Chapter 10- Molecular Biology of Genes

• Adenine• AIDS• Anticodon• Bacteriophages• Codon• Cytosine• DNA ligase• DNA polymerase• Double helix• Guanine• HIV• Lysogenic cycle• Lytic cycle• mRNA• Mutagen• Mutagenesis• Mutation• Nucleotides

• Phages• Promoter• Prophage• Reading frame• Retrovirus• Reverse transcriptase• RNA polymerase• rRNA

RNA splicing• Stop codon• Sugar-phosphate backbone• Terminator• Thymine• Transcription• Translation• Triplet code• tRNA• Uracil

Transformation Experiment• Griffith’s- 1928

– He wanted to learn what was transferred that made a bacteria become virulent and able to cause pneumonia

– He hypothesized some factor (maybe a gene) was transferred– 2 strains of bacteria

• S- strain = killed mice, R-strain = mice lived

– Transformation occurred- harmless bacteria was turned into disease-causing bacteria

Griffith’s Experiment

Next problem:

• 1940’s- – scientists knew that DNA and protein made up

chromosomes but they didn’t know which one was the genetic material

• Much evidence at first pointed to protein

DNA (not protein)• Hershey and Chase- 1952

– Used bacteriophages- virus that infects bacteria• They are composed of DNA, RNA, protein coat• Virus adheres to bacteria and injects genetic information into it, viral genes

act to produce new bacteriophages, cell bursts and new virus come out

– Needed to figure out which part entered the cell so they would know if genes were made of protein or DNA

• Grew viruses in cultures with radioactive phosphorus or sulfur, used as markers

• DNA contains no S, protein contains no P• If S found in bacteria – viral protein was injected• If P found in bacteria - viral DNA was injected

– Concluded DNA was the genetic material (not protein)

What do we know about DNA?

• Made up of nucleotides– Phosphate group– 5-C sugar (deoxyribose)– Nitrogen base- A, T, C, G– Backbone is made up of sugar and phosphate– Bases stick off the sides

The bases:

• Purines- have 2 rings– Adenine, Guanine

• Pyrimidines- have 1 ring– Cytosine, Thymine

There was a race to determine DNA structure

• Chargaff– Percentages of A’s and T’s were almost equal

as were percentages of C’s and G’s

• Franklin– used X-ray diffraction and recorded pattern,

gave clues to the structure• Pattern showed that it twisted around itself

• Watson and Crick-tried to figure out structure with models

• When they saw Rosalind’s photos they figured out the structure of DNA, which solved the mystery of how it carries information and how it was copied

– the double helix “twisted ladder”• Held together by hydrogen bonds between bases• This pairing of bases explained Chargraff’s rule

DNA Replication

• Each strand serves as a template to make a new complementary strand

• Each DNA strand has 3’ and 5’ end– DNA can only grow in the 5’ 3’

direction

DNA Replication

• Origin of replication- site where DNA replication starts– Replication proceeds in both directions forming a “bubble”

DNA Replication

• DNA polymerase- adds new nucleotides onto existing ones

• DNA ligase- joins new pieces of DNA together

• DNA polyermase and ligase are also involved in proofreading and fixing damaged DNA

• DNA replication involves many other proteins, it is surprisingly fast and accurate

DNA Replication

How information is transferred in a cell:

• Central dogma of biology

• DNA RNA protein

• Transcription- transfer of information from DNA to an RNA molecule

• Translation- transfer of RNA information to a protein molecule

• Beadle and Tatum’s experiment about lack of growth without certain enzymes yielded the 1 gene = 1 polypeptide hypothesis

RNA

• Made during transcription

• Single-stranded nucleic acid

• Bases: A, U (uracil), C, G

3 Types of RNA• mRNA- messenger

– Processed before it leaves the nucleus– G cap and tail chain of A’s is added– Introns are spliced out (RNA splicing)– Introns- intervening sequence in a gene (junk)– Exons- expressed part of a gene

• tRNA- transfer– match amino acid with correct codons– One end of tRNA has anticodon (complementary

sequence to codon) – tRNA’s are available for each amino acid– Uses ATP to drive binding reaction

• rRNA- ribosomal– along with proteins make up the ribosome where

protein synthesis takes place

The letters make a code• Transcription and translation rely on 3-

letter “words” called codons

• Each codon codes for 1 amino acid

Where does transcription and translation occur?

• Prokaryotes: both happen in the cytoplasm

• Eukaryotes: transcription- in the nucleus and translation- in the cytoplasm

Transcription

• Occurs in nucleus (we will be discussing eukayotes)• 1 strand serves as template, RNA polyermase binds and

RNA nucleotides join together to create a n RNA molecule

• Promoters- sequences that tell RNA pol where to bind• Stages:

– Initiation- RNA pol binds and starts transcription, – Elongation- continuation of RNA synthesis, – Termination- RNA pol reaches terminator sequence

and RNA pol detaches

Translation• Initiation- mRNA binds to ribosomal subunit

– Initiator tRNA binds to start codon and is carrying Methionine– Larger ribosome subunit binds and initiator tRNA binds to P site

• Elongation- anticodon on tRNA recognizes codon on mRNA and pairs in A site– Peptide bond forms between amino acids in A and P site,

ribosome catalyzes bond– P site tRNA leaves and A site tRNA moves over, now next tRNA

can bind to A site

• Termination- stops when a stop codon is reached (UAG, UAA, UGA)

Why do through transcription and translation?

• DNA RNA protein– Controls the way genotypes produce

phenotypes

What happens if a mistake is made?

• Mutations- change in the DNA sequence– bases can be substituted for one another– Bases can be deleted or inserted– ** not always a bad thing, also creates new variety that could

possibly benefit – Reading frame- triplet codon, can be affected by insertions and

deletions– Mutatgenesis- creation of mutations– Mutatgen- sources of mutations, chemicals, radiation

Viruses can affect transcription and translation

• A virus is just a package of genetic information• Types of viral reproduction

– Lytic cycle- leads to a cell filled with viruses lysing (breaking open)

– Lysogenic cycle- cell with viral DNA in it’s genome divides• When phage DNA enters cell it is called a prophage

RNA Viruses

• glycoprotein attaches to host membrane• RNA enters cell• enzymes remove RNA protein coat• complementary strand is made• new strand has instructions to make new proteins and viral RNA• new protein coat assembles on new RNA• lastly viruses leave cell coated in host’s plasma cell membrane

Plant Viruses

• Can stunt growth, lower crop yields• Plant viruses must pass outer layer of plant, so wind

damage, injury or insects allow for entrance• Some insects also carry viruses to plants, as well as

pruning shears may hold virus

Animal Viruses

• Influenza, hepatitis, chicken pox, herpes, mumps, measles, AIDS, polio

How do new ones arise?

• Fragments of cellular nucleic acid that could move from one cell to another, possible on the surfaces of injured cells

• Today- mutations of existing viruses is the source of new ones

• Also passing existing viruses to new hosts, via dust containing viruses

• RNA viruses mutate faster because there is not DNA proofreading happening

HIV Virus

• Acquired immune deficiency syndrome• Human immunodeficiency virus

• Affects WBC’s of immune system, can’t fight off infection

• Contains 2 RNA copies, instead of usual 1• It’s a retrovirus, RNA virus that produces DNA using

reverse transcriptase (enzyme that does the process)• Viral DNA enters nucleus and then viral DNA is

transcribed and translated making more viral proteins which then assemble and leave cell to infect others