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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.

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Classical and Modern Genetics

Chapter 23

Great Idea:

All living things use the same genetic code to guide the chemical reactions in every cell.

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Chapter Outline

  • Classical Genetics

  • DNA and the Birth of Molecular Genetics

  • The Genetic Code

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Classical Genetics

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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.

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Chromosomes from the Indian muntjak

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

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Mendel crossed tall and dwarf pea plants: all offspring were tall.

Tall Dwarf




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Next, Mendel crossed some of these F1 plants among themselves. Of these offspring (the F2 generation), about 3/4 of the plants were tall and 1/4 were dwarf.

tall tall


F2 tall tall tall dwarf

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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 F1 plants looked as though they had inherited the trait of just one of their two parents, but in the F2 generation both traits always appeared -- and always in a 3 to 1 ratio.

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The trait which was expressed in the F1 generation was always about 3 times as numerous in the F2 generation as was the other one which was hidden in the F1's.

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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).

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Homozygous = same Heterzygous =different

Tall Dwarf Tall

TT tt Tt

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Mendel's original cross produced only tall offspring:

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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.

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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

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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.

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Classical Genetics

  • Mendel

    • Basic laws of inheritance

    • Classic pea plant experiments

      • Purebred

      • Hybrid

  • Results

    • First generation

    • Second generation

  • Gene

    • Dominant

    • Recessive

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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

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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.

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Genotype AO BO AB OO

Phenotype A B AB O

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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.

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Qualitative versus Quantitative Genetics

  • Qualitative

    • observational

  • Quantitative

    • Predictive model

    • Used to trace genetic disease

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DNA and the Birth of Molecular Genetics

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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)

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DNA Structure

  • Join nucleotides

    • Alternating phosphate and sugar

  • DNA

    • 2 strands of nucleotides

    • Joined by base pairs

  • Bonding pattern

    • Adenine:Thymine

    • Cytosine:Guanine

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DNA Structure

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RNA Structure

  • Differences

    • One string of nucleotides

    • Sugar is ribose

    • Thymine replaced by uracil

      • Uracil (U) bonds with adenine

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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

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The Genetic Code

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Transcription of DNA

  • Transcription

    • Information transport

    • Uses RNA

  • Process

    • Unzip DNA

    • RNA binds to exposed bases

    • RNA moves out of nucleus (mRNA)

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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

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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

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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

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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

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  • 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

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  • 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

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Viral Epidemics

  • Viruses

    • Cannot use medication

    • Use vaccination

  • Viruses evolve rapidly

    • HIV

    • Influenza

    • SARS

    • Bird flu

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DNA Fingerprinting

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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.

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(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.

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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.

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The steps in DNA fingerprinting

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