Week12-01 (Lec) Micro 20 (Microbial genetics).pptx

(1)

Chapter 9

(2)

9-2

Genetics and Genes

• Genetics – the study of heredity

• The science of genetics explores:

– _{Transmission of biological traits from parent to}

offspring

– Expression and variation of those traits

(3)

(4)

9-4

Microbial Genomes

Genome – sum total of genetic material (DNA)

in a cell

• Most exists as chromosomes

• Some appear in non-chromosomal sites:

– Mitochondria

– _Chloroplasts – _Plasmids

• Genome of cells – DNA

(5)

Chromosomes (1 of 2)

• Chromosome - discrete cellular structure

composed of a neatly packaged DNA molecule

– _{Eukaryotic chromosomes are located in the}

nucleus and are multiple and linear

(6)

9-6

(7)

Genotypes and Phenotypes (1 of 2)

• A chromosome is subdivided into genes, the fundamental unit of heredity responsible for a given trait

– _{Site on the chromosome that provides}

information for a certain cell function

– Segment of DNA that contains the necessary code to make a protein or RNA molecule

• Three basic categories of genes:

(8)

9-8

Genotypes and Phenotypes (2 of 2)

• All types of genes constitute the genetic makeup – genotype

(9)

Size and Packaging of Genomes

• Smallest virus – 4-5 genes

• _{E. coli}_{– single chromosome containing 4,288}

genes; 1 mm; 1,000X longer than cell

• Human cell – 46 chromosomes containing

(10)

9-10

The Packaging of DNA (1 of 2)

• The DNA molecule is compacted in the cell by supercoils, or superhelices:

– In prokaryotes, by the action of the enzyme DNA gyrase, which coils the chromosome into a tight bundle by reversible series of twists

into the DNA molecule

(11)

(12)

9-12

The Structure of DNA: Double

Helix (1 of 6)

• Basic unit of DNA structure is the nucleotide:

– A deoxyribose sugar

– A phosphate group

– A nitrogenous base: adenine (A), guanine (G), thymine (T), cytosine (C)

(13)

(14)

9-14

The Structure of DNA: Double

Helix (3 of 6)

• Nitrogenous bases covalently bond to the 1′ carbon of each sugar and span the center of the molecule to pair with a complementary base on the other strand:

– Adenine (A) to thymine (T) with 2 hydrogen bonds

– _{Guanine (G) to cytosine (C) with 3 hydrogen}

(15)

(16)

9-16

The Structure of DNA: Double

Helix (5 of 6)

• Antiparallel arrangement: in one strand, the helix runs in a 5′ to 3′ direction and the other side is oriented from 3′ to 5′

• Each strand provides a template for the exact copying of a new strand

(17)

(18)

9-18

The Significance of DNA Structure

1. Maintenance of code during reproduction

- Constancy of base pairing guarantees that the code will be retained. When strands are separated, each strand serves as a template for replication of the molecule into an exact copy.

2. Providing variety - order of bases

(19)

The Overall Replication Process

(1 of 2)

• Replication occurs on both strands simultaneously

• Semiconservative process:

1. The parent DNA molecule is uncoiled

2. The two strands are separated exposing the nucleotide sequence to serve as templates 3. Two new complementary strands are

(20)

9-20

(21)

Events in DNA Replication (1 of 9)

• All chromosomes have a specific origin of

replication site as the place where

replication will be initiated

• The origin of replication is AT-rich, thus less energy is required to separate the two

strands

(22)

9-22

Events in DNA Replication

(23)

(24)

9-24

Events in DNA Replication (8 of 9)

• When replication forks meet, ligases link the DNA fragments along the lagging strand

• As replication proceeds, one newly synthesized strand loops down

• When the forks have gone full circle, a termination site shuts replication down

• The two circular daughter molecules remain connected briefly but are nicked and

(25)

(26)

9-26

Enzymes Involved in DNA

Replication

TABLE 9.1 Some Enzymes Involved in DNA Replication and Their Functions

Enzyme Function

Helicase Unzipping the DNA helix Primase Synthesizing an RNA primer

DNA polymerase III Adding bases to new DNA chain; proofreading the chain _{for mistakes}

DNA polymerase I Removing RNA primers, replacing gaps between Okazaki fragments with correct nucleotides, repairing mismatched bases

(27)

Applications of the DNA Code (1 of 2)

• Genetic information in DNA molecules is conveyed to RNA through the process of

transcription

• The information contained in the RNA

molecule is then used to produce proteins in the process of translation

(28)

9-28

Gene-Protein Connection

• Each structural gene is an ordered sequence of nucleotides that codes for a protein’s

primary structure

• Groups of three consecutive bases, triplets

or codons, on one DNA strand are

transcribed into RNA sequence triplets

• Each triplet of nucleotides on the RNA specifies a particular amino acid

• A protein’s primary structure (chain of amino acids) determines its shape and function

(29)

RNAs: Major Participants in

Transcription and Translation

The general structure of the ribonucleic acid

(RNA) is different than that of the DNA molecule in several ways:

1. RNA is a single-stranded molecule that can assume secondary and tertiary levels of

complexity, leading to specialized forms of RNA (mRNA, tRNA, and rRNA)

(30)

9-30

Major Types of RNA

TABLE 9.2 Major Types of Ribonucleic Acid Involved In Protein Synthesis

RNA Type Contains Codes For Function in Cell Translated

Messenger (mRNA) Sequence of amino acids _{in protein} Carries the DNA master code to _{the ribosomes} Yes

Transfer (tRNA) Specifying a given amino _acid Carries amino acids to ribosomes _{during translation} No

Ribosomal (rRNA) Several large structural _{rRNA molecules} Forms the major part of ribosomes and participates in protein synthesis No

(31)

Messenger RNA (mRNA) (1 of 2)

• Transcribed version of a structural gene or genes in DNA

• Synthesized following complementary-base pairing by a process similar to synthesis of the leading strand during DNA replication

(32)

9-32

Messenger RNA (mRNA) (2 of 2)

a) Messenger RNA (mRNA)

A short piece of messenger RNA (mRNA)

(33)

Transfer RNA: tRNA (1 of 3)

• Acts as a translator of the mRNA code into protein

• 75 - 95 nucleotides in length bent into

hairpin loops to form a cloverleaf structure further packed into a complex helix

• Bottom loop of the cloverleaf exposes the

tRNA specific anticodon complementary to a mRNA codon

(34)

9-34

Transfer RNA: tRNA (2 of 3)

The figure of “Transfer RNA.”

b) Transfer RNA (tRNA)

Left: The tRNA strand loops back on itself to form intrachain hydrogen bonds. The result is a

cloverleaf structure, shown here in simplified

(35)

Transfer RNA: tRNA (3 of 3)

(36)

9-36

Ribosomal RNA: rRNA

• The prokaryotic (70S) ribosome is a particle composed of tightly packaged ribosomal RNA (rRNA) and protein

• Forms complex three-dimensional figures that

(37)

Major Events in Transcription (3 of 3)

(38)

9-38

Translation: The Second Stage of

Gene Expression

• All the elements needed to synthesize protein (mRNA, tRNA, amino acids) are brought together on the

ribosomes

• The process occurs in five stages: initiation,

(39)

The Master Genetic Code (1 of 2)

• Represented by mRNA codons and their specific amino acids

(40)

9-40

The Master Genetic Code (2 of 2)

*This codon initiates translation.

(41)

Interpreting the DNA Code

• Transcription produces mRNA complementary to the DNA gene

• During translation, tRNAs use their anticodon to

(42)

9-42

Translation

• Ribosomes assemble on the 5′ end of an mRNA transcript

• Ribosome scans the mRNA until it reaches the start codon, usually AUG

• A tRNA molecule with the complementary

(43)

Translation Termination

• Termination codons – UAA, UAG, and UGA –

are codons for which there is no corresponding tRNA

• When this codon is reached, the ribosome falls off and the last tRNA is removed from the

(44)

9-44

Polyribosomal Complex

(45)

Eukaryotic Transcription and

Translation

1. Do not occur simultaneously – transcription

occurs in the nucleus and translation occurs in the cytoplasm

2. Eukaryotic start codon is AUG, but it does not use formyl-methionine

3. Eukaryotic mRNA encodes a single protein, unlike bacterial mRNA which encodes many

4. Eukaryotic DNA contains introns– intervening

(46)

9-46

Splicing of Eukaryotic pre-mRNA

(1 of 2)

• Eukaryotes gene coding sequences, or

exons, are interrupted by segments called

introns

• Introns are transcribed but not translated, they are removed before translation

(47)

(48)

9-48

Regulation of Protein Synthesis and

Metabolism

• Genes are regulated to be active only when their products are required

• In prokaryotes this regulation is coordinated

by operons, a set of genes, all of which are

(49)

Mutations: Changes in the Genetic

Code

• A change in phenotype due to a change in

genotype (nitrogen base sequence of DNA) is called a mutation

• A natural, nonmutated characteristic is known as a wild type (wild strain)

• An organism that has a mutation is a mutant

strain, showing variance in morphology,

(50)

9-50

Causes of Mutations (1 of 2)

• Spontaneous mutations – random change

in the DNA due to errors in replication that occur without known cause

• Induced mutations – result from exposure

to known mutagens, physical (primarily

(51)

Causes of Mutations (2 of 2)

TABLE 9.3 Selected Mutagenic Agents and Their Effects

Agent Effect

Chemical

Nitrous acid, bisulfite Remove an amino group from some nitrogen bases Ethidium bromide Inserts between the paired bases

Acridine dyes Cause frameshifts due to insertion between base _pairs

Nitrogen base analogs Compete with natural bases for sites on replicating _DNA

(52)

9-52

Categories of Mutations

• Point mutation – addition, deletion, or substitution of a few bases

• _{Missense mutation}_{– causes change in a single}

amino acid

• Nonsense mutation – changes a normal codon into a stop codon

• Silent mutation – alters a base but does not change the amino acid

• _{Back-mutation}_{– when a mutated gene}

reverses to its original base composition

(53)

Effect of Major Types of Mutations

(1 of 5)

TABLE 9.4 Classification of Major Types of

Mutations.

I. Wild-type (nonmutated) sequence

Example: THE BIG BAD CAT ATE THE FAT RED BUD

The wild-type sequence of a gene is the

DNA sequence found in most organisms and is generally considered the “normal”

(54)

9-54

Effect of Major Types of Mutations

(2 of 5)

The “TABLE 9.4” continues on this slide.

II. Categories of mutations based on type of DNA alteration

A. Substitution mutations

1. Missense:

Example: THE BIG MAD CAT ATE THE FAT RED BUG

A missense mutation causes a different amino acid to be incorporated into a

(55)

Effect of Major Types of Mutations

(3 of 5)

2. Nonsense

Example: THE BIG BAD XXX (stop)

A nonsense mutation converts a codon to a stop codon, resulting in premature

(56)

9-56

Effect of Major Types of Mutations

(4 of 5)

B. Inversion mutations Example:

THE BIG ABD CAT ATE THE FAT RED BUG THE BIB GAD CAT ATE THE FAT RED BUG Inversion arise when adjacent letters

(57)

Effect of Major Types of Mutations

(5 of 5)

C. Frameshift Mutations

1. Insertion: Example: THE BIG BAB DCA TAT ETH EFA TRE DBU G 

2. Deletion: Example: THE BIG * ADC ATA TET HEF ATR EDB UG 

Insertion (addition of letter) and deletion (removal of letter) mutations cause a

(58)

9-58

Repair of Mutations

• Since mutations can be potentially fatal, the cell has several enzymatic repair mechanisms in place to find and repair damaged DNA

– _{DNA polymerase – proofreads nucleotides}

during DNA replication

– Mismatch repair – locates and repairs

mismatched nitrogen bases that were not repaired by DNA polymerase

– Light repair – for UV light damage

(59)

The Ames Test (1 of 2)

• Any chemical capable of mutating bacterial DNA can similarly mutate mammalian DNA

• Agricultural, industrial, and medicinal

compounds are screened using the Ames test

• Indicator organism is a mutant strain of Salmonella typhimurium that has lost the ability to synthesize histidine

(60)

9-60

The Ames Test (2 of 2)

(61)

Conjugation (1 of 5)

Transfer of a plasmid or chromosomal fragment from a donor cell to a recipient cell via direct

contact

• Gram-negative cell donor has a fertility

plasmid (F factor) that allows the synthesis of a conjugative pilus

• Donor (F+ cell) transfers fertility plasmid through pilus to recipient (F- cell), which becomes F+ cell

(62)

9-62

Conjugation (2 of 5)

Physical Conjugation

(1) The pilus of donor cell

(63)

Conjugation (3 of 5)

• High-frequency recombination – donor’s fertility plasmid is integrated into the

bacterial chromosome

(64)

9-64

Conjugation (4 of 5)

F Factor Transfer

(65)