Microbial Genetics Unit 9 Donna Howell Medical Microbiology Blacksburg High School

History of DNA Late 1800’s Late 1800’s – scientists discovered that DNA is in the nucleus of the cell – Walter Sutton proposed that hereditary material resided in the chromosomes in the nucleus – Frederick Griffith found out that hereditary material was transmitted somehow from one organism to another – Hershey and Chase found that DNA was the hereditary substance as opposed to a protein

Eukaryotic DNA Structure Chargaff’s Rules 1950’s – Erwin Chargaff came up with Chargaff’s Rules: A-T are present in same amounts C-G are present in same amounts Remember this rhyme: A to the T and C to the G Franklin and Wilkins produced an x-ray crystallography of a DNA molecule double helix 1953 – Watson and Crick proposed that DNA resembles a twisted ladder, and named it a double helix.

Eukaryotic DNA Structure DNA consists of long strands of nucleotides. A nucleotide contains the following: A sugar (deoxyribose) A phosphate group A nitrogen base (adenine, thymine, 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 Replication When a cell divides, how is more DNA made? replication DNA makes copies of itself through a process called replication: 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! semiconservative replication. Each new DNA molecule has ½ of the original strand, and a new strand, so semiconservative replication.

The Genetic Code 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? 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 another Are part of our structural support Can be hormones Can be part of the body’s defense against disease Many more! Therefore, our bodies must produce many different types of proteins!

Protein Synthesis protein synthesis Proteins are manufactured by our bodies in a process called protein synthesis. 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 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 proteins) for the physiological processes of the bacterial cell. the physiological processes of the bacterial cell.

RNA vs. DNA Before we begin this process, let’s review the differences between RNA and DNA: Number of Strands Type of Sugar Bases Present DNA RNA

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? 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 codon. 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”. protein Once you have a bunch of different amino acids produced, they are joined together to form a protein!

Protein Synthesis: The Process There are two main parts to the process of protein synthesis: Transcription Transcription – the process of transferring information from a strand of DNA to a strand of RNA in the nucleus. Translation Translation – the process where ribosomes synthesize proteins with the help of other molecules in the cytoplasm.

Step 1 - Transcription Occurs in the nucleus Here are the 5 steps 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 - Translation Occurs in the cytoplasm. Here are the steps involved: The first codon of the mRNA attaches to a ribosome Then, 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 double- helix of DNA in a closed loop Plasmid – a circular piece of DNA separate from the chromosome

Chromosome Replication Bacterial 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:Mutations Genetic recombination Let’s discuss each. The sun was hot but the old man did not get his hat.

Mutations Mutations are permanent changes in an organism’s DNA. Can occur in one of 3 ways: Spontaneous changes Spontaneous changes – this is just when DNA produces a defective DNA molecule through replication (cell mistakes) Mutagens Mutagens – these are outside factors that cause mutations, such as UV light, chemicals, antibiotics, etc. Transposons Transposons – 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:ConjugationTransductionTransformation

Conjugation This 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.

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

ComparisonTransformationConjugationTransduction Method of DNA transfer Movement across membrane Through a channel By a virus Amount of DNA transferred Few genes Few to a lot Few genes Plasmid transferred? YesYes Not likely Entire chromosome transferred? NoSometimesNo Virus required? NoNoYes Live bacteria required? YesYesYes Dead bacteria required? YesNoNo Used to acquire antibiotic resistance? YesYes Not likely

The End!