Molecular Basis of Heredity

Discovery of DNAStructure and Function of

DNAReplicationTranscriptionTranslation

Discovery of DNA:Frederick Griffith – 1928

Wanted to know how bacteria caused pneumonia

Injected mice with disease-causing strain mice died

Injected mice with harmless strain mice lived

Injected mice with heat killed, disease-causing strain and harmless strain mixed together mice died

Discovery of DNA:Frederick Griffith – 1928

Transformation:The heat-killed bacteria passed their

disease-causing ability to the harmless strain

One strain of bacteria was changed into another

Oswald Avery – 1944

Repeated Griffith’s work Treated the heat-killed bacteria to enzymes

that broke down everything but DNA bacteria still transformed

Treated the heat-killed bacteria to enzyme that broke down DNA bacteria did not transform

Discovered that DNA is the nucleic acid that stores and transmits genetic information

Alfred Hershey & Martha Chase – 1952

Bacteriophage – “bacteria eater” (a virus that infects and kills bacteria)

Placed a radioactive marker on phosphorus (DNA) and sulfur (protein)

Only radioactive phosphorus was found in the bacteria

Thus, the virus only injected DNA into the bacteria not protein

DNA Structure Discovery

Please review DNA discovery notes handed out in class.

Portfolio worthy narrative account = Watson, Crick, Wilkens, and Franklin

controversy newspaper article

Function of DNA

DNA carries information from one generation to the next

DNA determines the heritable characteristics of organisms

DNA is easily copied

Structure of DNA

Double helixAnti-parallelComplementarySugar phosphate backboneNitrogenous bases in the center held

together with hydrogen bondsChargaff’s Rule = A binds with T, C

binds with G

Structure of DNA

Nucleotide = A phosphate

group A deoxyribose

sugar (5 carbon) A nitrogenous

base• Adenine• Thymine• Cytosine• Guanine

Chromosome Structure

“supercoils”

DNA Replication Overview

Each strand of the double helix can be used as a template for a new strand of DNA “Semi-conservative” each new DNA

molecules contains one new strand and one old strand

Prokaryotes = replication is simple; typically one replication fork (circular DNA)

Eukaryotes = replication is more complex; hundreds of replication forks

The Cell Cycle

DNA is replicated during the S phase of the cell cycle

Replication Enzymes

Gyrase – Unwinds the supercoils Helicase – Unwinds the double helix Single-strand Binding Proteins –

stabilizes the DNA strands and keeps them apart

Primase – Attaches the RNA primer to the parent DNA strand to begin replication

Replication Enzymes (continued) DNA Polymerase (3 functions) –

1. Adds new nucleotides to the growing DNA strand

2. Proofreads and makes repairs when needed

3. Replaces RNA primer with DNA nucleotides

Ligase – joins and bonds the DNA fragments together to form a complete double helix

How Replication Occurs

DNA is synthesized in the 5’ 3’ direction only!!!This means that new nucleotides are

attached to the 3’ carbon of the deoxyribose molecule.

Replication occurs in the nucleus! View the DNA Replication streaming

video now and complete the replication activities

How Replication Occurs

Depending on how the replication fork opens :Continuous replication occurs on

the leading strand (new strand is made continuously in the 5’ 3’ direction)

Discontinuous replication occurs on the lagging strand (new strand is made in fragments called Okazaki fragments)

DNA Replication VIDEO

Watch DNA Replication streaming video from PBS.

http://player.discoveryeducation.com/index.cfm?guidAssetId=0CB6B02F-092A-4035-98B6-6378AF13F567&blnFromSearch=1&productcode=US

Telomeres Short repetitive sequence of DNA

• ex. TTTAAGGG (guanine rich)

Protect the ends of the chromosome from deterioration

Over time there is loss of DNA at the 5’ end of the lagging strands

• RNA primers cannot be replaced with DNA if there is no DNA after it for DNA polymerase to bind!

Causes aging in somatic (body) cells!• Telomerase (enzyme that regenerates telomeres) only

occurs in germ cells (sex cells) and malignant cells!

Turn and Talk

What is the consequence of losing telomeres on the 5’ end of the lagging strands of DNA molecules?

What could happen if we could prevent that loss?

Transcription and Translation

Structure of RNA

RNA Nucleotide = 5-carbon sugar (Ribose) Phosphate group Nitrogenous base

• Adenine, cytosine, guanine, uracil• No thymine (only in DNA)

Single stranded molecule Not a double helix like DNA

Blueprint of DNA (DNA is the Master plan)

Types of RNA

3 main types = Messenger RNA (mRNA)

• Carries copy of DNA message to the ribosome to be made into a protein

Transfer RNA (tRNA)• Transfers amino acids to the ribosome

based on the mRNA coded messageRibosomal RNA (rRNA)

• Reads the mRNA coded message like a decoder ring

Transcription Overview

Transcription begins in the nucleus and ends in the cytoplasm

To make mRNA RNA polymeraseBinds to DNA and uses one strand as

a template for a molecule of mRNA How does it know where to bind?

Promoters specific sequences in DNA that signal RNA polymerase to bind there (also tells when to stop)

mRNA Editing

mRNA must be edited before moving from the nucleus to the cytoplasm Introns – these intervening (non-coding)

sequences must be cut out Exons – Coding sequences that encode for a

specific protein No clear understanding why introns must be

removed Only the mature (“edited”) mRNA moves to

the cytoplasm

The Genetic Code Proteins are made using amino acids joined together by

peptide bonds 20 different amino acids The code consists of 4 letters:

A, U, C, and G (RNA bases) The genetic code is read 3 letters at a time

mRNA “Codon” = 3 bases (AUG) tRNA “Anti-codon” = 3 complimentary bases (UAC) 64 possible 3-base codons (some amino acids have

more than one codon that codes for it) Each amino acid has an amino group, a carboxyl group,

and an R-group. The R-group gives the amino acid it’s unique

personality!!! The peptide bond forms between the amino group of

one amino acid and the carboxyl group of another!

The Genetic Code (continued)

Start codon (for all proteins) =AUG methionine

Several stop codons (do not code for an amino acid…allows for release of the protein from the ribosomal complex)UGA, UAA, UAG

Translation (or protein synthesis) Overview mRNA serves as instructions for the protein to be

made (made during transcription) Translation begins when an mRNA molecule attaches

to the ribosomal complex and begins with the 1st codon (AUG)

tRNA (the “anticodon”) transfers the corresponding amino acid to the ribosome.

As each codon is read tRNA brings the corresponding amino acids to the ribosome

The amino acids are bonded to each other via a peptide bond

Once a stop codon is reached the protein molecule is released

Ribosomal complex

Mutations

Mutations Changes in the genetic material

2 Types:Gene MutationChromosome Mutation

Gene Mutations

Point mutations Occurs at a single point in the DNA sequence Could change one of the amino acids

Example: AAA TTT (normal) AAC TTT (mutation)

Frameshift mutations Addition or deletion of a nucleotide in the DNA Changes the “reading frame” of the code Consequences more serious

Example: AAA TTT (normal) AAT TT (mutation)

Chromosomal Mutations

Involves changes in the number or structure of chromosomes.DeletionDuplicationInversionTranslocation

Gene Regulation Genes are not always “on” Genes are regulated to turn “on” and

“off” In Prokaryotes:

The Lac Operon (a series of genes that work together) breaks down lactose if present into galactose and glucose.

These genes are turned off by repressors and are only turned on by the presence of lactose.

Eukaryotic Gene Regulation

Genes are controlled individually Have regulatory sequences that are much

more complex than prokaryotic gene regulation

Why are they more complex? Cell specialization!!!

• Each cell has DNA for the whole organism’s functioning, however, only liver cells need to produce liver proteins (etc.)

Regulation and Development Differentiation

Cells become specialized in structure and function

Hox genes Controls the differentiation of cells and

tissues in the embryo (controls the “body plan”)

• Example: Mouse eye gene inserted into the “knee” of a fly gene fly grew an eye on its leg!!!

Genes have descended from a common ancestor

Genetic Engineering

Selective Breeding

Humans take advantage of naturally occurring genetic variations Select desired traits to pass on to the next

generation (domestic animals) Hybridization

Cross dissimilar individuals to bring out the best of both organisms (“Hybrid vigor”)

Inbreeding Maintains the desired characteristics of a line

of organisms (although not without risk) Example: Dog breeds

Increasing Variation

Breeders can increase variation in a population by inducing mutations Radiation and chemicals Many mutations are harmful to the organism

New Kinds of Bacteria Development of useful strains of bacteria

(digestion of oil) New Kinds of Plants

Produces polyploid (multiple sets of chromosomes) individuals in plants, larger and stronger than diploid individuals (fatal in animals)

Manipulating DNA

Different techniques are used to:Extract DNA from cellsCut DNA into smaller piecesIdentify the sequence of bases in a

DNA moleculeMake unlimited copies of DNA

Tools of Molecular Biology Makes changes in the DNA code of a living organism DNA Extraction (SLE A1: banana DNA extraction lab)

DNA is separated from the rest of the cell using a simple chemical procedure

Cutting DNA Restriction enzymes cuts specific

sequences of nucleotides Separating DNA

Gel electrophoresis a DNA sample is placed at one end of a porous gel and an electric current is applied making the DNA fragments separate according to size

• Large fragments move more slowly than short fragments

Using the DNA sequence

Reading the sequence Creates a series of dye-tagged copies from

which the order tells the exact sequence of DNA

Cutting and Pasting Recombinant DNA DNA molecules

produced by combining DNA from different sources (DNA synthesizers)

Making copies Polymerase Chain Reaction (PCR) makes

several copies of the same gene by repeated heating and cooling

Applications of Genetic Engineering

Transgenic Organisms (contains genes from other organisms) Transgenic bacteria

• Useful for health (bacteria can be transformed to create human insulin and other forms of proteins) and industry (raw materials for plastics and synthetic fibers)

Transgenic animals• Used to study genes (example: mice with human

immune systems) and improve the food supply Transgenic plants

• Important part of our food supply (25% corn and 52% soybeans have been modified)

Cloning A member of a population of genetically identical cells. Easy to do with microorganisms/Hard with multicellular

organisms • There are ethical concerns too!