Unit 2 The Molecule of Life Genes and Heredity

What is a gene?

Definitions of the gene The 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.

Summary Mendel’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 Topics Describe the DNA molecule (i.e. what it is made of, importance/purpose, etc) Explain how DNA contributes to the diversity of life forms Describe the chemical nature of chromosomes (proteins and nucleic acids) Summarize the work of Watson and Crick in discovering the DNA molecule Explain the structure of DNA (include a diagram)

Criteria for a material to be hereditary: separate particle that can function independently found in the nuclear material of all cells every cell has the same kind and amount of material located in a safe place protected from damage able to be passed on to the next generation able to copy itself accurately carries instructions in chemical code able to translate messages from chemical codes into agents that cause things to happen within the cell/body

The DNA Molecule Found in all cells of each living organism It 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 ones Chemical messages carry information from the nucleus to the cytoplasm where the directions are carried out

DNA and Diversity Offspring 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 Protein Basic Unit = amino acid Nucleic Acid Basic Unit = nucleotide

Nucleotide Made of: Phosphates Sugar (deoxyribose) One of four different nitrogen bases Adenine Thymine Guanine Cytosine Genetic code lies in the sequence of nucleotides, not the proteins

Watson and Crick Scientists 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 stranded Twisted into helix Has a beginning and an end Nucleotides are found in all organisms but the proportion of nitrogen bases (A, G, T, C) varied from species to species A=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 Structure DNA 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 group 2. Pentose sugar 3. Nitrogenous base

Nucleotides Phosphate Pentose Sugar Nitrogenous Base Genetic code lies in the sequence of nucleotides

Nucleotides The 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 Adenine T Thymine G Guanine C Cytosine

Nucleotides Each base will only bond with one other specific base. Adenine (A) Thymine (T) Cytosine (C) Guanine (G) 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 Structure To 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 G T C C - G A G - T T C - A G G - A T C

A 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 Genetics The Structure of the DNA Molecule Build a DNA Molecule The Discovery of the Molecular Structure of DNA The Discovery of the Molecular Structure of DNA

Summary of the Chemical Nature of Chromosomes Made up of proteins and nucleic acids Protein Basic Unit = amino acid Nucleic Acid Basic Unit = nucleotide Nucleotide made of: Phosphates Sugar (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 Task Draw 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.