Chapter 12: DNA & RNA

Section 12-1: DNA & RNA

• After it was established that traits were passed from one generation to the next, scientists wanted to know how this information passed from parental to offspring.

• The most important experiments done to answer this question were performed by:– Frederick Griffith– Oswald Avery– Alfred Hershey & Martha Chase

Experiment Summary• Frederick Griffith:

– Used isolated bacteria that cause pneumonia.• Disease-causing bacteria• Harmless bacteria

– Showed that heat-killed bacteria could “transform”” harmless into disease-causing bacteria.

• Oswald Avery:– Repeated Griffith’s experiment, using an extract from heat-killed bacteria.

• Treated extract with chemicals to eliminate macromolecules.– Showed that DNA was the molecule causing the “transformation” in

harmless bacteria.

• Hershey-Chase:– Used bacteriophages and radioactive markers to corroborate previous

results.• Experiment would discriminate if what caused the “transformation” was nucleic

acid or protein.– Showed that DNA was the transferred material and not protein.

Griffith’s Experiment

Avery’s Experiment

Hershey-Chase Experiment

Structure of DNA

• DNA is composed of 4 Nucleic Acids, a deoxyribose and a phosphate group– Depending on their chemical composition,

they are classified in one of two categories:• Purines – two rings

– Adenine– Guanine

• Pyrimidines – one ring– Thymine– Cytosine

Structure of DNA

Nucleic Acids DNA Backbone

Chargaff’s Rule

• Nucleic Acids pair between a purine and a pyrimidine.– Adenines with Thymines– Guanines with Cytosines

• The percent of Guanines and Cytosines are almost equal and the same is true for Adenine and Thymine.– This is known as Chargaff’s

Rule.

X-Ray Evidence

• Research done by Rosalind Franklin using X-Ray diffraction, showed the pattern that latter helped Watson and Crick determine the 3-D structure of DNA.

The Double Helix

• In 1953 Francis Crick and James Watson proposed the model that sustained the previously stated and it explained the DNA’s properties.– Watson and Crick’s model

of the DNA was a double helix, in which two strands were wound around each other.

The Double Helix

DNA Structure’s Summary

• Watson and Crick’s model explained all the previous data presented:– The double helix explains the paring of

nucleic acids (base paring).• Chargaff’s Rule

– The helix structure explains the shape that Rosalind Franklin saw.

Section 12-2: Chromosomes and DNA Replication

• DNA molecules are quite long.– The smallest human

chromosome contains more than 30 million base pairs of DNA.

• To achieve the level of compaction needed, DNA is arranged into chromosomes.

DNA to Chromosome

DNA Replication

• When Watson & Crick presented their structural model of the DNA, their proposed structure also gave them insight into a possible method of coping the molecule.– Each of the strands can

serve as a mold for the new strand using base paring rules.

DNA Duplication

• Before each cell division, the DNA is copied by the process called replication.

• During DNA replication, the DNA molecule separates into two strands, then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand.

DNA Duplication

DNA Duplication

Prokaryotes vs. Eukaryotes

Prokaryote Eukaryote

The enzyme DNA Polymerase is involved in DNA Replication.

This enzyme besides adding nucleotides it performs “poof-reading” to maximize correct pairing of bases

12-3: RNA and Protein Synthesis

• RNA’s structure is like DNA’s with three major differences:– RNA is generally single strand (DNA is

double)– RNA has the nucleotide Uracil (DNA has

Thymine)– RNA has the sugar ribose (DNA has

deoxyribose)

Types of RNA

• There are 3 major classes of RNA:– mRNA: messenger RNA, takes information

from DNA to the rest of the cell.

– rRNA: ribosomal RNA, combines with proteins to form the ribosomes.

– tRNA: transfer RNA, transfer amino acids to the ribosome to build proteins based on information in the mRNA.

Types of RNA

Messenger RNA (mRNA)

Types of RNA

Messenger RNA (mRNA)

RNA (orange); Protein (blue)

Ribosomal RNA (rRNA)

Types of RNA

Messenger RNA (mRNA)

RNA (orange); Protein (red)

Ribosomal RNA (rRNA) Transfer RNA (tRNA)

Types of RNA

Transcription

• Process to make an mRNA using the DNA as template.– Process mediated by enzyme, RNA Polymerase

Transcription

RNA Editing

Genetic Code

• mRNA is read in groups of 3 called triplets.– Each triplet is called a codon.

• Each codon specify for a specific amino acid.– Some amino acids can be specified by more

than one codon.

Genetic Code

• Important codons:– Start (AUG)– Stop (UGA - UAA – UAG)

• Some amino acids have more than one codon.– This means that the code is

redundant.• The reason why some codons

code the same amino acid is because of the 3rd position of the codon.

Translation

• Process of “reading” the mRNA into protein.

• Process occurs within the ribosome (combination of rRNA and Protein).– The ribosomes contains

various sites within its structure for the entrance of the mRNA, the tRNA and the exit of the growing protein.

Translation

Genes and Proteins

• Genes contain the information to assemble proteins.

• Depending on the protein or changes done to these proteins, changes in phenotype can be noticed.– Proteins catalyze the reactions that produce

the pigment for the eyes or the antigens in the surface of red-blood cells.

12-4: Mutations

• Mutations are changes in the DNA sequence that affects genetic information.– Mutations can occur either at the level of:

• Genes– Point mutations (Substitutions)– Frameshift mutations (Insertions)

• Chromosomes– Deletion– Duplication– Insertion– Translocation

Genetic Mutations

• Gene mutations are changes in the DNA sequence or changing the sequence of triplets read.– Substitutions: changing

a nucleotide of another.• Missense & Nonsense

– Frameshift: changes in the reading frame.

• Addition or Deletions

Chromosomal Mutations

• Chromosomal mutations are changes in the structural make-up of the chromosomes.– Inversion: changes in the

order of genes with the chromosome.

– Deletions: elimination of a segment of a chromosomes.

– Translocation: exchange from one chromosome to another (not crossing over).

– Insertion: a piece of a chromosome is relocated to another chromosome.

12-5: Gene Regulation

• Only a handful o genes are expressed at a particular time.– An expressed gene is

a gene that is transcribed.

– Non-transcribed genes can be considered “silent genes”.

– Genes can turn “on” or “off”.

Before

After

Gene Regulation

• Not all sequences of DNA are genes, some sequences serve as promoters or binding sites for proteins, or signals for regulation (start and stop).– These sequences that do not code for a transcript are called

regulatory sites.

Gene Regulation in Prokaryotes

• Prokaryotes although simpler than Eukaryotes, utilize an operon to control the expression of their genes.– An operon is a group of genes that operate

(turn on or off) together.

– An example of a prokaryotic operon is the Lac Operon.

The Lac Operon

• The Lac gene does not need to be turned on all the time.– Bacteria can use glucose

as a source of energy.

• In the absence of glucose and presence of galactose, the Lac operon must turn on to produce the protein that will break down galactose.– Gal Glu + Glu

The Lac Operon

Repressor – gene off

Inducer – gene on

• Most eukaryotic genes are controlled individually and have regulatory sequences that are much more complex than those of the lac operon.

Gene Regulation in Eukaryotes

Regulation and Development

• With gene regulation, studies have shown that there specific genes that control the development of embryos.– Hox genes are a series of genes that control

organ and tissues that develop in various parts of the embryo.

– These genes determine the basic body plan for an animal.

Hox genes

Chordate Embryonic Comparison