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Explore the fascinating world of genes and chromosomes with a focus on Mendel's inheritance laws, chromosome behavior, genetic linkage, and DNA structure. Understand how genes control traits and how chromosomes carry genetic information. Delve into the chemical nature of DNA and its pivotal role in heredity.
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CHAPTER 10 The Nature of the Gene and the Genome
Introduction • Hereditary factors consist of DNA and reside on chromosomes. • The collective body of genetic information in an organism is called the genome.
10.1 The Concept of a Gene as a Unit of Inheritance (1) • Mendel’s work became the foundation for the science of genetics. • He established the laws of inheritance based on his studies of pea plants.
The Concept of a Gene as a Unit of Inheritance (2) • Characteristics of organisms are governed by units of inheritance called genes. • Each trait is controlled by two forms of a gene called alleles. • Alleles could be identical or nonidentical. • When alleles are nonidentical, the dominant allele masks the recessive allele.
The Concept of a Gene as a Unit of Inheritance (3) 2. A reproductive cell (gamete) contains one gene for each trait. a)Somatic cells arise by the union of male and female gametes. b) Two alleles controlling each trait are inherited; one from each parent. 3. The pairs of genes are separated (segregated) during gamete formation. 4. Genes controlling different traits segregate independently of each (independent assortment).
10.2 Chromosomes: The Physical Carriers of Genes (1) • The Discovery of Chromosomes • Chromosomes were first observed in dividing cells, using the light microscope. • Chromosomes are divided equally between the two daughter cells during cell division. • Chromosomes are doubled prior to cell division.
Chromosomes: The Physical Carriers of Genes (2) • Chromosomes as the Carriers of Genetic Information • Chromosomes are present as pairs of homologous chromosomes. • During meiosis, homologous chromosomes associate and form a bivalent; then separate into different cells. • Chromosomal behavior correlates with Mendel’s laws of inheritance.
Chromosomes: The Physical Carriers of Genes (3) • The chromosome as a linkage group • Genes that are on the same chromosome do not assort independently. • Genes on the same chromosome are part of the same linkage group. • The traits analyzed by Mendel occur on different chromosomes.
Chromosomes: The Physical Carriers of Genes (4) • Genetic Analysis in Drosophila • Morgan was the first to use fruit flies in genetic research. • Morgan only had available wild type flies but one he developed his first mutant, it became a primary tool for genetic research. • Mutation was recognized as a mechanism for variation in populations. • Studies with Drosophila confirmed that genes reside on chromosomes.
Chromosomes: The Physical Carriers of Genes (5) • Crossing Over and Recombination • Linkage between alleles on the same chromosome is incomplete. • Maternal and paternal chromosomes can exchange pieces during crossing over or genetic recombination.
Chromosomes: The Physical Carriers of Genes (6) • Crossing over and recombination • Percentage of recombination between a pair of genes is constant. • Percentage of recombination between different pairs of genes can be different. • The positions of genes along the chromosome (loci) can be mapped. • Frequency of recombination indicates distance, and increases as distance increases.
Chromosomes: The Physical Carriers of Genes (7) • Mutagenesis and Giant Chromosomes • Exposure to a sublethal dose of X-rays increases the rate of spontaneous mutations. • Cells from the salivary gland of Drosophila have giant polytene chromosomes. • Polytene chromosomes have been useful to observe specific bands correlated with individual genes. • “Puffs” in polytene chromosomes allow visualization of gene expression.
10.3 The Chemical Natureof the Gene (1) • DNA is the genetic material in all organisms. • The Structure of DNA: • The nucleotide is the building block of DNA. • It consists of a phosphate, a sugar, and either a pyrimidine or purine nitrogenous base. • There are two different pyrimidines: thymine (T) and cytosine (C). • There are two different purines: adenine (A) and guanine (G).
The Chemical Nature of the Gene (2) • Nucleotides have a polarized structure where the ends are called 5’ and 3’ . • Nucleotides are linked into nucleic acids polymers: • Sugar and phosphates are linked by 3’,5’-phosphodiester bonds. • Nitrogenous bases project out like stacked shelves.
The Chemical Nature of the Gene (3) • Chargaff established rules after doing base composition analysis: • Number of adenine = number of thymine • Number of cytosine = number of guanine • [A] + [T] ≠ [G] + [C]
The Chemical Nature of the Gene (4) • The Watson-Crick Proposal • The DNA molecule is a double helix. • DNA is composed of two chains of nucleotides. • The two chains spiral around each other forming a pair of right-hand helices. • The two chains are antiparallel, they run in opposite directions. • The sugar-phosphate backbone is located on the outside of the molecule. • The bases are inside the helix.
The Chemical Nature of the Gene (5) • The Watson-Crick Proposal (continued) • The DNA is a double helix • The two DNA chains are held together by hydrogen bonds between each base. • The double helix is 2 nm wide. • Pyrimidines are always paired with purines. • Only A-T and C-G pairs fit within double helix. • Molecule has a major groove and a minor groove. • The double helix makes a turn every 10 residues. • The two chains are complementary to each other.
The Chemical Nature of the Gene (6) • The Importance of the Watson-Crick Proposal • Storage of genetic information. • Replication and inheritance. • Expression of the genetic message.
The Chemical Nature of the Gene (7) • DNA Supercoiling • DNA that is more compact than its relaxed counterpart is called supercoiled.
The Chemical Nature of the Gene (8) • DNA Supercoiling (continued) • Underwound DNA is negatively supercoiled, and overwound DNA is positively supercoiled. • Negative supercoiling plays a role in allowing chromosomes to fit within the cell nucleus.
The Chemical Nature of the Gene (9) • DNA Supercoiling (continued) • Enzymes called topoisomerases change the level of DNA supercoiling. • Cells contain a variety of topoisomerases. • Type I – change the supercoiled state by creating a transient break in one strand of the duplex. • Type II – make a transient break in both strands of the DNA duplex.
10.4 The Structure of the Genome (1) • The genome of a cell is its unique content of genetic information. • The Complexity of the Genome • One important property of DNA is its ability to separate into two strands (denaturation).
The Structure of the Genome (2) • DNA Renaturation • Renaturation or reanneling is when single-stranded DNA molecules are capable of reassociating. • Reanneling has led to the development of nucleic acid hybridization in which complementary strands of nucleic acids form different sources can form hybrid molecules.
The Structure of the Genome (3) • The Complexity of Viral and Bacterial Genomes • The rate of renaturation of DNA from bacteria and viruses depends on the size of their genome.
The Structure of the Genome (4) • The Complexity of the Eukaryotic Genome • Reanneling of eukaryotic genomes shows three classes of DNA: • Highly repeated • Moderately repeated • Nonrepeated
The Structure of the Genome (5) • Highly Repeated DNA Sequences – represent about 1-10% of total DNA. • Satellite DNAs – short sequences that tend to evolve very rapidly. • Minisatellite DNAs – unstable and tend to be variable in the population; form the basis of DNA fingerprinting. • Microsatellite DNAs – shortest sequences and typically found in small clusters; implicated in genetic disorders.
Fluorescence in situ hybridization and localization of satellite DNA
The Structure of the Genome (6) • Moderately Repeated DNA Sequences • Repeated DNA Sequences with Coding Functions – include genes that code for ribosomal RNA and histones. • Repeated DNA Sequences that Lack Coding Functions – do not include any type of gene product; can be grouped into two classes: SINEs or LINEs. • Nonrepeated DNA Sequences – code for the majority of proteins.
The Human Perspective: Diseases That Result from Expansion of Trinucleotide Repeats (1) • Mutations occur in genes containing a repeating unit of three nucleotides. • The mutant alleles are highly unstable and the number of repeating units tends to increase as the gene passes from parent to offspring. • Type I disease are all neurodegenerative disorders resulting form expansion of CAG trinucleotides.
The Human Perspective: Diseases That Result from Expansion of Trinucleotide Repeats (2) • Huntington’s disease (HD) result from ≥ 36 glutamine repeats in the huntingtin gene. • The molecular basis of HD remains unclear but it is presumed that expanded glutamine repeats are toxic to brain cell. • Type II diseases arise from a variety of trinucleotide repeats, and are present in parts of the gene that do not code for amino acids (i.e. fragile X syndrome).
10.5 The Stability of the Genome (1) • Whole Genome Duplication (Polyploidization) • Polyploidization (or whole genome duplication) occurs when offspring receive more than two sets of chromosomes from their parents. • Could be the result of hybrids from closely related parents. • Could result from duplicate chromosomes not separated in embryonic cells.
The Stability of the Genome (2) • Duplication and Modification of DNA Sequences • Gene duplication occurs within a portion of a single chromosome. • Duplication may occur by unequal crossing over between misaligned homologous chromosomes. • Duplication has played a major role in the evolution of multigene families.
