CHAPTER 12

DNA and RNA

12-1: DNA

How was DNA discovered? Fredrick Griffith Oswald Avery Hershey & Chase Watson & Crick

• Frederick Griffith

Griffith (cont.) Experimented with bacteria and mice Cultured both harmless and pneumonia

causing bacteria. Exposed mice to a mixture of both

harmless and heat killed pneumonia causing bacteria Mice came down with disease

Conclusion: Griffith called it transformation

Cells can be transformed when coming into contact with other types of cells

Harmless bacteria transformed into disease causing strains.

What was this disease causing agent and how did it transform the other cell?

Oswald Avery Set out to answer the previous question. Repeated Griffith's work, took extract from heat

killed bacteria. Treated extract with enzymes that break down:

Proteins CHO’s Lipids Even RNA

Same results Then repeated with enzymes that break down DNA Result:

No transformation

Hershey and Chase (cont.) Used radioactive markers to determine if

proteins or DNA was injected by viruses The tracers could be followed from the virus

to the bacteria. Injected with:

Phosphorus-32 (in DNA, not in proteins) Sulfur-35 (in proteins, not in DNA)

Results: Only Phosphorus-32 was transferred

At this point, scientists knew DNA was the “culprit” for where genes are contained.

Still needed to know:1. How did DNA carry genes generation to

generation?2. How did DNA code for traits?3. How was copied?

The Components and Structure of DNA

Nucleotides 5-carbon sugar

Deoxyribose Ribose

Phosphate Nitrogen base

Chargaff’s rule Determined complementary nature of DNA.

X-rays were used to “see” the general structure of DNA

Appeared like this:

Watson & Crick Determined the

shape had to be a double helix

Two strands with complementary base pairs in between that are bonded together.

12-2: Chromosomes and DNA Replication

Chromosome structure Histones

Proteins that chromatin is wrapped around Chromatin

Condensed DNA

Nucleosome = histones + chromatin

Duplicating DNA “Replication” Each new cell after

mitosis gets exact copy

Steps of replication

1. DNA is “unzipped” by protein called DNA helicase (breaks hydrogen bonds between compl. bases)

2. DNA polymerase reads the sequence and adds base pairs that complement.

3. DNA polymerase also proofreads along the existing strand.

4. DNA ligases “put together” chunks of copied base segements.

12-3 RNA and Protein Synthesis

RNA Structure

Single-stranded Ribose-phosphate backbone Contains nitrogen base uracil instead of thymine

It bonds with adenine on DNA

Types of RNA mRNA – messenger RNA

transcription rRNA – ribosomal RNA

Make up components of the ribosomes tRNA – transfer RNA

translation

Transcription DNA gives code to RNA for making proteins Similar to replication except now code is

copied to RNA (has uracil) RNA polymerase unzips the DNA strand and

begins to add bases that complement one of the strands.

YouTube - Transcription

How does it know where to start? Promoters

Specific sequences of base pairs that RNA polymerase can only bind to in order to initiate transcription.

RNA editing Introns and Exons

Introns Sequences that code for nothing

Exons Sequences that directly code for sections of

proteins Sequences that are “expressed” as protein

Eventually enzymes go back and cut the introns out and splice together the exons to have a fully functioning mRNA.

The Genetic Code 20 different amino acids Each is coded for by a segment of 3 base

pairs Codon Most amino acids have multiple codons Also codons for starting and stopping

transcription

Translation Copying mRNA into a sequence of amino

acids. mRNA attaches to ribosome Ribosome “reads” looks for a “start codon” Two tRNA with “anticodon” that

complement the strand are attached to mRNA by the ribosome.

Temporary hydrogen bonds allow the tRNA to be bound to mRNA long enough to form a “peptide bond” between the two amino acids.

YouTube - Translation

Genes and Proteins

These proteins that are produced ultimately go to make: Strucutral proteins (eye color, physical

features) Enzymes (control all cellular activities, ex:

digesting lactose) Hormones (producing

testosterone/estrogen) Combined all of these factors ultimately

make us what we are.

12-4 Mutations

Mutations Changes in the genetic sequence

Types of mutations Point mutations

Occur in one (or few) base(s) of the DNA sequence

Include: Substitutions

Sometimes little to no effect on amino acid sequence, however sometimes can be cataclysmic

Sickle-cell anemia

Frameshift mutations Caused by insertions or deletions of bases,

shifting the way the mRNA is read. Shifts the “reading frame” Usually has dramatic effects on the

formation of the protein – often rendering it useless

Chromosomal mutations Deletions

Duplications

Inversions

Translocations

Significance of mutations Most often mutations ultimately show little

no effect on the protein that is supposed to be made.

However, when it does have an effect the new protein formed can be:

DetrimentalIncreases organisms chance of

dying an not passing mutated gene on.

BeneficialIncreases the organisms chance of

survival and reproductibility – therefore passing it down

CREATES GENETIC VARIABILITY!This is often how asexually

reproducing organisms evolveSlow process

12-5: Gene Regulation

How does an organism “know” when to turn the gene on or off? Example: How does your body turn on the

lactase gene?

Regulation of the lac operon in E.Coli Operon

Groups of genes that code for specific protein lac operon in E.Coli codes for protein that

breaks down lactose

Again, transcription begins at the sequence called the promoter

Just “below” the promoter are “operator” (O) sites These sites are areas where “repressors” can

bind In most cases repressors are bound to O site,

preventing transcription Turning off the gene Just like a room, when you are not in it –

TURN OFF THE LIGHT!

When lactose is present, it binds with the repressor changing its shape, forcing it off the O site

Allows RNA polymerase to begin transcription

YouTube - Lac Operon

How is this done in eukaryotic cells? Much more complex than lac operon. Invovles sequence of base pairs near

promoter called “TATA box” Sequence of TATATATA… or TATAAA….

Positions RNA polymerase

All cells contain the entire Genetic Code BUT any one cell will use a small fraction of those genes. Heart cells use different genes than brain cells.

Development and Differentiation

Cells that change into specific types of specialized cells

Hox genes Genes that control this differentiation early in

development Mutations involving hox genes can have HUGE

effect on outcome of organism Pax 6

Gene found in Drosophila and mice that controls eye development

Inserted mouse Pax 6 gene into the “knee” of Drosophila embryo Grew an eye on its leg

YouTube - Evolution Genetic Tool Kit