Chapter 9 Topics - Genetics - Flow of Genetics - Regulation - Mutation - Recombination

Genetics Genome Chromosome Gene Protein Genotype Phenotype

The sum total of genetic material of a cell is referred to as the genome. Fig. 9.2 The general location and forms of the genome

Chromosome Procaryotic Eucaryotic Histonelike proteins condense DNA Eucaryotic Histone proteins condense DNA Subdivided into basic informational packets called genes

Genes Three categories Genotype Phenotype Structural Regulatory Encode for RNA Genotype sum of all gene types Phenotype Expression of the genotypes

Flow of Genetics DNA =>RNA=>Protein Replication Transcription Translation

Representation of the flow of genetic information. Fig. 9.9 Summary of the flow of genetic information in cell.

DNA Structure Replication

DNA is lengthy and occupies a small part of the cell by coiling up into a smaller package. Fig. 9.3 An Escherichia coli cell disrupted to release its DNA molecule.

Structure Nucleotide Double stranded helix Phosphate Deoxyribose sugar Nitrogenous base Double stranded helix Antiparallel arrangement

Nitrogenous bases Purines Adenine Guanine Pyrimidines Thymine Cytosine

Purines and pyrimidines pair (A-T or G-C) and the sugars (backbone) are linked by a phosphate. Fig. 9.4 Three views of DNA structure

Replication Semiconservative Enzymes Leading strand Lagging strand Okazaki fragments

Semiconservative New strands are synthesized in 5’ to 3’ direction

Semiconservative replication of DNA synthesizes a new strand of DNA from a template strand. Fig. 9.5 Simplified steps to show the semiconservative replication of DNA

Enzymes Helicase DNA polymerase III Primase DNA polymerase I Ligase Gyrase

The function of important enzymes involved in DNA replication. Table 9.1 Some enzymes involved in DNA replication

Leading strand RNA primer initiates the 5’ to 3’ synthesis of DNA in continuous manner

Lagging strand Multiple Okazaki fragments are synthesized Okazaki fragments are ligated together to form one continuous strand

The steps associated with the DNA replication process. Fig. 9.6 The bacterial replicon: a model for DNA Synthesis

Replication processes from other biological systems (plasmids, viruses) involve a rolling cycle. Fig. 9.8 Simplified model of rolling circle DNA Replication

RNA Transcription Codon Message RNA (mRNA) Transfer RNA (tRNA) Ribosomal RNA (rRNA) Codon

Transcription A single strand of RNA is transcribed from a template strand of DNA RNA polymerase catalyzes the reaction Synthesis in 5’ to 3’ direction

mRNA Copy of a structural gene or genes of DNA Can encode for multiple proteins on one message Thymidine is replaced by uracil The message contains a codon (three bases)

The synthesis of mRNA from DNA. Fig. 9.12 The major events in mRNA synthesis

tRNA Copy of specific regions of DNA Complimentary sequences form hairpin loops Amino acid attachment site Anticodon Participates in translation (protein synthesis)

Important structural characteristics for tRNA and mRNA. Fig. 9.11 Characteristics of transfer and message RNA

rRNA Consist of two subunits (70S) A subunit is composed of rRNA and protein Participates in translation

Ribosomes bind to the mRNA, enabling tRNAs to bind, followed by protein synthesis. Fig. 9.9 Summary of the flow of genetics

Codons Triplet code that specifies a given amino acid Multiple codes for one amino acid 20 amino acids Start codon Stop codons

The codons from mRNA specify a given amino acid. Fig. 9.14 The Genetic code

Representation of the codons and their corresponding amino acids. Fig. 9.15 Interpreting the DNA code

Protein Translation Protein synthesis have the following participants mRNA tRNA with attached amino acid Ribosome

Participants involved in the translation process. Fig. 9.13 The “players” in translation

Translation Ribosomes bind mRNA near the start codon (ex. AUG) tRNA anticodon with attached amino acid binds to the start codon Ribosomes move to the next codon, allowing a new tRNA to bind and add another amino acid Series of amino acids form peptide bonds Stop codon terminates translation

The process of translation. Fig. 9.16 The events in protein synthesis

For procaryotes, translation can occur at multiple sites on the mRNA while the message is still being transcribed. Fig. 9.17 Speeding up the protein assembly line in bacteria

Transcription and translation in eucaryotes Similar to procaryotes except AUG encodes for a different form of methionine mRNA code for one protein Transcription and translation are not simultaneous Pre-mRNA Introns Exons

The processing of pre-mRNA into mRNA involves the removal of introns. Fig. 9.18 The split gene of eucaryotes

Regulation Lactose operon Repressible operon Antimicrobials sugar Amino acids, nucleotides Antimicrobials

The regulation of sugar metabolism such as lactose involves repression in the absence of lactose, and induction when lactose is present. Fig. 9.19 The lactose operon in bacteria

The regulation of amino acids such as arginine involves repression when arginine accumulates, and no repression when arginine is being used. Fig. 9.20 Repressible operon

Antimicrobials Ex. Antibiotics and drugs can inhibit the enzymes involved in transcription and translation

Mutations Changes made to the DNA Spontaneous – random change Induced – chemical, radiation. Point – change a single base Nonsense – change a normal codon into a stop codon Back-mutation – mutation is reversed Frameshift – reading frame of the mRNA changes

Examples of chemical and radioactive mutagens, and their effects. Table 9.3 Selected mutagenic agents and their effects

Repair of mutations involves enzymes recognizing, removing, and replacing the bases. Fig. 9.22 Excision repair of mutation by enzymes

The Ames test is used to screen environmental and dietary chemicals for mutagenicity and carcinogenicity without using animal studies. Fig. 9.23 The Ames test.

Effects of mutations Positive effects for the cell Allow cells to adapt Negative effects for the cell Loss of function Cells cannot survive

Recombination Sharing or recombining parts of their genome Conjugation Transformation Transduction

Conjugation Transfer of plasmid DNA from a F+ (F factor) cell to a F- cell An F+ bacterium possesses a pilus Pilus attaches to the recipient cell and creates pore for the transfer DNA High frequency recombination (Hfr) donors contain the F factor in the chromosome

Conjugation is the genetic transmission through direct contact between cells. Fig. 9.24 Conjugation: genetic transmission through direct contact

Transformation Nonspecific acceptance of free DNA by the cell (ex. DNA fragments, plasmids) DNA can be inserted into the chromosome Competent cells readily accept DNA

DNA released from a killed cell can be accepted by a live competent cell, expressing a new phenotype. Fig. 9.25 Griffith’s classic experiment in transformation

Transduction Bacteriophage infect host cells Serve as the carrier of DNA from a donor cell to a recipient cell Generalized Specialized

Genetic transfer based on generalized transduction. Fig. 9.26 Generalized transduction

Genetic transfer based on specialized transduction. Fig. 9.27 Specialized transduction

Transposon “Jumping genes” Exist in plasmids and chromosomes Contains genes that encode for enzymes that remove and reintegrate the transposon Small transposons are called insertion elements

Movement of transposons can occur in plasmids and chromosomes. Fig. 9.28 Transposons: shifting segments of the genome