Principles of Genetics (Biol 4041) PDF

Bahir Dar University College of Science Department of Biology Principles of Genetics (Biol 4041) Sileshi A. (Ph.D)...

Bahir Dar University College of Science Department of Biology Principles of Genetics (Biol 4041) Sileshi A. (Ph.D) http://bdu.edu.et Friday, December 20, 2024 1 www.facebook.com/bduethiopia  Chapter One UNIT 1 INTRODUCTION Introduction 1.1 What is genetics? – is the study of inheritance of traits along generations and the mechanism of variation at molecular, individual and population level and even to the evolution of species. – “gene- GR.- to become” – Genetics is the science of coming into being  The term was coined by William Bateson-1905 As a science genetics is a very young discipline  Genes are units of biological information that serve as hereditary vehicle  The science of genetics deals with:  Mechanisms of inheritance Molecular nature, and functions of the genetic materials  Regulation of gene expression  Distribution of genes in natural populations Heredity is the passing of traits from parents to their offspring.  Genetics is one of the central pillars of biology and overlaps with many other areas, such as » Molecular biology, Cell biology, Evolution, Biotechnology etc. Traits are determined by the genes on the chromosomes. A gene is a segment of DNA that determines a trait. http://bdu.edu.et Friday, December 20, 2024 4 www.facebook.com/bduethiopia  1.2 Branches of Genetics There are 3 major branches:  Transmission Genetics: - deals with the mechanism of transmission of genes along generation and their recombination  Molecular Genetics: deals with the structure and functions of genes (replication, transcription, translation and regulation) at a molecular level  Population Genetics: deals with the behavior and distribution of genes in natural populations Population genetics focuses on the degree and pattern of genetic diversity in the gene pool of the natural populations and the mechanisms of transmission and preservation of the variation.  1.3 Applications of Genetics Genetics has got a myriads of applications in the different sectors:  Agriculture: - Crop & animal improvement/breeding  Medicine: molecular basis of diseases, Diagnostics (rapid, cheap and sensitive), pharmaceuticals  Forensics: DNA finger printing for criminal and court cases  Eugenics: misuse of the knowledge of genetics on human species E.g. Segrigation (Nazism)  Euphenics: medical/genetic intervention to reduce the impact of defective genotypes on individuals  1.4 Historical Milestones The field of Genetics is relatively a very young field of science about a century old It was established as a distinct science in 1900 when Mendel’s findings were re-discovered by Devries, Correns and Tschermak Since the start of civilization, humankind has known about genetics; Domestication of animals and plants via selective breeding E.g Horses, camels, cattle and dogs- 8000-1000 BC Cultivation of maize, wheat etc around 5000 BC Ability to indentify a person as a member of a particular family by certain physical traits  However, the mechanisms of heredity, remained mystery until the 19th century. Many scientists and naturalists were thriving to unlock the mechanism of inheritance A number of schools of thought forwarded theories regarding the mechanism of inheritance. Some of them are: Pre-Mendelian Ideas about Heredity  Hippocrates (500-400 BC) – the males semen is formed in different parts of the body and transported to the testicles through the blood vessels - active humors are bearers of the hereditary traits  Aristotle (384-322 BC):- the vital heat contained in the semen generates offspring by cooking and molding the menstrual blood  Theory of epigenesis: - William Harvey (1578-1657) - An organism is derived from germ cells that contain definate but undifferentiated substances that after fertilization organize to complex organs  Theory of pre-formation: Swammerdam (17th C) - Sex cells has miniature copy of adults and the embryonic development is actually the enlargement of the pre-existed organism - Some scientists (Hartsoeker-1694 and Dalempotius-1699) imagined that they could see a miniature copy- homumculus - Disproved by casperwolf (1733-1794) favoring the theory of epigenesis  Theory of pangenesis: Charles Darwin-1868 - Every cell/tissue produces minute particles (Pangenes/gemmules) discharged to the bloodstream and deposited to the sex cells. Fusion of pangenes will give rise to a child during fertilization  Gregor John Mendel published on the particulate nature of the genetic material in 1866  He was an Austrian Monk who experimented with garden peas  He noticed that:  Certain traits seemed to be passed from one generation to another  The hereditary material was of particulate nature  The behavior of their transmission can statistically be predicted  He is known as the “father of genetics”  However, his works were not appreciated until they were re-discovered by other scientists http://bdu.edu.et Friday, December 20, 2024 10 www.facebook.com/bduethiopia  2. The Mendelian Experiments worked with garden peas (Pisum sativum). a. He selected strains that differed in particular traits (e.g., purple or white flowers) b. Develop true breeding lines by raising the plants for many generations c. M a d e c r o s s e s a n d c o u n t e d t h e appearance of traits in the progeny d. He concluded that each organism contains two copies of each gene, one from each parent the alternative versions of the genes (alleles) exist (e.g., pea flower color alleles are purple( P) & white (p).  He was not the first to conduct hybridization experiments however his success was attributed to:  He chose good model plant (garden pea)- easy to obtain varieties, take little space and time and self pollinated  He performed his experiments carefully by tracing only selected characters  He kept excellent records  Fortunately studied traits transmitted independently (non-linked)  Used mathematical models to analyze his data http://bdu.edu.et Friday, December 20, 2024 12 www.facebook.com/bduethiopia  Traits he studied 1. Plant height (Tall Vs dwarf) 5. Pod shape (Inflated vs constricted) 2. Seed coat color (Colored vs white) 6. Seed color (Yellow vs green) 3. Flower position (Axial vs terminal) 7. Seed shape (Round vs wrinkled) 4. Pod color (Green vs yellow) http://bdu.edu.et Friday, December 20, 2024 13 www.facebook.com/bduethiopia  Cross Breeding Experiments Steps He raised true-breeding lines for each contrasting pair of characters He carefully removed the anthers (emasculated) from the pea plants Dust pollen grains from the female parent into the stigma of the male flower First generation pea plants were called parental generation, P0, while the following generations were called filial, Fn The ratio of characteristics in the P0 :F1:F2 generations became the basis for Mendel’s postulates. http://bdu.edu.et Friday, December 20, 2024 14 www.facebook.com/bduethiopia  MonohybridCrosses He let the F1 plants self-fertilize to give F2 generation Found that 7055 plants had violet flowers and 224 had white flowers This was a ratio of 3.15 violet flowers to 1 white flower/approximately 3:1 This 3:1 was no fluke He did same for the other six characteristics He observed that one of the two traits would disappear completely from the F1 in a ratio of roughly 3:1 http://bdu.edu.et Friday, December 20, 2024 15 www.facebook.com/bduethiopia  Conclusions  Based on his results Mendel came up with a model for the inheritance of individual characteristics a. parents pass their traits unchanged along “heritable factors," which we now call genes, b. Each parent has two copies of a given gene, such as the gene for seed color (Y gene) shown below. c. If these copies represent different versions, or alleles, of the gene, one allele /the dominant one/may mask the other allele—the recessive one. E.g. For seed color, the dominant yellow allele Y hides the recessive green allele y http://bdu.edu.et Friday, December 20, 2024 16 www.facebook.com/bduethiopia  Definition of Terms  Filial: of a generation or generations descending from a specific previous one  parental: the generation of organisms that produce a hybrid  Gene: a particulate hereditary unit factor that control a given character  Allele - one alternative form of a given pair; tall and dwarf more than two alleles can exist for any specific gene, but only two of them will be found within any individual  Dominant allele: the allele that was visible in the F1 generation (violet flowers) by masking the other allele  Recessive allele: the allele that was hidden or lost (white flowers) at F1  Allelic pair - the combination of two alleles which comprise the gene pair http://bdu.edu.et Friday, December 20, 2024 17 www.facebook.com/bduethiopia  Genotype - the specific allelic combination for a certain gene or set of genes  True Breeding line/pure line: a line that give the same phenotype along generations (indefinitely) i.e. a line that do not segregate  Phenotype: an organism's observable features that is affected by the environment in many real-life cases, though this did not have an impact on Mendel's work http://bdu.edu.et Friday, December 20, 2024 18 www.facebook.com/bduethiopia  Homozygote - an individual which contains only one allele at the allelic pair; e.g. DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest Heterozygote - an individual which contains one of each member of the gene pair e.g. Dd heterozygote Genetic variation is the heritable component of phenotypic variation that arises due to reshuffle, crossover & alter genes. This variation is the raw material for evolution. http://bdu.edu.et Friday, December 20, 2024 19 www.facebook.com/bduethiopia  The law of segregation The previous model doesn't still account for the 3:1 ratio. For that, he added Mendel's law of segregation (First law of inheritance). States that during gamate formation the paired unit factors segregate randomly from each other When an egg and a sperm join in fertilization, they form a new organism that contain genes derived from both parents http://bdu.edu.et Friday, December 20, 2024 20 www.facebook.com/bduethiopia  The Law of Independent Assortment Mendel also designed experiments to predict the inheritance of two characteristics associated with two different genes For this he conducted di-hybrid crosses/two factors cross to know whether two genes are inherited independently or not Do they "ignore" one another when they're sorted into gametes, or whether they "stick together" and get inherited as a unit? He found that different genes were inherited independently of one another, giving rise to the law of independent assortment/Mendel’s second rule of inheritance States that the hereditary factors of one pair assorts itself with either one of the other pair independently of its own pair. http://bdu.edu.et Friday, December 20, 2024 21 www.facebook.com/bduethiopia  Dihybrid cross http://bdu.edu.et Friday, December 20, 2024 22 www.facebook.com/bduethiopia  The ratio was the key clue that led Mendel to the law of independent assortment Because a 9:3:3:1 ratio is exactly what we'd expect to see if the F1 plant made 4 types of gametes with equal frequency: YR, Yr, yR, and yr. In other words, this is the result we'd predict if each gamete randomly got a Y or y allele, and, in a separate process, also randomly got an R or r allele http://bdu.edu.et Friday, December 20, 2024 23 www.facebook.com/bduethiopia  The test cross Mendel also came up with a way to figure out whether an organism with a dominant phenotype was a heterozygote or a homozygote This technique is called a test cross and is still used by plant and animal breeders today. In a test cross, the organism with the dominant phenotype is crossed with an organism that is homozygous recessive If the test organism is homozygous, then all of the F1 offspring will get a dominant allele but if it was a heterozygote, the F1 offspring will be half heterozygotes (dominant phenotype) and half recessive homozygotes (recessive phenotype) The fact that we get a 1:1 ratio in this second case is another confirmation of Mendel’s law of segregation. http://bdu.edu.et Friday, December 20, 2024 24 www.facebook.com/bduethiopia  A test cross designed to identify whether a pea plant with yellow pod color (dominant) has got a homozygous or heterozygous genotype Punnet square: an approach to analyze possible genotypes resulting from a given cross http://bdu.edu.et Friday, December 20, 2024 25 www.facebook.com/bduethiopia  Independent assortment vs. linkage Question: what was the alternative possibility to independent assortment? That is, what would happen if two genes didn't follow independent assortment? What if the genes for seed color and seed shape might have always been inherited as a pair? This condition is called Linkage Is the tendency for two or more genes that are found on the same chromosome to inherit together. we'll explore this idea further in the next chapter http://bdu.edu.et Friday, December 20, 2024 26 www.facebook.com/bduethiopia  Linkage Condition http://bdu.edu.et Friday, December 20, 2024 27 www.facebook.com/bduethiopia  Finding genotypic, phenotypic and gametic frequencies from a given cross There are two approaches: 1. Punnet square: Tedious and lengthy especially when the crosses involve more than two characters at a time 2. Fork line diagram: is a simple and effective technique  It involves two steps:  Taking each loci separately  Calculating the ratios by us which which states that the probability of two (or more) independent events occurring together can be calculated by multiplying the individual probabilities of the events. For example: If you roll two dice at once, your chance of getting two sixes is: (probability of a six on die 1) x (probability of a six on die 2) http://bdu.edu.et = (1/6)⋅(1/6)=1/36 Friday, December 20, 2024 www.facebook.com/bduethiopia 28  Some genetics problems one may need to calculate the probability that any one of several events will occur In this case, the summation rule will be applied which states the probability that any of several mutually exclusive events will occur is equal to the sum of the events’ individual probabilities. For example: if you roll a six-sided die, the chance of getting either a one or a six will be (1/6)+(1/6) http://bdu.edu.et Friday, December 20, 2024 29 www.facebook.com/bduethiopia   Given a cross involves n number of heterozygous pairs one can calculate the expected genotypic and phenotypic ratios assuming complete dominance and independent assortment using general formula: # Class of gametes = 2n # Gamate Combinations = 4n # Different genotypes = 3n # Homozygous genotypic classes = 2n # Heterozygous genotypic classes = 3n- 2n # Phenotypic Classes = 2n Example: AaBbCc n=3 http://bdu.edu.et Friday, December 20, 2024 30 www.facebook.com/bduethiopia  Exceptions to Mendelian Rules Mendelian rules still form the foundation of our understanding of inheritance However, we now know of some exceptions, extensions, and variations, which must be added to fully explain the inheritance patterns we see around us Multiple alleles: real populations often have multiple alleles of a given gene. Incomplete dominance: Two alleles may produce an intermediate phenotype when both are present, rather than one fully determining the phenotype. Codominance: Two alleles may be simultaneously expressed when both are present, rather than one fully determining the phenotype. Pleiotropy: Some genes affect many different characteristics, not just a single characteristic. Lethal alleles: Some genes have alleles that prevent survival when homozygous or heterozygous. Sex linkage: some genes show different inheritance patterns in different sexes http://bdu.edu.et Friday, December 20, 2024 31 www.facebook.com/bduethiopia  Summary Some of the key elements of Mendel’s original model were: 1. Traits are determined by heritable factors, now called genes. Genes come in pairs (present in two copies in an organism) 2. Genes come in different versions, now called alleles. When an organism has two different alleles of a gene, one (the dominant allele) will hide the presence of the other (the recessive allele) and determine appearance 3. During gamete production, each egg or sperm cell receives just one of the two gene copies present in the organism, and the copy allocated to each gamete is random (law of segregation) 4. Genes for different traits are inherited independently of one another (law of independent assortment). http://bdu.edu.et Friday, December 20, 2024 32 www.facebook.com/bduethiopia  2. The cytological Basis of Inheritance Where are genes found in a cell? Medndel’s works didn’t get the attention of the scientific community sine 1902 Scientists Walter Sutton and Theodor Boveri began to study Mendel’s long-ignored work Re-evaluate his model in terms of the behavior of chromosomes Sutton, who was American, studied chromosomes and meiosis in grasshoppers. Boveri, who was German, studied the same things in sea urchins. In 1902 and 1903, Sutton (grass hoppers) and Boveri (sea urchins) published independent papers proposing what we now call the chromosome theory of inheritance http://bdu.edu.et Friday, December 20, 2024 33 www.facebook.com/bduethiopia  Chromosomal Theory of Inheritance States that individual genes are found at specific locations on particular chromosomes, and that the behavior of chromosomes during meiosis – Parallelism between the behaviors of chromosomes and mendelian factors during cell division – Can explain why genes are inherited according to Mendel’s laws – Chromosomes are carriers of the genetic material http://bdu.edu.et Friday, December 20, 2024 34 www.facebook.com/bduethiopia  Observations that support the chromosome theory of inheritance: Chromosomes, like Mendel's genes, come in pairs (homologous) one member of the pair comes from the mother and one from the father Members of a homologous pair separate (segregate) during meiosis, so each sperm or egg receives just one member. Members of different pairs are sorted into gametes independently of one another in meiosis The chromosome theory of inheritance was controversial at first until confirmedby Thomas Hunt Morgan who studied the genetics of fruit flies http://bdu.edu.et Friday, December 20, 2024 35 www.facebook.com/bduethiopia  Hunt’s Experiment He found a mutation in a gene affecting fly eye color This mutation made a fly's eyes white, rather than their normal red. Unexpectedly, Morgan found that the eye color gene was inherited in different patterns by male and female flies – Male flies have an X and a Y chromosome (XY), while female flies have two X chromosomes (XX) – He realized that the eye color gene was being inherited in the same pattern as the X chromosome. This may have come as a surprise to Morgan, who had been a critic of the chromosome theory http://bdu.edu.et Friday, December 20, 2024 36 www.facebook.com/bduethiopia  Chromosomes are thread-like structures located inside the nucleus of animal and plant cells Each chromosome is made of protein (Histone) and a single molecule of deoxyribonucleic acid (DNA) DNA contains the specific instructions that make each type of living creature unique The histone protiens play structural role The term chromosome (Gr.- color (chroma) and body (soma) as they get strongly stained by some colorful dyes used in research. http://bdu.edu.et Friday, December 20, 2024 37 www.facebook.com/bduethiopia  Eukaryotic chromosome contains a single DNA molecule of enormous length in a highly coiled stable complexes of DNA and protein called chromatin Prokaryotic cells contain single naked chromosome 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 http://bdu.edu.et Friday, December 20, 2024 38 www.facebook.com/bduethiopia  The chromosome complement (Karyotype) = the complete set of chromosomes of plants and animals The nucleus of each somatic cell contains a fixed number of chromosomes typical of the particular species The number of chromosomes vary tremendously among species and have little relationship to the complexity of the organism http://bdu.edu.et Friday, December 20, 2024 39 www.facebook.com/bduethiopia  The number of sets of chromosomes of an organism – 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 Heterochromatin regions are gene poor regions (heavily staining regions) Euchromatin regions are gene rich regions (poorly staining regions) http://bdu.edu.et Friday, December 20, 2024 40 www.facebook.com/bduethiopia  The Cell Cycle Is a series of events that takes place in a cell as it grows and divides In eukaryotic cells, the stages of the cell cycle are divided into two major phases: interphase and the mitotic (M) phase.  During interphase, the cell grows and makes a copy of its DNA and prepares for cell division  During the mitotic (M) phase, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells. A cell spends most of its time in interphase The resulting cells, known as daughter cells, each enter their own interphase and begin a new round of the cell cycle http://bdu.edu.et Friday, December 20, 2024 41 www.facebook.com/bduethiopia  http://bdu.edu.et Friday, December 20, 2024 42 www.facebook.com/bduethiopia  Interphase Happens in three steps: G1 phase: also called the first gap phase the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps S phase. The cell synthesizes a complete copy of the DNA in its nucleus It also duplicates a microtubule-organizing structure called the centrosome. that help separate DNA during M phase G2 phase: or the second gap phase, The cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis G2 phase ends when mitosis begins. The G1, S, and G2 phases together are known as interphase http://bdu.edu.et Friday, December 20, 2024 43 www.facebook.com/bduethiopia  M phase During this phase, the cell divides its copied DNA and cytoplasm to make two new cells M phase involves three distinct division-related processes: mitosis, karyokinesis and cytokinesis. In mitosis, the nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a specialized structure made out of microtubules Mitosis takes place in four stages: prophase, metaphase, anaphase, and telophase In cytokinesis, the cytoplasm of the cell is split in two, making two new cells Cytokinesis usually begins just as mitosis is ending, with a little overlap Cytokinesis takes place differently in animal and plant cells. http://bdu.edu.et Friday, December 20, 2024 44 www.facebook.com/bduethiopia  In animals, cell division occurs when a band of cytoskeletal fibers called the contractile ring contracts inward and pinches the cell in This process is called contractile cytokinesis The indentation produced as the ring contracts inward is called the cleavage furrow This is because animal cells can be pinched as they’re relatively soft and squishy. http://bdu.edu.et Friday, December 20, 2024 45 www.facebook.com/bduethiopia  Plant cells are much stiffer than animal cells as they’re surrounded by a rigid cell wall and have high internal pressure Cytokinesis is conducted by dividing the cells into two by building a new structure down the middle of the cell This structure, known as the cell plate, is made up of plasma membrane and cell wall components delivered in vesicles, and it partitions the cell in two http://bdu.edu.et Friday, December 20, 2024 46 www.facebook.com/bduethiopia  The G0 phase Some types of cells like embryonic cells and tumor cells divide rapidly enter another round of the cell cycle Other types of cells divide slowly or not at all (remain non- proliferative) These cells may exit the G1 and enter a resting state called G0 Such a cell is not actively preparing to divide, it’s just doing its job e.g. a neuron The G0 is a permanent state for some cells, while others may re- start division if they get the right signals http://bdu.edu.et Friday, December 20, 2024 47 www.facebook.com/bduethiopia  Mitosis is a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to itself i.e. equational division The objective of mitosis is to make two genetically identical cells from a single cell In the cells of our body, we start with 46 chromosomes in a single cell and end up with 46 chromosomes in two cells. Obviously, replicating the chromosomes is a prerequisite to mitosis Remember, replication takes place during interphase Mitosis is an organized process that allows the replicated chromosomes to be properly divided into two identical cells http://bdu.edu.et Friday, December 20, 2024 48 www.facebook.com/bduethiopia  Significance: – populates an organism’s body with cells, and throughout an organism’s life during development and growth – Replaces old, worn-out cells with new ones – For single-celled eukaryotes like yeast, mitotic divisions are actually a form of reproduction, adding new individuals to the population. In all of these cases, the “goal” of mitosis is to make sure that each daughter cell gets a perfect, full set of chromosomes Thereby it restores a characteristic chromosome number of each species Friday, December 20, 2024 http://bdu.edu.et 49 www.facebook.com/bduethiopia  Cells with too few or too many chromosomes usually don’t function well as they may not survive, or cause cancer So, when cells undergo mitosis, they don’t just divide their DNA at random and toss it into piles rather they split up their duplicated chromosomes in a carefully organized series of steps Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase that occur in strict sequential order http://bdu.edu.et Friday, December 20, 2024 50 www.facebook.com/bduethiopia Prophase Mitosis  chromosomes start to condense  Each chromosome is already doubled (chromatid) and held together at the centromere. Metaphase  chromosomes line up at the center of the cell  Mitotic spindle (a bunch of microtubules) attaches the kinetochores to the centrosomes Anaphase  The two sister chromatids move toward opposite poles (each become chromosome again). Telophase  A nuclear envelope re-forms  The chromosomes undergo decondense, 51  http://bdu.edu.et Friday, December 20, 2024 52 www.facebook.com/bduethiopia  Meiosis Is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information 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) Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II) 53 Fig. 3.5 54  Phases of Meiosis Prophase I chromosomes condense (leptotene), homologous pairs of chromosomes synapse (zygotene) crossovers or exchanges occur between non-sister chromatids (pachytene). The crossovers, called chiasmata, become visible as the chromosomes separate a bit (diplotene), and condense a bit more (diakinesis). http://bdu.edu.et Friday, December 20, 2024 55 www.facebook.com/bduethiopia  synapsis, allows the homologous chromosomes to crossover and exchange identical parts The impact of crossing over is that genes that are on the same chromosome can be recombined – they are not always inherited together. The tetrad or bivalent formed during synapsis remains assembled as prophase progresses The tetrad therefore moves as a unit to the center of the cell. http://bdu.edu.et Friday, December 20, 2024 56 www.facebook.com/bduethiopia  Metaphase I chromosome pairs line up at the center of the cell. Oriented randomly. Metaphase I starts when the tetrads are at the center of the cell The tetrads have stayed together which insures that during the first division, each cell will get one chromosome from each homologous pair. The chromosomes remain at the center of the cell until the homologous pairs are ready to move away from each other. http://bdu.edu.et Friday, December 20, 2024 57 www.facebook.com/bduethiopia  Anaphase I the two bivalent chromosome pairs move toward opposite poles The chromosomes that make up each tetrad separate during anaphase I However, the sister chromatids will stay connected at the centromere. Anaphase I proceeds until the chromosomes are pulled into a bundle at opposite ends of the cell. http://bdu.edu.et Friday, December 20, 2024 58 www.facebook.com/bduethiopia  Telophase I the chromosomes usually only partially decondense, and the second division begins The cell divides into two cells during telophase I The bundle of chromosomes may have a nuclear envelope develop around them. The germline cells in some organisms such as human females, go through the first four stages of meiosis prior to birth. The germline cells remain at telophase I for some time. The second round of division occurs when the gamete is needed for reproduction. In other situations, telophase I is an abbreviated stage, and the second round of division proceeds without delay. http://bdu.edu.et Friday, December 20, 2024 59 www.facebook.com/bduethiopia 60  Prophase II chromosomes condense The spindle apparatus moves the chromosomes to the middle of the cell The centromeres are still holding the sister chromatids together http://bdu.edu.et Friday, December 20, 2024 61 www.facebook.com/bduethiopia  Metaphase II chromosomes line up at the center of the cell In metaphase II the chromosomes are aligned at the center of the cell This time there are not homologous chromosomes to be paired with This metaphase looks similar to metaphase of mitosis but there is a key difference. What is the difference? http://bdu.edu.et Friday, December 20, 2024 62 www.facebook.com/bduethiopia  Anaphase II the chromatids are pulled apart by the spindle fibers. Now they are classified as chromosomes, not chromatids. The chromosomes move apart to opposite ends of the cell http://bdu.edu.et Friday, December 20, 2024 63 www.facebook.com/bduethiopia  Telophase II A nuclear envelope re-forms around the bundle of chromosomes the chromosomes decondense and cell division occurs Now four cells exist that originated from one germline cell. Each cell is a gamete with half the number of chromosomes and genes as a somatic cell. http://bdu.edu.et Friday, December 20, 2024 64 www.facebook.com/bduethiopia 65  http://bdu.edu.et Friday, December 20, 2024 66 www.facebook.com/bduethiopia 2 Ways of Generating Variation (Recombinant Progeny) Random assortment of different molecules Crossing over (molecular recombination) 67 68