Classical and Modern Genetics Chapter 23 Great Idea: All living things use the same genetic code to guide the chemical reactions in every cell.

Chapter Outline Classical Genetics DNA and the Birth of Molecular Genetics The Genetic Code

Classical Genetics

Chapter 23- Part 1 Classical Genetics Genetics got it’s start as the study of inheritance. Charles Darwin proposed that favorable traits could be passed from generation to generation resulting in natural selection. However, Darwin did not know how these traits were passed on.

Chromosomes from the Indian muntjak

It remained for the Austrian monk Gregor Mendel, in 1865, to carry out the definitive experiments.

Mendel crossed tall and dwarf pea plants: all offspring were tall. Tall Dwarf x F1 tall

Next, Mendel crossed some of these F 1 plants among themselves. Of these offspring (the F 2 generation), about 3/4 of the plants were tall and 1/4 were dwarf. tall tall x F 2 tall tall tall dwarf

Mendel tested 6 other traits of pea plants: traits for seed shape (wrinkled or smooth) seed color (yellow or green), etc. In each case, all of the F 1 plants looked as though they had inherited the trait of just one of their two parents, but in the F 2 generation both traits always appeared -- and always in a 3 to 1 ratio.

The trait which was expressed in the F 1 generation was always about 3 times as numerous in the F 2 generation as was the other one which was hidden in the F 1 's.

Homozygous = same Heterzygous = different When both alleles for a trait are identical, say that the organism is homozygous for that trait. When the 2 alleles are different, is heterozygous. TT = Homozygous Tt = Heterozygous tall tall Tall is dominant over dwarf; dwarf is said to be a recessive trait (i.e. can only be expressed when there are two copies of it).

Homozygous = same Heterzygous =different Tall Dwarf Tall TT tt Tt

Mendel's original cross produced only tall offspring:

However, in the second generation the rules of probability dictate that 1/4 of the plants will be tt = dwarf and 3/4 will have at least one T and hence be tall.

Mendel Studied Many Traits in Pea Plants-- Seed shape- smooth or wrinkled Seed color- green or yellow Pod shape- smooth or bumpy Pod color- green or yellow Flower location- at leaf or tip of branch

Many traits are passed on by genes. The genes encode the information for proteins. The genes are segments of DNA. Mendel found that two factors determine traits. These are alternate forms of genes- one from each parent. These are now called alleles.

Classical Genetics Mendel –Basic laws of inheritance –Classic pea plant experiments Purebred Hybrid Results –First generation –Second generation Gene –Dominant –Recessive

Rules of Classical Genetics Traits (genes) are passed from parent to offspring –mechanism unknown Two genes for each trait –One from each parent There are dominant and recessive genes –Dominant expressed

Alleles: two different forms of the gene. For many hereditary traits, genes exist in two or more different forms called alleles. On each pair of chromosomes, there is one allele for a particular gene on each. ex. A, B, O blood groups. In humans there are 3 alleles: A, B, and O.

Genotype AO BO AB OO Phenotype A B AB O

Genotype- genetic composition Phenotype- physical characteristics Genotype AO BO AB OO Phenotype A B AB O Ex. ABO blood groups. A and B are codominant and O is recessive.

Qualitative versus Quantitative Genetics Qualitative –observational Quantitative –Predictive model –Used to trace genetic disease

DNA and the Birth of Molecular Genetics

Nucleotides: The Building Blocks of Nucleic Acids Nucleotide –Three molecules Sugar –DNA: deoxyribose –RNA: ribose Phosphate ion Base –Adenine (A) –Guanine (G) –Cytosine (C) –Thymine (T)

DNA Structure Join nucleotides –Alternating phosphate and sugar DNA –2 strands of nucleotides –Joined by base pairs Bonding pattern –Adenine:Thymine –Cytosine:Guanine

DNA Structure

RNA Structure Differences –One string of nucleotides –Sugar is ribose –Thymine replaced by uracil Uracil (U) bonds with adenine

The Replication of DNA DNA replication –Occurs before mitosis & meiosis Process –DNA double helix splits –New bases bond to exposed bases –Result Two identical DNA strands

The Genetic Code

Transcription of DNA Transcription –Information transport –Uses RNA Process –Unzip DNA –RNA binds to exposed bases –RNA moves out of nucleus (mRNA)

The Synthesis of Proteins tRNA –Reads message –Structure Amino acid 3 bases Process –mRNA moves to ribosome –rRNA aligns mRNA and tRNA –tRNA matches codon on mRNA –Amino acid chain forms Basis for protein

Protein synthesis cont. One gene codes for one protein Protein drives chemical process in cell DNA –Introns –Exons All living things on Earth use the same genetic code

Mutations and DNA Repair Mutations –Change in DNA of parent –Causes Nuclear radiation X-rays UV light DNA Repair –10,000 ‘hits’ per day –Cells repair damage

Why Are Genes Expressed? Gene control –Turning genes on and off –Each cell contains same genes –Not all cells have same function –Certain genes activated Scientists currently studying how

Viruses Virus –Not alive –No metabolism –Cannot reproduce on own Structure –Short DNA or RNA –Protein coating How it works –Taken into cell –Takes over cell –Produces more copies –Kills cell

HIV Human Immunodeficiency Virus (HIV) –Contains RNA –Codes back to DNA –DNA incorporated into cell –Makes new viruses –Cell dies Complex –Two protein coats Outer coat fits T cell receptors Inner coat encloses RNA

Viral Epidemics Viruses –Cannot use medication –Use vaccination Viruses evolve rapidly –HIV –Influenza –SARS –Bird flu

DNA Fingerprinting

The polymerase chain reaction (PCR) copies a sequence of DNA. (a) A strand of DNA is mixed in solution with DNA precursors (nucleotides), a primer that targets a specific piece of DNA, and an enzyme (polymerase) that helps to assemble DNA. The mix is heated to 200°F to separate DNA strands.

(b) When cooled to 140°F, primers attach to the DNA strands. (c) At 160°F nucleotides begin to attach to the DNA strands. (d) At the end you have two copies of the desired DNA.

DNA fingerprinting requires breaking DNA into short fragments, tagging those fragments with radioactive tracers, and then mixing the fragments in a gel. In an electric field, smaller fragments move farther along the gel, and the distribution of fragments can be recorded on a photographic film (b). Because each person ’ s DNA sequence is unique, each DNA fingerprint is distinctive.

The steps in DNA fingerprinting