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Chapter 02 Reproduction and Chromosome Transmission Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Reproduction and Chromosome Transmission are Key to Genetics • Reproduction – the process by which new cells or organisms are produced • Requires the transmission of chromosomes from parent to offspring • When eukaryotic cells divide, they must sort their chromosomes so each cell receives the correct number 2 2.1 General Features of Chromosomes Definition of the term chromosome Key differences between prokaryotic and eukaryotic cells Procedure for making a karyotype Similarities and differences between homologous chromosomes 3 Chromosomes • Chromosomes are structures within living cells that contain the genetic material - the genes • Chromosomes are composed of o DNA, the genetic material o Proteins, to provide an organized structure • In eukaryotes the DNA-protein complex is called chromatin Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 4 Two Types of Cells • Cells are classified as one of two types: o Prokaryotes - Bacteria and archaea o Eukaryotes - Protists, fungi, plants and animals • Protists and fungi may be single-celled, but are still eukaryotes because they have a membrane-bound nucleus Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 5 • Prokaryotes 1 mm o No nucleus o No membrane-bound organelles o Usually contain a single type of circular chromosome • Found in the nucleoid o Outside the membrane is a rigid cell wall o May contain other structures • Outer membrane • Flagellum Ribosomes in cytoplasm Outer Cell wall Plasma membrane membrane (also known as inner membrane) Flagellum Nucleoid (where bacterial chromosome is found) 6 • Eukaryotes o o o o o Have a nucleus – DNA surrounded by membrane Two or more linear chromosomes May have flagella and other structures May have cell wall, different than prokaryotes Membrane-bounded organelles such as: • Mitochondria – have DNA • Chloroplasts – have DNA Microfilament Golgi Nuclear Nucleolus Chromosomal • Lysosomes body envelope DNA • Golgi apparatus Nucleus Polyribosomes Ribosome Rough endoplasmic reticulum Cytoplasm Membrane protein Plasma membrane Smooth endoplasmic reticulum Lysosome Mitochondrial DNA Mitochondrion Centriole Microtubule 7 Chromosomes are Examined Cytogenetically to Produce a Karyotype • Cytogenetics – The field of genetics that involves the microscopic examination of chromosomes • A cytogeneticist typically examines the chromosomal composition of a particular cell or organism o This allows the detection of individuals with abnormal chromosome number or structure o Provides a way to distinguish between two closely-related species 8 • Animal cells are of two types o Somatic cells • Body cells, other than gametes • Ex: Blood cells o Gametes (germ cells) • Sperm and egg cells • Precursor cells that give rise to sperm and egg • To get a complete karyotype, the cytogeneticist examines somatic cells o Usually blood cells 9 • During cell division chromosomes can be seen with a light microscope. • Each chromosome has a unique size, shape and banding pattern (light and dark areas) • A karyotype is a set of images of the chromosomes 10 Eukaryotic Chromosomes Are Inherited in Sets • Most eukaryotic species are diploid o Two sets of chromosomes • For example: o o o o Humans – 46 total chromosomes (23 per set) Dogs – 78 total chromosomes (39 per set) Fruit fly – 8 total chromosomes (4 per set) Tomato – 24 total chromosomes (12 per set) Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 11 • Members of a pair of chromosomes are called homologs o The two homologs form a homologous pair • The two chromosomes in a homologous pair o o o o Are nearly identical in size Have the same banding pattern Have the same centromere location Have the same genes • But not necessarily the same alleles Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 12 • The DNA sequences on homologous chromosomes are also very similar o There is usually less than 1% difference between homologs • Nevertheless, these slight differences in DNA sequence provide the allelic differences in genes o Eye color gene • Blue allele vs. brown allele Homologous pair of chromosomes Genotype: The physical location of a gene on a chromosome is called its locus ( plural: loci ). Gene loci (location) A b c A B c AA Bb cc Homozygous Heterozygous Homozygous for the for the dominant recessive 13 allele allele • The sex chromosomes (X and Y) are not homologous o They differ in size and genetic composition • They do have short regions of homology, though 14 2.2 Cell Division The process of binary fission in bacteria Phases of the eukaryotic cell cycle Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 15 Cell Division • One purpose of cell division is asexual reproduction o How some unicellular organisms make new individuals o Examples: • Bacteria • Amoeba • Baker’s Yeast (Saccharomyces cerevisiae) • Cell division also allows multicellularity o Plants, animals and some fungi derive from a single cell that has undergone repeated cell divisions o Ex: Humans • Start as a fertilized egg, grow to several trillion cells Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 16 Bacteria Reproduce Asexually by Binary Fission • The capacity of bacteria to divide is really quite astounding o Escherichia coli can divide every 20-30 minutes • Prior to division, the bacterial cell replicates its chromosome • Then the cell divides into two daughter cells by a process called binary fission • Binary fission is asexual – does not involve genetic contributions from two different gametes Mother cell Bacterial chromosome Replication of bacterial chromosome FtsZ protein Septum Two daughter cells 17 Eukaryotic Cell Cycle • Eukaryotic cells must use a more complex process o Each daughter cell must receive the right number of each type of chromosome o Series of phases is called the cell cycle • Cell cycle: o o o o G1 phase – Gap 1 S phase – Synthesis G2 phase – Gap 2 M phase – Mitosis Interphase Mother cell Interphase Restriction point S G1 G0 (Nondividing cell) M Mitosis G2 Chromosome Nucleolus Formation of two daughter cells 18 • G1 phase o The cell prepares to divide o Restriction point – the point at which molecular changes have accumulated to commit the cell to proceed through cell division • S phase o o o o Chromosomes are replicated After replication the copies are called chromatids The two sister chromatids are joined at the centromere to form a dyad Kinetochore proteins on the centromere help with sorting • G2 phase o The cell accumulates the material for nuclear and cell division Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 19 • M phase o When mitosis occurs o Distributes replicated chromosomes to produce two identical daughter cells o Cytokinesis – the process that divides the cell into two daughters A pair of sister chromatids (a dyad) Centromere (DNA that is hidden beneath the kinetochore proteins) One chromatid Kinetochore (proteins attached to the centromere) One chromatid (a monad) 20 • A cell may remain for long periods of time in the G0 phase • A cell in G0 phase has either o Postponed making a decision to divide o Or made the decision to never divide again • Ex: Terminally differentiated cells like neurons Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 21 2.3 Mitosis and Cytokinesis Structure and function of the mitotic spindle Phases of mitosis Key differences in cytokinesis between plants and animals Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 22 The Mitotic Spindle Apparatus • Mitotic spindle apparatus o Organizes and sorts eukaryotic chromosomes o Forms from microtubule-organizing centers (MTOCs) • In animals, the two MTOCs are called centrosomes o Centrosomes lie at each spindle pole o A pair of centrioles is within each centrosome • Plants do not have centrosomes o The nuclear envelope functions as an MTOC Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23 • The mitotic spindle has three types of microtubules o Aster microtubules • From the centrosome to the plasma membrane • Help position the spindle o Polar microtubules • Project from the centrosomes to the middle • Help to push the spindle poles away from each other o Kinetochore microtubules • Attach to the kinetochore on the centromere of the chromosomes Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 24 Mitosis • Mitosis is the process of organizing and sorting the chromosomes into two daughter cells during the cell cycle • Mitosis takes place in five phases o o o o o Prophase Prometaphase Metaphase Anaphase Telophase Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 25 • In these diagrams, the original mother cell is diploid (2n) o It contains a total of 6 chromosomes o Three per set (n = 3) • Before mitosis begins, the cell is in Interphase • Refer to Figure 2.8 for the phases of mitosis Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 26 27 28 29 30 31 32 • Cytokinesis o In most cases, mitosis is quickly followed by cytokinesis to produce the separate daughter cells S o In animals • Formation of a cleavage furrow G1 G2 o In plants • Formation of a cell plate Cleavage furrow 150 mm 33 • Mitosis and cytokinesis ultimately produce two daughter cells with the same number of chromosomes as the mother cell • The two daughter cells are genetically identical o Except for rare mutations • Thus, mitosis ensures genetic consistency from one cell to the next • The development of multicellularity relies on the repeated processes of mitosis and cytokinesis 34 2.4 Meiosis Phases of meiosis Key differences between mitosis and meiosis 35 Meiosis • Meiosis produces haploid cells from a cell that was originally diploid • Like mitosis, the cell has progressed through G1, S, and G2 • Unlike mitosis, meiosis involves two successive divisions called Meiosis I and Meiosis II, each subdivided into • • • • • Prophase Prometaphase Metaphase Anaphase Telophase Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 36 37 38 Prophase of Meiosis I • Prophase I is further subdivided into periods known as o o o o o Leptotene Zygotene Pachytene Diplotene Diakinesis Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 39 Prophase of Meiosis I • Leptotene o Replicated chromosomes begin to condense • Zygotene o Via the process of synapsis, homologous chromosomes recognize each other and align, forming the synaptonemal complex • Pachytene o Homologs are aligned o Pairs of sister chromatids – four total! – are called bivalents or tetrads o Crossing over occurs Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 40 Crossing Over • During pachytene of Prophase I • Crossing over involves a physical exchange of chromosome pieces that result in exchange of genetic information • On each chromosome, may occur a couple times or a couple dozen times, depending on size and species -- About twice per chromosome in human sperm • During crossing over, a chiasma forms (plural: chiasmata) Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 41 Prophase of Meiosis I • Diplotene o The synaptonemal complex starts to dissociate, allowing the bivalent to separate slightly • Diakinesis o The synaptonemal complex has disappeared Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 42 43 Prometaphase of Meiosis I o Pairing and crossing over are completed at the end of prophase o In prometaphase I, the spindle apparatus is formed o Chromatids are attached to the spindle via kinteochore microtubules Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 44 45 Metaphase of Meiosis I o At metaphase, the bivalents (or tetrads) are organizaed along the metaphase plate o Pairs of sister chromatids form a double row (not a single row as in mitosis) o Sister chromatids may be aligned in a very large number of possible ways • 2n possible alignments • Humans would have 223 – more than 8 million possible arrangements! 46 47 Anaphase of Meiosis I and Cytokinesis o The two pairs of sister chromatids within a bivalent separate from one another o However, the connection between the sister chromatids does not break • The joined pair of sister chromatids moves to the pole • In other words, the two dyads within a tetrad separate and move to opposite poles 48 49 Telophase of Meiosis I and Cytokinesis o The dyads have separated to opposite poles • The chromatids may decondense and the nuclear membrane may reform at this point o Meiosis I ends with two cells, each with three pairs (in this example) of sister chromatids o This is a reduction division, and the cells are considered haploid, because they only carry one set of homologous chromosomes 50 Meiosis II • The sorting events that occur during meiosis II are similar to those that occur during mitosis, however the starting point is different o For a diploid organism with six chromosomes • Mitosis begins with 12 chromatids joined as six pairs of sister chromatids • Meiosis II begins with 6 chromatids joined as three pairs of sister chromatids • There is no chromosomal replication prior to meiosis II • Meiosis II proceeds through prophase, prometaphase, metaphase, anaphase, and telophase Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 51 52 Mitosis vs. Meiosis o Mitosis produces two diploid daughter cells o Meiosis produce four haploid daughter cells o Mitosis produces cells that are genetically identical o Meiosis produces cells that are not genetically identical • The daughter cells contain only one homologous chromosome from each pair • The daughter cells contain many different combinations of the single homologs 53 2.5 Sexual Reproduction Definition of sexual reproduction How animals make sperm and egg cells How plants alternate between haploid and diploid generations Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 54 Sexual Reproduction • The most common way for eukaryotic organisms to produce offspring o Parents make gametes with half the amount of genetic material • These gametes fuse with each other during fertilization to begin the life of a new organism • The process of forming gametes is called gametogenesis Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 55 • Some simple eukaryotic species are isogamous o They produce gametes that are morphologically similar • Example: Many species of fungi and algae • Most eukaryotic species are heterogamous o These produce gametes that are morphologically different • Sperm cells o Relatively small and mobile • Egg cell or ovum o Usually large and nonmobile o Stores a large amount of nutrients, in animal species Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 56 • Gametes are typically haploid o They contain a single set of chromosomes • Gametes are 1n, while diploid cells are 2n o A diploid human cell contains 46 chromosomes o A human gamete only contains 23 chromosomes • During meiosis, haploid cells are produced from diploid cells o Chromosomes must be correctly distributed • Each gamete must receive one chromosome from each pair Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 57 Spermatogenesis • The production of sperm • In male animals, it occurs in the testes • A diploid spermatogonial cell divides mitotically to produce two cells o One remains a spermatogonial cell o The other becomes a primary spermatocyte • The primary spermatocyte progresses through meiosis I and II MEIOSIS I MEIOSIS II Secondary spermatocyte Primary spermatocyte (diploid) Spermatids Sperm cells (haploid) 58 Meiois I yields two haploid secondary spermatocytes Meiois II yields four haploid spermatids Each spermatid matures into a haploid sperm cell Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS I MEIOSIS II Secondary spermatocyte Primary spermatocyte (diploid) Spermatids (a) Spermatogenesis Sperm cells (haploid) 59 • The structure of a sperm includes o A long flagellum o A head • The head contains a haploid nucleus o Capped by the acrosome • In human males, spermatogenesis is a continuous process o A mature human male produces several hundred million sperm per day Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 60 Oogenesis • The production of egg cells • In female animals, it occurs in the ovaries • Early in development, diploid oogonia produce diploid primary oocytes o In humans, for example, about 1 million primary oocytes per ovary are produced before birth Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 61 • The primary oocytes initiate meiosis I • However, they enter into a dormant phase o They are arrested in prophase I until sexual maturity • At puberty, primary oocytes are periodically activated to progress through meiosis I o In humans, one oocyte per month is activated • The division in meiosis I is asymmetric producing two haploid cells of unequal size o A large secondary oocyte and a small polar body Secondary oocyte Primary oocyte (diploid) 62 First polar body • The secondary oocyte enters meiosis II but is quickly arrested in it • It is released into the oviduct o An event called ovulation • If the secondary oocyte is fertilized o Meiosis II is completed o A haploid egg and a second polar body are produced Second polar body Egg cell (haploid) 63 • The haploid egg and sperm nuclei then fuse to create the diploid nucleus of a new individual • Note that only one of the four cells produced in this meiosis becomes an egg Secondary oocyte Primary oocyte (diploid) Second polar body Egg cell (haploid) First polar body 64 Plants Alternate Between Haploid and Diploid Generations • The life cycles of plant species alternate between two generations o Haploid generation is called the gametophyte o Diploid generation is called the sporophyte Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 65 • Meiosis produces haploid cells called spores o Spores divide by mitosis to produce the gametophyte • In simpler plants, like mosses: o Spores develop into gametophytes that have large numbers of cells • In flowering plants: o Spores develop into gametophytes that have only a few cells – they are tiny o What we see as “the plant” is the sporophyte Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 66 • Meiosis occurs within two different structures of the sporophyte o Anthers • Produce the male gametophyte – the pollen grain o Ovaries • Produce the female gametophyte – the embryo sac 67 Animals vs. Plants • Animals produce gametes by meiosis • Plants produce spores by meiosis o The spores develop into gametophytes o The haploid gametophyte becomes multicellular by mitotic cell divisions o The mutlicellular gametophyte then goes on to produce certain specialized cells as gametes Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 68