Unit 2The Molecule of LifeGenes and Heredity

DNAStructur

e

What is a gene?

Definitions of the geneThe gene is to genetics what the atom is to

chemistry.The gene is the unit of genetic information

that controls a specific aspect of the phenotype.

The gene is the unit of genetic information that specifies the synthesis of one polypeptide.

Are genes both the basic functional unit and the smallest genetic structural unit?Until 1940, the gene was considered as the

basic unit of genetic information as defined by three criteria.The unit of function, controlling the

inheritance of one “character” or phenotypic attribute.

The unit of structure, operationally defined by recombination and by mutation.

Further studies of intergenic recombination in bacteriophage and bacteria (where billions, instead of thousands, of progeny can be analyzed) showed that recombination occurs between adjacent nucleotide pairs.

So the nucleotide, not the gene, is the basic unit of genetic structure.

SummaryMendel’s work established the concept of the

gene.This concept has evolved from:

the unit that can mutate to cause a specific block in metabolism…

to the unit specifying one enzyme… to the sequence of nucleotide pairs in DNA

encoding one polypeptide chain.

What is DNA? The Basics

Notes TopicsDescribe the DNA molecule (i.e. what it is

made of, importance/purpose, etc)Explain how DNA contributes to the

diversity of life formsDescribe the chemical nature of

chromosomes (proteins and nucleic acids)Summarize the work of Watson and Crick

in discovering the DNA moleculeExplain the structure of DNA (include a

diagram)

Criteria for a material to be hereditary:

separate particle that can function independentlyfound in the nuclear material of all cellsevery cell has the same kind and amount of materiallocated in a safe place protected from damageable to be passed on to the next generationable to copy itself accuratelycarries instructions in chemical codeable to translate messages from chemical codes into

agents that cause things to happen within the cell/body

The DNA MoleculeFound in all cells of each living

organismIt is the only molecule that can

replicate itself which permits cell division

Provides directions that guide the repair of damaged cells or production of new onesChemical messages carry information from the nucleus to the cytoplasm where the directions are carried out

DNA and DiversityOffspring are genetically different from

their parents due to exchange of genes during crossing over and mutations

In both cases, the original DNA code is changed to produce a genetically different offspring

What is a gene?

Chemical Nature of Chromosomes

Chromosomes are made up of proteins and nucleic acids

ProteinBasic Unit = amino acid

Nucleic Acid Basic Unit = nucleotide

NucleotideMade of:

PhosphatesSugar (deoxyribose)One of four different nitrogen bases

Adenine Thymine Guanine Cytosine

Genetic code lies in the sequence of nucleotides, not the proteins

Watson and CrickScientists who discovered the structure of DNA:• Knowing structure leads to knowledge of how it

functions

• Used X-Ray Crystallography technology to discover that the structure was a double helix

• Studied DNA of different organisms.

• They presented a 3-D model to scientific community in 1953

What they found out…..DNA is Double strandedTwisted into helixHas a beginning and an

endNucleotides are found in all organisms but the proportion of nitrogen bases (A, G, T, C) varied from species to speciesA=T, C=G suggested they are arranged in pairs

DNA has 4 levels of arrangement (structures) dependent on the stage of cell cycle

Example:as chromatin it is in its primary structure

as a chromosome in prophase it is in its quaternary structure

DNA StructureDNA consists of two molecules that are arranged

into a twisted ladder-like structure called a Double Helix.

A molecule of DNA is made up of millions of tiny subunits called Nucleotides = Basic unit of DNA

Each nucleotide consists of:1. Phosphate group2. Pentose sugar3. Nitrogenous base

Nucleotides

Phosphate

Pentose

Sugar

Nitrogenous

Base

Genetic code lies in the sequence of nucleotides

NucleotidesThe phosphate and sugar alternate to

form the backbone of the DNA molecule The bases are joined by hydrogen bonds

form the “rungs”.

There are four types of nitrogenous bases.

Nucleotides

A

AdenineT

Thymine

G

GuanineC

Cytosine

NucleotidesEach base will only bond with one

other specific base.

Adenine (A)Thymine (T)

Cytosine (C)Guanine (G)

Form a base pair.

Form a base pair.

DNA Structure

Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand.

G

G

A

T

T

A

A

C

T

G

C

A

T

C

Break it Down….

What holds them together?

DNA StructureTo crack the genetic code found in

DNA we need to look at the sequence of bases.

The bases are arranged in triplets called codons.

A G G - C T C - A A G - T C C - T A GT C C - G A G - T T C - A G G - A T C

DNA StructureA gene is a section of DNA that codes for a

protein.

Each unique gene has a unique sequence of bases.

This unique sequence of bases will code for the production of a unique protein.

It is these proteins and combination of proteins that give us a unique phenotype.

Supplementary Resources Molecular GeneticsThe Structure of the DNA MoleculeBuild a DNA MoleculeThe Discovery of the Molecular Struc

ture of DNA

Summary of the Chemical Nature of Chromosomes

Made up of proteins and nucleic acidsProtein

Basic Unit = amino acid

Nucleic Acid Basic Unit = nucleotide

Nucleotide made of:PhosphatesSugar

(deoxyribose)One of four

different nitrogen bases

adenine thymine guanine cytosine

*Genetic code lies in the sequence of nucleotides, not the proteins

Protein

DNA

Gene

Trait

Your TaskDraw a flow chart to

show how to get from:

The DNA of eukaryotic cell is tightly bound to small basic proteins (histones) that package the DNA in an orderly way in the cell nucleus.

suggest that the binding of proteins to DNA in chromatin protects the regions of DNA from nuclease digestion, so that enzyme can

attack DNA only at sites separated by approximately 200 base pairs.