Microbial GeneticsUnit 9

Donna HowellDonna HowellMedical Medical

MicrobiologyMicrobiologyBlacksburg High Blacksburg High

SchoolSchool

History of DNA

Late 1800’sLate 1800’s – scientists discovered that DNA is in the nucleus of the cell

19021902 – Walter Sutton proposed that hereditary material resided in the chromosomes in the nucleus

19281928 – Frederick Griffith found out that hereditary material was transmitted somehow from one organism to another

19521952 – Hershey and Chase found that DNA was the hereditary substance as opposed to a protein

Eukaryotic DNA Structure

1950’s – Erwin Chargaff came up with Chargaff’s RulesChargaff’s Rules:

A-T are present in same amountsC-G are present in same amountsRemember this rhyme: A to the T and C to the G

Franklin and Wilkins produced an x-ray crystallography of a DNA molecule1953 – Watson and Crick proposed that DNA resembles a twisted ladder, and named it a double helixdouble helix.

Eukaryotic DNA Structure

DNA consists of long strands of nucleotides. A nucleotide contains the following:

A sugar (deoxyribose)A sugar (deoxyribose)

A phosphate groupA phosphate group

A nitrogen base A nitrogen base (adenine, thymine, (adenine, thymine, cytosine, guanine)cytosine, guanine)

Nucleotide

DNA in the Nucleus

Scientists estimate the if you were to stretch the DNA out in each cell into one line, it would measure 3 meters in length!So how is so much DNA packed into one cell?It is supercoiled! The “ladder” is first twisted, then it winds around histones (proteins), then it coils again until it forms the familiar “X” shaped chromosomes.

DNA ReplicationWhen a cell divides, how is more DNA made?DNA makes copies of itself through a process called replicationreplication:

First, the DNA helix unwinds.Next, enzymes break the hydrogen bonds that hold the base pairs together, sort of like taking a chainsaw and slicing down through the middle of the ladder’s rungs.Then, each strand serves as a template for a new strand. Another enzyme moves along the separated DNA strands, and matches bases from the parent strand to the new complementary strand.Last, hydrogen bonds form between bases, and you have 2 new DNA molecules!Each new DNA molecule has ½ of the original strand, and a new strand, so semiconservative replication.semiconservative replication.

The Genetic CodeWhat is a gene?What is a gene?

A region of DNA on a chromosome that controls the production of a protein, of which we have many in our bodies.

What does a protein do?What does a protein do?Proteins are used in various body functions, and each protein has a specific job in our bodies:

Can be enzymes which assist chemical reactions Can transport substances from one place to anotherAre part of our structural supportCan be hormonesCan be part of the body’s defense against diseaseMany more!

Therefore, our bodies must produce many different types of proteins!

Protein Synthesis

Proteins are manufactured by our bodies in a process called protein protein synthesissynthesis. It is a two part process that involves RNA and DNA.

Why Do Bacteria Need Proteins?

Bacteria, like humans, must have the capability of producing proteins. What do bacteria use proteins for? Well, there are two reasons:

    

1. To provide structural proteins for the cell 1. To provide structural proteins for the cell

membrane, cell wall, etc. of the replicating membrane, cell wall, etc. of the replicating

bacterial cell. bacterial cell.

2. To provide enzymes (all enzymes are 2. To provide enzymes (all enzymes are proteins) for proteins) for

the physiological processes of the bacterial the physiological processes of the bacterial cell.cell.

RNA vs. DNA

Before we begin this process, let’s review the differences between RNA and DNA:

NumbeNumber of r of

StrandStrandss

Type of Type of SugarSugar

Bases Bases PresentPresent

DNADNA 22 DeoxyriboDeoxyribosese

A, A, TT, C, , C, GG

RNARNA 11 RiboseRibose A, A, UU, , C, GC, G

Types of RNA

Also before we begin, let’s review the three types of RNA:

Messenger RNA (mRNA)Messenger RNA (mRNA) – carries the coded instructions for protein synthesis from the DNA in the nucleus to the ribosome in the cytoplasm

Transfer RNA (tRNA)Transfer RNA (tRNA) – brings the amino acids to the ribosome in the correct order so that they can be built into the new protein

Ribosomal RNA (rRNA)Ribosomal RNA (rRNA) – works with several proteins to make up the structure of the ribosomes

What is a Codon?

One last quick review before we delve into protein synthesis: each amino acid is coded for by a sequence of 3 bases called a codoncodon. Each codon produces a specific amino acid, depending on the sequence of the bases.We can figure out which amino acid will be produced by looking at the “codon wheel”.Once you have a bunch of different amino acids produced, they are joined together to form a proteinprotein!

Protein Synthesis: The Process

There are two main parts to the process of protein synthesis:

TranscriptionTranscription – the process of transferring information from a strand of DNA to a strand of RNA in the nucleus.TranslationTranslation – the process where ribosomes synthesize proteins with the help of other molecules in the cytoplasm.

Step 1 - Transcription

Occurs in the nucleusOccurs in the nucleusHere are the 5 steps Here are the 5 steps involved:involved:

The DNA strand in nucleus unwinds and separates.The ½ of the strand that contains the gene for a protein acts as the template.An enzyme matches RNA base pairs with their complementary DNA base pair.The nucleotides of the RNA are bonded together to form a strand of mRNA, which contains the complete genetic code!mRNA leaves the nucleus and moves into the cytoplasm for the second step.

Step 2 - TranslationOccurs in the cytoplasm.Occurs in the cytoplasm.Here are the steps involved:Here are the steps involved:

The first codon of the mRNA attaches to a ribosomeThen, tRNA molecules, each carrying a specific amino acid, approaches the ribosome.The tRNA with the complementary anticodon pairs with the mRNA codon, joining together.Often, the first codon to be translated is the “start” codon, which tells the whole process to begin.The mRNA then slides along the ribosome to the next codon, and the process is repeated until a “stop” codon is reached.Each amino acid produced is joined with the next one, until you have a long string of amino acids (polypeptide). This is a protein!

Prokaryotic DNA

So now we have talked about eukaryotic DNA. Let’s now concentrate on prokaryotic DNA.Remember that in bacteria, DNA exists in two forms:

Chromosome – is a Chromosome – is a double-helix of DNA in a double-helix of DNA in a closed loopclosed loop

Plasmid – a circular Plasmid – a circular piece of DNA separate piece of DNA separate from the chromosomefrom the chromosome

Chromosome ReplicationBacterial chromosomes replicate themselves in a process called binary fission. Here’s how it happens:

Changes in the Genome

Humans usually go through their lives with the same set of genes; bacteria do not.Bacteria can change their genes by one of the following methods:

MutationsMutationsGenetic recombinationGenetic recombination

Let’s discuss each.

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T hes unw ash otb utt heo ldm and idn otg eth ish at.

Mutations

Mutations are permanent changes in an organism’s DNA.Can occur in one of 3 ways:

Spontaneous changesSpontaneous changes – this is just when DNA produces a defective DNA molecule through replication (cell mistakes)

MutagensMutagens – these are outside factors that cause mutations, such as UV light, chemicals, antibiotics, etc.

TransposonsTransposons – these are small segments of DNA that can move from one position in a DNA strand to another – also called “jumping genes”. Causes incorrect proteins to be formed.

Genetic Recombination

Bacteria can transfer genes to other bacteria in one of 3 ways:

Conjugation Conjugation

TransductionTransduction

TransformationTransformation

ConjugationThis is a process where two live bacterial cells come together and transfer genetic material by transferring plasmids:

Transduction

This is a process where transfer occurs with the assistance of a bacterial virus called a bacteriophage. The virus actually transfers DNA from the donor cell to the recipient cell.

TransformationThis is a process where one bacterium acquires fragments of DNA from a dead bacterium and incorporates it into it’s own genome.

ComparisonTransformatioTransformatio

nnConjugationConjugation TransductionTransduction

Method of Method of DNA transferDNA transfer

Movement Movement across across

membranemembrane

Through a Through a channelchannel

By a virusBy a virus

Amount of Amount of DNA DNA transferredtransferred

Few genesFew genes Few to a lotFew to a lot Few genesFew genes

Plasmid Plasmid transferred?transferred?

YesYes YesYes Not likelyNot likely

Entire Entire chromosome chromosome transferred?transferred?

NoNo SometimesSometimes NoNo

Virus Virus required?required?

NoNo NoNo YesYes

Live bacteria Live bacteria required?required?

YesYes YesYes YesYes

Dead bacteria Dead bacteria required?required?

YesYes NoNo NoNo

Used to Used to acquire acquire antibiotic antibiotic resistance?resistance?

YesYes YesYes Not likelyNot likely

The End!The End!