THE GENETIC CODE

Chapter 8

Freshman Honors Biology

Semester Two

Discovery

Where does our inheritance come from?Thought to be either DNA or proteinSeveral experiments were performed by

various scientists Conclusion: What molecule is the hereditary

molecule? Read p. 228 Hershey and Chase experiments.

1. What did they do?

2. What did they find out?

What does DNA look like?

Rosalind Franklin (1951)

Chargaff Noticed that the percentage of adenine and

thymine were similar in DNA samples Also, the percentages of cytosine and

guanine were similar Conclusion…?

Watson & Crick (1953) Won the Nobel Prize

for determining the structure of DNA

Proposed that it was a double helix with bases pairing in the middle (like a twisted ladder)

Nucleic Acid: Basic Structure

Nucleotide- monomer of a nucleic acid (polymer)

Three partsPhosphateSugarNitrogen Base

Phosphate

Sugar Nitrogen Base

DNA Nucleotide

= Phosphate = Deoxyribose

PURINES PYRIMIDINES

= Adenine

= Guanine

= Thymine

= Cytosine

DNA-Nitrogen Bases Pyrimidine

Single ring structure; C and T Purine

Double ring structure; A and G Base pairing

One purine and one pyrimidine Why does this work?

A = TG = C

A T

G C

DNA Structure Sugar-Phosphate

BackboneDouble strandedAlternating Inverted strands/anti-

parallelBase attached to

sugar Base Pairing

A pairs with TC pairs with GWeak Hydrogen bonds

hold them together

Nucleic Acids

Location of DNA

ProkaryotesFloating in cytoplasmOccurs as a ring

EukaryoteLocated in NucleusChromatin- form present during interphaseChromosomes- form present during

mitosis/meiosis

Chromatin and Chromosome StructureChromatin

DNA is coiled around histone proteins

Looks like a beaded necklace

Provides access to genes during interphase

Chromosome DNA is super-coiled Compacts the DNA for

more efficient movement Less chance of damage

or mistakes

DNA Replication DNA Formation of a new DNA molecule Occurs in the nucleus during S phase of

interphase Goal- to create a copy of every piece of DNA

before cell division Semi-conservative

Each original strand serves as a template Ending DNA molecules have one original strand and one

new strand

Process of DNA Replication DNA is unzipped

Done by an enzyme (helicase)Creates replication forksMany replication forks along length of DNA

strand

Process of DNA Replication (continued) Synthesis of new DNA strands

Done by enzyme (DNA polymerase)Occurs in opposite directions on the two strandsBases are added according to base pairing rules

(A-T and C-G)

Process of DNA Replication (continued) Finishing Touches

Backbone is sealed (done by enzyme: ligase)Proofread (done by enzyme: DNA polymerase)

RNA: The Other Nucleic Acid RNA Found in both the nucleus and the cytoplasm Also composed of nucleotides Functions to

Turn DNA instructions into a proteinRegulate gene function

Differences from DNASugar is ribose instead of deoxyriboseThymine is replaced with uracilSingle strand (backbone)

RNA NUCLEOTIDE

= Phosphate = Ribose

Purines Pyrimidines

= Adenine

= Guanine

= Uracil

= Cytosine

RNA STRUCTURE

•Single-stranded

Types of RNA All are made as copies of the DNA Messenger RNA (mRNA)

Single strand forms a stringCarries DNA instructions for 1 protein to the ribosome

Transfer RNA (tRNA)Single strand folds into clover leaf shapeCarries amino acids to the ribosome to build the

protein Ribosomal RNA (rRNA)

Single strand folds into 3D shapeCreates ribosome structure

Protein Synthesis: Central Dogma

DNA RNA Protein

TranscriptionDNA gene is transcribed (copied) into mRNA

mRNA bases are added by complementary base pairing rules

Occurs in nucleusDone by enzyme (RNA polymerase)

Separates DNA strands Uses one strand as template to base pair When finished, mRNA breaks off and DNA binds together

againmRNA is processedLeaves the nucleus through pores Travels to ribosome in cytoplasm

Complementary Base Pairing

DNA NitrogenBases

A=UT=AC=GG=C

RNA Nitrogen Bases

Translation mRNA is translated (decoded) into a

protein molecule at ribosome in cytoplasm mRNA instructions are read three bases at

a time- codon Every codon matches with a tRNA

anticodon tRNA is attached to a specific amino acid

(protein monomer) Amino acids are joined at the ribosome to

form a protein

Genetic Code Every mRNA codon

codes for a specific amino acid

This is called the genetic code

64 possible codons- only 20 amino acids

Start codon- AUG Stop codons-

UGA, UAA, UAG

Mutations Change to the genetic material Mutations within a gene

Point Mutation- occurs at one point in the DNA, changes one nucleotide○ Insertion- extra base added to gene○ Deletion- base removed from gene○ Substitution- one base is exchanged with another

Frameshift Mutation- moves all remaining bases forward or backward; changes all of the codons after it (insertion and deletion)

Mutations

Chromosomal Mutation Change in genetic material that can be seen

on a chromosomal level Duplication- part of a chromosome has been

repeated Deletion- part of a chromosome has been lost Inversion- part of a chromosome has been

flipped Translocation- part of a chromosome has

broken off and attached to another chromosome

Consequences of Mutations Some don’t do anything

Genetic change causes no change to protein

Protein change causes no change to function

Some are harmfulProtein change causes loss of function or

improper function

Gene Regulation

All somatic cells of an individual contain the same DNA

Different cell types just use different parts of the DNA library

Since these cells use different proteins, they look and act differently (cell specialization or differentiation)