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Reproduction and Chromosome Transmission
LECTURE 6 Reproduction and Chromosome Transmission (Chapter 3) Slides 1-3; 9-12; 25-44 On your own: Slides 4-8; 13-24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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INTRODUCTION In this chapter we will survey reproduction at the cellular level We will examine chromosomes at the microscopic level This examination provides us with insights to help understand the inheritance patterns of traits Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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3.1 GENERAL FEATURES OF CHROMOSOMES
Chromosomes are structures within living cells that contain the genetic material They contain the genes Biochemically, chromosomes are composed of DNA, which is the genetic material Proteins, which 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
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3.1 GENERAL FEATURES OF CHROMOSOMES
First, let’s consider the distinctive cellular differences between the two types of cells 1. Prokaryotes Bacteria and archaea 2. Eukaryotes Protists, fungi, plants and animals Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Prokaryotes Do not contain a nucleus
Usually contain a single type of circular chromosome Found in the nucleoid Cytoplasm is enclosed by a plasma membrane Regulates nutrient uptake and waste excretion Outside the membrane is a rigid cell wall For protection from breakage May contain other structures Outer membrane Flagella Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Figure 3.1 (a) Bacterial cell
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 mm Ribosomes in cytoplasm Flagellum Outer membrane Cell wall Plasma membrane (also known as inner membrane) Nucleoid (where bacterial chromosome is found) (a) Bacterial cell This example is typical of bacteria such as Escherichia coli, which has an outer membrane and flagella.
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Eukaryotes Have a nucleus
Contains most of the genetic material in the form of linear chromosomes Bounded by two membranes Have membrane-bounded organelles with specific functions These include Mitochondria ATP synthesis Contain their own DNA Lysosomes Plays a role in degradation of macromolecules Golgi apparatus Plays a role in protein modification and trafficking Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Figure 3.1 (b) Animal cell Microfilament Golgi body Nuclear envelope
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Microfilament Golgi body Nuclear envelope Nucleolus Chromosomal DNA Nucleus Polyribosomes Ribosome Rough endoplasmic reticulum Cytoplasm Membrane protein Plasma membrane Smooth endoplasmic reticulum Lysosome Figure 3.1 (b) Animal cell Mitochondrial DNA Mitochondrion Centriole Microtubule (b) Animal cell
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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 This allows the detection of individuals with abnormal chromosome number or structure This also provides a way to distinguish between two closely-related species Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Eukaryotic Chromosomes Are Inherited in Sets
Most eukaryotic species are diploid Have two sets of chromosomes For example Humans 46 total chromosomes (23 per set) Dogs 78 total chromosomes (39 per set) Fruit fly 8 total chromosomes (4 per set) Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Eukaryotic Chromosomes Are Inherited in Sets
Members of a pair of chromosomes are called homologs The two homologs form a homologous pair The two chromosomes in a homologous pair Are nearly identical in size Have the same banding pattern and centromere location Have the same genes But not necessarily the same alleles Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Eukaryotic Chromosomes Are Inherited in Sets
The DNA sequences on homologous chromosomes are also very similar There is usually less than 1% difference between homologs Nevertheless, these slight differences in DNA sequence provide the allelic differences in genes Eye color gene Blue allele vs. brown allele Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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The process of mitosis is shown in Figure 3.8
Mitosis was first observed microscopically in the 1870s by the German biologist, Walter Flemming He coined the term mitosis From the Greek mitos, meaning thread The process of mitosis is shown in Figure 3.8 The original mother cell is diploid (2n) It contains a total of six chromosomes Three per set (n = 3) One set is shown in blue and the homologous set in red Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Mitosis is subdivided into five phases Prophase Prometaphase Metaphase
Anaphase Telophase Following are the stages of mitosis from Figure 3.8 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Chromosomes are decondensed
By the end of interphase, the chromosomes have already replicated But the six pairs of sister chromatids are not seen until prophase The centrosome, the attachment point of the mitotic spindle, divides Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two centrosomes, each with centriole pairs Nuclear membrane Nucleolus Chromosomes INTERPHASE
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Nuclear envelope dissociates into small vesicles
Chromatids condense into more compact structures Centrosomes begin to separate The mitotic spindle apparatus is formed Composed of mircotubules (MTs) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Microtubules forming mitotic spindle Sister chromatids PROPHASE
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Microtubules are formed by rapid polymerization of tubulin proteins
There are three types of spindle microtubules 1. Aster microtubules Important for positioning of the spindle apparatus 2. Polar microtubules Help to “push” the poles away from each other 3. Kinetochore microtubules Attach to the kinetochore , which is bound to the centromere of each individual chromosome Refer to Figure 3.7
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3-36 Figure 3.7 Contacts the centromere Contacts the other two
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Contacts the centromere Inner plate Centromeric DNA Contacts the other two Kinetochore Middle layer Outer plate Kinetochore microtubule Contacts the kinetochore microtubule Spindle pole: a centrosome with 2 centeriorles Figure 3.7 Aster microtubules Kinetochore Metaphase plate Sister chromatids Polar microtubules Kinetochore microtubules 3-36
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Spindle fibers interact with the sister chromatids
Centrosomes move to opposite ends of the cell, forming the spindle poles Spindle fibers interact with the sister chromatids Kinetochore microtubules grow from the two poles If they make contact with a kinetochore, the sister chromatid is “captured” If not, the microtubule depolymerizes and retracts to the centrosome The two kinetochores on a pair of sister chromatids are attached to kinetochore MTs on opposite poles Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nuclear membrane fragmenting into vesicles Mitotic spindle Spindle pole PROMETAPHASE
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Pairs of sister chromatids align themselves along a plane called the metaphase plate
Each pair of chromatids is attached to both poles by kinetochore microtubules Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Astral microtubule Metaphase plate Polar microtubule Kinetochore proteins attached to centromere Kinetochore microtubule METAPHASE
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The connection holding the sister chromatids together is broken
Each chromatid, now an individual chromosome, is linked to only one pole As anaphase proceeds Kinetochore MTs shorten Chromosomes move to opposite poles Polar MTs lengthen Poles themselves move further away from each other Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chromosomes ANAPHASE
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Chromosomes reach their respective poles and decondense
Nuclear membrane reforms to form two separate nuclei In most cases, mitosis is quickly followed by cytokinesis In animals Formation of a cleavage furrow In plants Formation of a cell plate Refer to Figure 3.9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nuclear membrane re-forming Cleavage furrow Chromosomes decondensing TELOPHASE AND CYTOKINESIS
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Figure 3.9 Cleavage furrow S G1 G2 150 um
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cleavage furrow S G1 G2 Cytokinesis 150 um © Dr. David M. Phillips/Visuals Unlimited (a) Cleavage of an animal cell Phragmoplast Cell plate 10 um © Ed Reschke (b) Formation of a cell plate in a plant cell
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The two daughter cells are genetically identical to each other
Mitosis and cytokinesis ultimately produce two daughter cells having the same number and complement of chromosomes as the mother cell The two daughter cells are genetically identical to each other Barring rare mutations Thus, mitosis ensures genetic consistency from one cell to the next The development of multicellularity relies on the repeated process of mitosis and cytokinesis
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3.3 SEXUAL REPRODUCTION Sexual reproduction is the most common way for eukaryotic organisms to produce offspring 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
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Some simple eukaryotic species are isogamous
They produce gametes that are morphologically similar Example: Many species of fungi and algae Most eukaryotic species are heterogamous These produce gametes that are morphologically different Sperm cells (male gametes) Relatively small and mobile Egg cell or ovum (female gametes) Usually large and nonmotile Stores a large amount of nutrients (animal species) Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Gametes are typically haploid
They contain a single set of chromosomes Gametes are 1n, while diploid cells are 2n A diploid human cell contains 46 chromosomes A human gamete contains only 23 chromosomes During meiosis, haploid cells are produced from diploid cells Thus, the chromosomes must be correctly sorted and distributed to reduce the chromosome number to half its original value In humans, for example, a gamete must receive one chromosome from each of the 23 pairs Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Tribble: B_D_S_XOXo
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MENDEL'S LAWS AND MEIOSIS
Law of Segregation Refers to situations in which a single gene is being followed through a cross Each diploid adult has two alleles for every gene but passes only one allele to each of its haploid gametes AA makes gametes containing only A Aa makes gametes containing A or a (half of each) aa makes gametes containing only a A a AA Aa aa
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MENDEL'S LAWS AND MEIOSIS
Law of Independent Assortment Refers to situations in which more than one gene is being followed through a cross Assumes that meiosis includes independent assortment of homologues but NO CROSSING OVER Under these circumstances, the number of different gametes produced depends only on the number of genes being followed in the cross # gametes = 2(# hybrid loci)
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For AaBb: # gametes = 22 = 4 AB Ab aB For AaBbCc: # gametes = 23 = 8 ABC ABc AbC Abc aBC aBc abC abc
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For the following cross: AaBb x aabb
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MEIOSIS Like mitosis, meiosis begins after a cell has progressed through interphase of the cell cycle Unlike mitosis, meiosis involves two successive divisions These are termed Meiosis I and Meiosis II Each meiotic division is subdivided into Prophase Prometaphase Metaphase Anaphase Telophase Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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MEIOSIS Prophase I is further subdivided into five stages known as
Leptotene Zygotene Pachytene Diplotene Diakinesis Refer to Figure 3.10 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Figure 3.10 tetrad A physical exchange of chromosome pieces
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. STAGES OF PROPHASE OF MEIOSIS I LEPTOTENE ZYGOTENE PACHYTENE DIPLOTENE DIAKINESIS Nuclear membrane Bivalent forming Chiasma Nuclear membrane fragmenting tetrad Synaptonemal complex forming Replicated chromosomes condense. Synapsis begins. A bivalent has formed and crossing over has occurred. Synaptonemal complex dissociates. End of prophase I A physical exchange of chromosome pieces Figure 3.10
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The Synaptonemal Complex
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Synaptonemal Complex Formed between homologous chromosomes May not be required for pairing Precise role not clearly understood Synaptonemal complex Bound to chromosomal DNA of homologous chromatids Lateral element Central element Chromatid Figure 3.11 Transverse filament Provides link between lateral elements
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Figure 3.12 Spindle apparatus complete
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Spindle apparatus complete Chromatids attached via kinetochore microtubules MEIOSIS I Centrosomes with centrioles Mitotic spindle Sister chromatids Bivalent Synapsis of homologous chromatids and crossing over Nuclear membrane fragmenting EARLY PROPHASE LATE PROPHASE PROMETAPHASE Cleavage furrow Metaphase plate METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS MEIOSIS II Four haploid daughter cells PROPHASE PROMETAPHASE METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS
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Bivalents are organized along the metaphase plate in Meiosis I
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bivalents are organized along the metaphase plate in Meiosis I Pairs of sister chromatids are aligned in a double row, rather than a single row (as in mitosis) Metaphase plate Figure 3.12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The arrangement is random with regard to the (blue and red) homologs Furthermore One pair of sister chromatids is linked to one of the poles And the homologous pair is linked to the opposite pole Kinetochore Figure 3.13
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The two pairs of sister chromatids separate from each other.
However, the connection that holds sister chromatids together does not break. Sister chromatids reach their respective poles and decondense. Nuclear envelope reforms to produce two separate nuclei. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cleavage furrow Metaphase plate METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Figure 3.12 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Meiosis I is followed by cytokinesis and then meiosis II
The sorting events that occur during meiosis II are similar to those that occur during mitosis However the starting point is different 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 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Figure 3.12 Four haploid daughter cells PROPHASE PROMETAPHASE
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Four haploid daughter cells PROPHASE PROMETAPHASE METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Figure 3.12
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Mitosis vs Meiosis Mitosis produces two diploid daughter cells
Meiosis produces four haploid daughter cells Mitosis produces daughter cells that are genetically identical Meiosis produces daughter 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 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Spermatogenesis The production of sperm
In male animals, it occurs in the testes A diploid spermatogonial cell divides mitotically to produce two cells One remains a spermatogonial cell The other becomes a primary spermatocyte The primary spermatocyte progresses through meiosis I and II Refer to Figure 3.14a Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Figure 3.14 (a) Each spermatid matures into a haploid sperm cell
Meiois I yields two haploid secondary spermatocytes Meiois II yields four haploid spermatids Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS I MEIOSIS II Primary spermatocyte (diploid) Spermatids Sperm cells (haploid) (a) Spermatogenesis Figure 3.14 (a)
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The structure of a sperm includes
A long flagellum A head The head contains a haploid nucleus Capped by the acrosome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The acrosome contains digestive enzymes - Enable the sperm to penetrate the protective layers of the egg Sperm cells (haploid) In human males, spermatogenesis is a continuous process A mature human male produces several hundred million sperm per day Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Oogenesis The production of egg cells
In female animals, it occurs in the ovaries Early in development, diploid oogonia produce diploid primary oocytes 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
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The primary oocytes initiate meiosis I
However, they enter into a dormant phase They are arrested in prophase I until the female becomes sexually mature At puberty, primary oocytes are periodically activated to progress through meiosis I In humans, one oocyte per month is activated The division in meiosis I is asymmetric producing two haploid cells of unequal size A large secondary oocyte A small polar body Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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It is released into the oviduct If the secondary oocyte is fertilized
The secondary oocyte enters meiosis II, but is arrested at metaphase II It is released into the oviduct An event called ovulation If the secondary oocyte is fertilized Meiosis II is completed A haploid egg and a second polar body are produced The haploid egg and sperm nuclei then fuse to create the diploid nucleus of a new individual Note that only one of the cells produced in this meiosis becomes an egg Refer to Figure 3.14b Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Unlike spermatogenesis, the divisions in oogenesis are asymmetric
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Secondary oocyte Second polar body Primary oocyte (diploid) Egg cell (haploid) First polar body (b) Oogenesis Figure 3.14 (b) Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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