromosome contains a single DNA molecule of enormous length in a highly coiled stable complexes of DNA and protein called chromatin The basic structural unit of chromatin is the nucleosome, a core particle of histone proteins that the DNA wraps around in ~200bp segments Each nucleosome particle consists of an octamere of pairs each of four histone proteins H2A, H2B, H3, and H4; a fifth histone protein, H1, binds the core particle to the linker DNA 37 Chromosome Structure (gene poor regions) (heavily staining regions) (gene rich regions) (poorly staining regions) 38  Part 3. Cell Division 2 types: mitosis and meiosis Mitosis – happens in somatic (body) cells – 1 cell division results in 2 daughter cells – chromosome number is maintained (diploid) Meiosis – happens in cells in the sexual cycle – 2 cell divisions, 4 products – chromosome number is halved (haploid) 39 Cell Cycle 40  Mitosis Prophase chromosomes condense. Each chromosome is already doubled (each is called a chromatid for some reason) and held together at a specific region of the chromosome called the centromere. Metaphase chromosomes line up at the center of the cell. mitotic spindle (a bunch of microtubules) microtubules attaches the kinetochores to the centrosomes Anaphase the two sister chromatids move toward opposite poles (each sister chromatid is called a chromosome again). Telophase A nuclear envelope re-forms around each compact group of chromosomes, the chromosomes undergo decondense, and the cell divides in two 41  Meiosis In animals, meiosis takes place in specific cells called meiocytes. The oocytes form egg cells and the spermatocytes form sperm cells. In the females of both animals and plants, only one of the four products develops into a functional cell (the other three disintegrate). 42 Meiosis Prophase I chromosomes condense (leptotene), homologous pairs of chromosomes synapse (zygotene) and crossovers or exchanges occur between nonsister chromatids (pachytene). The crossovers, called chiasmata, become visible as the chromosomes separate a bit (diplotene), and condense a bit more (diakinesis). Metaphase I chromosome pairs line up at the center of the cell. Oriented randomly. Anaphase I the two bivalent chromosome pairs move toward opposite poles Telophase I the chromosomes usually only partially decondense, and the second division begins Prophase II chromosomes condense Metaphase II chromosomes line up at the center of the cell Anaphase II the the two sister chromatids move to opposite poles Telophase II A nuclear envelope re-forms the chromosomes decondense and cell division occurs 43 Part 4. Biological Life Cycles Biological life cycle - a series of changes in form that an organism undergoes, returning to the starting state. 3 Types: 1. Haplontic life cycle — the haploid stage is multicellular and the diploid stage is a single cell, meiosis is "zygotic". 2. Diplontic life cycle — the diploid stage is multicellular and haploid gametes are formed, meiosis is "gametic". 3. Haplodiplontic life cycle (also referred to as diplohaplontic, diplobiontic, or dibiontic life cycle) — multicellular diploid and haploid stages occur, 44 meiosis is "sporic".  Gametic Meiosis Meiosis occurs during gamete formation. Results in haploid gametes. Gametes fuse during fertilization to form zygote. Zygote grows into diploid multicellular organism. Example: sexually- reproducing organisms (animals) 45  Sporic Meiosis Meiosis occurs during sporogenesis. Results in haploid spores. Spores divide to form gametophytes. Gametic fusion results in diploid sporophyte. Example: plants 46  Zygotic Meiosis Meiosis occurs during zygote germination. The zygote is the only diploid (2n) stage; organism spends most of its life cycle in haploid (1n) condition. Example: fungi, some algae, protists 47