CHAPTER 10 Molecular Biology of the Gene Overview: DNA & RNA Structure DNA replication DNA-> RNA-> Protein Viruses

Saboteurs Inside Our Cells The invasion and damage of cells by the herpesvirus can be compared to the actions of a saboteur intent on taking over a factory The herpesvirus hijacks the host cell’s molecules and organelles to produce new copies of the virus; the virus remains permanently latent in the body.

Experiments showed that DNA is the genetic material THE STRUCTURE OF THE GENETIC MATERIAL Experiments showed that DNA is the genetic material The Hershey-Chase experiment showed that certain viruses reprogram host cells to produce more viruses by injecting their DNA

THE STRUCTURE AND REPLICATION OF DNA Was known as a chemical in cells by the end of the nineteenth century Has the capacity to store genetic information Can be copied and passed from generation to generation

DNA and RNA: Polymers of Nucleotides DNA and RNA are nucleic acids They consist of chemical units called nucleotides The nucleotides are joined by a sugar-phosphate backbone

DNA and RNA are polymers of nucleotides DNA is a nucleic acid, made of long chains of nucleotides

The four nucleotides found in DNA Differ in their nitrogenous bases Are thymine (T), cytosine (C), adenine (A), and guanine (G) RNA has uracil (U) in place of thymine

RNA is also a nucleic acid RNA has a slightly different sugar RNA has U instead of T

DNA is a double-stranded helix James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin

The structure of DNA consists of two polynucleotide strands wrapped around each other in a double helix 1 chocolate coat, Blind (PRA) The model of DNA is like a rope ladder twisted into a spiral

Hydrogen bonds between bases hold the strands together Each base pairs with a complementary partner A pairs with T G pairs with C

Detailed representations of DNA Notice that the bases pair in a complementary fashion

DNA replication depends on specific base pairing In DNA replication, the strands separate Enzymes use each strand as a template to assemble the new strands

Untwisting and replication of DNA

DNA can be damaged by ultraviolet light The enzymes and proteins involved in replication can repair the damage

DNA replication Begins at specific sites on a double helix Proceeds in both directions

Each strand of the double helix is oriented in the opposite direction

How DNA daughter strands are synthesized 3 How DNA daughter strands are synthesized The daughter strands are identical to the parent molecule

THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN DNA functions as the inherited directions for a cell or organism How are these directions carried out?

How an Organism’s DNA Genotype Produces Its Phenotype An organism’s genotype, its genetic makeup is the sequence of nucleotide bases in DNA The phenotype is the organism’s specific traits

DNA specifies the synthesis of proteins in two stages Transcription Translation

From Nucleotide Sequence to Amino Acid Sequence: An Overview The one gene–one protein hypothesis states that the function of an individual gene is to dictate the production of a specific protein From Nucleotide Sequence to Amino Acid Sequence: An Overview The information, or “language,” in DNA is ultimately translated into the language of polypeptides

What is the language of nucleic acids? In DNA, it is the linear sequence of nucleotide bases The “words” of the DNA “language” are triplets of bases called codons The codons in a gene specify the amino acid sequence of a polypeptide

When DNA is transcribed, the result is an RNA molecule RNA is then translated into a sequence of amino acids in a polypeptide What is the correspondence between the nucleotides of an RNA molecule and the amino acids of a polypeptide? Triplets of bases Specify all the amino acids Are called codons

The Genetic Code The genetic code is the set of rules relating nucleotide sequence to amino acid sequence

Transcription produces genetic messages in the form of RNA

Transcription: From DNA to RNA In transcription Genetic information is transferred from DNA to RNA An RNA molecule is transcribed from a DNA template

Initiation of Transcription The “start transcribing” signal is a nucleotide sequence called a promoter The first phase of transcription is initiation RNA polymerase attaches to the promoter RNA synthesis begins RNA Elongation The second phase of transcription is elongation The RNA grows longer

Termination of Transcription The third phase of transcription is termination RNA polymerase reaches a sequence of DNA bases called a terminator

In transcription, the DNA helix unzips RNA nucleotides line up along one strand of the DNA following the base-pairing rules The single-stranded messenger RNA peels away and the DNA strands rejoin

The Processing of Eukaryotic RNA The eukaryotic cell processes the RNA after transcription

RNA processing includes Adding a cap and tail Removing introns (Noncoding segments) Splicing exons together

Translation: Translation Is the conversion from the nucleic acid language to the protein language

Transfer RNA molecules serve as interpreters during translation In the cytoplasm, a ribosome attaches to the mRNA and translates its message into a polypeptide The process is aided by transfer RNAs

Each tRNA molecule has a triplet anticodon on one end and an amino acid attachment site on the other

Transfer RNA (tRNA) tRNA Acts as a molecular interpreter Carries amino acids Matches amino acids with codons in mRNA using anticodons

Ribosomes Ribosomes Are organelles that actually make polypeptides Are made up of two protein subunits Contain ribosomal RNA (rRNA)

A fully assembled ribosome holds tRNA and mRNA for use in translation

Translation: The Process Translation is divided into three phases Initiation Elongation Termination

Initiation The first phase brings together The mRNA The first amino acid with its attached tRNA The two subunits of the ribosome

An mRNA molecule has a cap and tail that help it bind to the ribosome

Elongation Step 1, codon recognition Step 2, peptide bond formation The anticodon of an incoming tRNA pairs with the mRNA codon Step 2, peptide bond formation The ribosome catalyzes bond formation between amino acids Step 3, translocation A tRNA leaves the P site of the ribosome The ribosome moves down the mRNA

Review: DNA RNA Protein Termination Elongation continues until the ribosome reaches a stop codon Review: DNA RNA Protein The flow of genetic information in a cell

In eukaryotic cells Transcription occurs in the nucleus Translation occurs in the cytoplasm

Mutations A mutation Is any change in the nucleotide sequence of DNA These are caused by errors in DNA replication or by mutagens Physical or chemical agents

Types of Mutations Mutations within a gene Can be divided into two general categories Can result in changes in the amino acids in proteins

Insertions and deletions Can have disastrous effects Change the reading frame of the genetic message

VIRUSES: GENES IN PACKAGES Viruses sit on the fence between life and nonlife They exhibit some but not all characteristics of living organisms

Bacteriophages Bacteriophages, or phages Attack bacteria Bacterial cell

Connection: Many viruses cause disease in animals Many viruses have RNA, rather than DNA, as their genetic material Example: flu viruses

Plant Viruses Viruses that infect plants Most plant viruses have RNA Can stunt growth and diminish plant yields Can spread throughout the entire plant Most plant viruses have RNA Example: tobacco mosaic disease Genetic engineering methods Have been used to create virus-resistant plants

Connection: Emerging viruses threaten human health The deadly Ebola virus causes hemorrhagic fever Each virus is an enveloped thread of protein-coated RNA Hantavirus is another enveloped RNA virus

The AIDS virus makes DNA on an RNA template HIV is a retrovirus

HIV is a retrovirus A retrovirus is an RNA virus that reproduces by means of a DNA molecule It copies its RNA to DNA using reverse transcriptase

Inside a cell, HIV uses its RNA as a template for making DNA to insert into the host chromosome

AIDS is Acquired immune deficiency syndrome The disease caused by HIV infection Treated with the drug AZT Virus studies help establish molecular genetics Molecular genetics helps us understand viruses such as HIV, seen here attacking a white blood cell

How do new viruses arise? Mutation of existing viruses Spread to new host species