BIO 205 Chapter 7 Lecture

8/14/2019 BIO 205 Chapter 7 Lecture

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BIO 205

Chapter 7


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Prokaryotic

Genetics


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The Four Nucleotides Composing DNA


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DNA Base Pairs


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DNA occupies around 1/3 of the total volume of the cell.

DNA within a Chromosome Is Highly Compacted.

The chromosome is located in the nucleoid.

Most of the genetic information in prokaryotic cells is contained in thechromosome.

Prokaryotic DNA Is Organized within the Nucleoid:

Prokaryotic DNA


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The chromosome is supercoiled twisted and tightlypacked by nucleoid-associated proteins

Prokaryotic DNA Continued


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R plasmids carry genes for antibiotic resistance

F plasmids allow genetic material to be transferred from a

donor cell to a recipient

They replicate independently from the chromosome

Plasmids are stable extrachromosomal DNA elements thatcarry nonessential genetic information

Many Prokaryotic Cells also Contain Plasmids.

Plasmids


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As the DNA unzips, two replication forks form and move inopposite directions away from the origin

The point where replication starts is the origin ofreplication (oriC)

The first stage of prokaryotic replication is initiation,where DNA unwinds and the strands separate.

DNA Replication Is a Highly Regulated Process

DNA Replication


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Mistakes that occur in DNA replication are calledmutations

The lagging strand is synthesized in Okazaki fragments,

which are joined by DNA ligase

DNA polymerase III can move only in a 3 to 5 direction,

creating a leading strand and a lagging strand

Insertion of complementary nucleotides on the templatestrand is carried out by DNA polymerase III

The second stage is elongation, when enzymessynthesize a new strand to pair with each original strand.


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The pairing of one old strand with one new strand is

called semiconservative replication

When the replication forks reach the termination point

(terC), terminator proteins block any further replication

The third stage is termination, when the two DNA

helices separate from each other


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RNA is translated into proteins through translation

DNA is expressed as RNA through transcription

The Central Dogma Identifies the Flow of GeneticInformation

Protein Synthesis


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Messenger RNA (mRNA) carries the information of what

protein will be synthesized

Only one of the two DNA strands is transcribed

RNA polymerase is the enzyme that synthesizes RNA from

the DNA template

Transcription Copies Genetic Information into RNA

Transcription


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It corresponds to a specificamino acid.

The anticodon is a sequence of 3nitrogenous bases complementaryto the codon on the mRNA.

Transfer RNA (tRNA) contains theanticodon.

Ribosomal RNA (rRNA) are the

framework of ribosomes


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Some codonsdo not code for amino acids. They can terminate theaddition of amino acids to a polypeptide (stop codon)

AUG is the start codon.

More than one codon specifies a particular amino acid.

The Genetic Code Is Degenerate.


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Translation begins with the addition of the tRNA whose anticodon iscomplementary to AUG

The mRNA codons are exposed to tRNA binding sites in the ribosome

Translation occurs as the ribosome moves along the mRNA.

Translation Is the Process of Making the Polypeptide.

Translation


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Chaperones help polypeptides fold into the correctshape.

Releasing factors bind to the codon and trigger releaseof the polypeptide.

Polypeptide elongation continues until the stop codonis reached.

Each amino acid in the chain is attached to itsneighbors by a peptide bond.

Antibiotics Interfere with Protein Synthesis.


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Coupled transcription and translation in prokaryotes

Operons


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This prevents transcription of structural genes (negativecontrol)

The repressor protein binds to the operon.

DNA also contains regulatory genes that code for a repressorprotein

A promoter to which RNA polymerase binds in initiation ofstructural gene transcription.

An operator, which controls transcription of structural genes.

Each operon consists of a cluster of structural genes thatcarry the genetic information for proteins involved in metabolicfunctions.

Segments of prokaryotic DNA are organized into operons.

Protein Synthesis Can Be Controlled in Several Ways.

Operons


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Base analogs bear a close resemblance to nitrogenous bases andcan cause replication errors.

Hypoxanthine base pairs with cytosine instead of thymine.

Nitrous acid is a chemical mutagen that converts adenine bases tohypoxanthine.

P i t M t ti


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Point Mutations

Base-pair deletion orinsertion results in anincorrect number ofbases.

Base-pair substitutionsresult in an incorrect basein transcribed mRNAcodons

A point mutation affectsjust one base pair in agene.

Point Mutations Are a Result of Spontaneous or InducedMutations

DNA R i M h i


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Mismatch repair involves DNA polymerase proofreadingthe new strandand removing mismatched nucleotides.

Repair Mechanisms Attempt to Correct Mistakes orDamage in the DNA.

DNA Repair Mechanisms


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Replacing it with correctnucleotides

Cutting out damaged DNA

Excision repair involves:


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End of Chapter 7

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BIO 205 Chapter 7 Lecture