Genetics and Molecular Biology Chapter 13

Chapter 13: Genetics and Molecular Biology

• This chapter covers genetics and molecular biology, specifically focusing on plant biology.
• The content includes an introduction to genetics and molecular biology, structure of DNA, DNA functions, cytogenetics, changes in chromosome structure and chromosome number, Mendelian genetics (Mendel's studies, monohybrid cross, dihybrid cross, the backcross, the testcross), incomplete dominance, interactions among genes, quantitative traits, extranuclear DNA, linkage and mapping, the Hardy-Weinberg Law, and mutations.
• Molecular genetics:
  • The discovery of DNA structure in 1953 by Watson and Crick revolutionized this field.
  • Genes are sequences of nucleic acids, distinguishable and characterizable.
  • Mutations are also better understood, thanks to understanding this DNA structure.
• DNA structure:
  • Chromosomes are composed of large molecules, DNA and proteins.
  • DNA is organized into a chain of nucleotides.
  • A nucleotide contains a nitrogenous base, a 5-carbon sugar (deoxyribose), and a phosphate group.
  • There are four types of DNA nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T).
  • Purines (A and G) have a double-ring structure, while pyrimidines (C and T) have a single-ring structure.
• DNA Function:
  • DNA stores genetic information in the sequence of nucleotides.
  • Genes are segments of DNA that direct protein synthesis, important for cell function, protein structure, and enzyme production.
  • The genome encompasses the total DNA in an organism's chromosomes.
• DNA Replication:
  • Replication occurs in the S phase of the cell cycle.
  • DNA strands unzip.
  • The single strands act as templates for new strands.
  • DNA polymerase adds new nucleotides in a precise G-C and A-T sequence.
  • Replication is semi-conservative
• Gene Expression:
  • Different genetic subsets are read in diverse cells.
  • A cell's environment influences which genes are expressed.
  • Transcription copies a gene's message from DNA using RNA building blocks.
  • RNA differs from DNA in having ribose instead of deoxyribose and uracil instead of thymine also RNA is typically single-stranded, unlike DNA's double-strand.
  • Translation utilizes this RNA (transformed into protein) in the cytoplasm.
• Translation:
  • Chromosomes contain genes for building tRNA.
  • tRNA acts as a translator during translation.
  • It has one end that binds to mRNA, and another to a specific amino acid (this binding is crucial for matching codon to amino acid).
  • Each tRNA has a unique anticodon loop that recognizes a specific mRNA codon.
  • Ribosomes are constructed from rRNA.
  • Ribosomes act as workbenches during translation.
• Genetic Code
  • The genetic code provides the instructions for building proteins from the order of nucleotides within mRNA.
  • The code uses codons, sequences of three nucleotides, to specify amino acids.
  • There are 64 possible codons for 20 amino acids.
  • The order of nucleotides determines the amino acid sequence. This code is universal across many life forms.
• Central Dogma of Molecular Genetics:
  • Information flows unidirectionally in the central dogma: from DNA to RNA to protein.
  • Transcription converts the genetic information from DNA to RNA and happens in the nucleus.
  • Translation uses the RNA information for protein synthesis, which occurs in the cytoplasm.
• Mutations:
  • Mutations alter DNA sequences.
  • Mutagens like UV light, ionizing radiation, or some chemicals induce these changes.
  • DNA repair enzymes attempt to fix these damages.
  • Mutations are divided into two types: somatic (occur in body cells) and germline (occur in cells producing sex cells).
• Cytogenetics:
  • Cytogenetics studies chromosome behavior and structure.
  • Inversions involve chromosomal pieces breaking and rearranging.
  • Translocations involve one chromosome breaking off and attaching to another.
• Changes in Chromosome Number
  • Mistakes during chromosome pairing and separation can result in extra or missing chromosomes.
  • Aneuploid cells carry too few or too many chromosomes.
  • polyploidy involves additional sets of chromosomes. If meiosis fails to halve the chromosomes resulting in 2n gametes, they fuse in polyploidy.
• Mendelian Genetics:
  • Gregor Mendel conducted experiments on pea plants around 1860, discovering the basic principles of inheritance, now known as Mendelian genetics.
  • He focused on traits like plant height, seed color, and flower color, finding patterns in their inheritance.
• Mendel's Studies:
  • Mendel selected plants with different forms of a trait.
  • He observed the patterns of inheritance across generations (Parental, First filial, Second filial).
  • Mendel's studies established the foundation of genetics.
• Mendelian Laws:
  • The law of unit characters states that genes come in pairs.
  • The law of dominance, one allele (dominant) can mask the expression of another (recessive).
  • The phenotype is an organism's physical appearance, based on its genotype (genetic composition).
  • Homozygotes have identical alleles, while heterozygotes have contrasting alleles.
• Monohybrid Cross:
  • A monohybrid cross involves parents differing in one trait.
  • Results in a 3:1 phenotypic ratio (3 dominant, 1 recessive individuals) and 1:2:1 genotypic ratio.
• Dihybrid Cross:
  • A dihybrid cross includes parents differing in two traits.
  • The law of independent assortment describes how traits are independently inherited (not linked).
  • Results in a 9:3:3:1 phenotypic ratio.
• The Backcross:
  • This cross involves a hybrid and one of its parents.
  • A phenotypic ratio of 1:1 is expected in the offspring.
• The Testcross:
  • This cross involves a plant with a dominant phenotype and a homozygous recessive plant.
  • This distinguishes between homozygous and heterozygous dominant forms.
• Incomplete Dominance:
  • This is when the heterozygote shows a phenotype intermediate between the two homozygotes.
• Interactions Among Genes
  • Multiple genes often influence a single characteristic.
  • The expression of a trait is the consequence of interactions between genes.
• How Genotype Controls Phenotype:
  • A dominant allele codes for a protein that catalyzes a reaction.
  • A recessive allele represents a mutant form, that cannot effectively catalyze the reaction nor produces the functional product.
• Quantitative Traits
  • These traits exhibit a range of phenotypes (continuous variation), unlike the discrete phenotypes Mendel typically observed.
• Extranuclear DNA
  • DNA in organelles like mitochondria and chloroplasts.
  • Inheritance is maternal, because generally, sperm do not pass on mitochondrial/chloroplast DNA.
• Linkage and Mapping
  • Linked genes are found close together on a chromosome.
  • The closer genes are, the more likely they are to be inherited together.
  • Recombinant types result from crossing-over.
  • Crossing-over frequencies are used to create genetic maps of chromosomes.
• The Hardy-Weinberg Law:
  • This law describes gene frequencies in stable populations (those undergoing no changes/mutations).
  • The frequency of certain alleles should remain consistent across generations under certain conditions.
  • However, factors such as small population sizes, or selection, can change these proportions.