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

Classical Genetics

• Gregor Mendel– Basic laws of inheritance– Classic pea plant experiments

•Purebred•Hybrid

Classical Genetics – cont.

• Results– Parent generation– First generation– Second generation

• Gene– Dominant– Recessive

Typical Mendel Genetic Cross

Pea Plants

Hybrid Cross

9:3:3:1 Distribution

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

Qualitative versus Quantitative Genetics

• Qualitative– Observational

• Quantitative– Predictive model

– Used to trace genetic disease

Qualitative Aspects

Science in the Making

• Mendel lost and found– 8 years and 28,000 experiments

• Karl Correns, Erich von Tschermak, and Hugo de Vries– Independently deduced Mendel’s laws

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); uracil (U) in RNA

Ribose and Deoxyribose

Nucleotide

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 and meiosis

• Process– DNA double helix splits– New bases bond to exposed bases– Result

•Two double-stranded DNA molecules, each identical to the original molecule

DNA Replication

Electron Micrograph of DNA

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)

Transcription of DNA

tRNA

• tRNA– Reads message– Structure

•Amino acid•3 bases

The Synthesis of Proteins

• Process– mRNA moves to ribosome– rRNA aligns mRNA and tRNA– tRNA matches codon on mRNA– Amino acid chain forms

•Basis for protein

Computer-Generated Model of tRNA

The Interaction of mRNA and tRNA

The Formation of a Protein

The Genetic Code

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

Protein Production from DNA

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

Viruses – cont.

• How it works– Taken into cell– Takes over cell– Produces more copies– Kills cell

Bacterial Virus

Herpes Virus

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

HIV

Viral Epidemics

• Viruses– Cannot use medication– Use vaccination

• Viruses evolve rapidly– HIV– Influenza – SARS– Bird flu

Role of Phagocytes

The Human Genome

• Human Genome Project– DNA sequencing– 3 billion bases

Wooly Mammoth

DNA, Genes, Chromosomes, and Genomes

Science in the Making

• Connecting genes and DNA– Thomas Hunt Morgan– Alfred Sturtevant

Technology

• New ways to sequence– J. Craig Venter

•Faster sequencing

Thinking More about Genetics

• The ethics of genes– What should we do with genetic information?