DNA: Structure and Replication

Two Types of Nucleic Acids

Nucleic Acids: carry the genetic instructions for all life

Nucleic Acid

Stands for Type of Sugar

# of strands

Function

DNA Deoxyribonucleic acid

deoxyribose 2 “double

helix”

Stores genetic material

RNA Ribonucleic acid ribose 1 single Transfers genetic material

Slide # 2

Discovery of Nucleic Acid

1.1869: Friedrich Miescher discovered nuclein– Noticed that nuclein

had a lot of phosphate

2. He found nuclein in every type of cell he studied.

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Miescher’s lab

Griffith’s Experiment

1. 1928: Frederick Griffith

a. Showed that bacteria could be transformed

b. transformation: process in which bacteria can take up the genes of another bacteria and express those genes

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Avery’s ExperimentSlide # 5

Avery showed that DNA is the transforming agent!

Oswald Avery continued Griffith’s experiment!

Hershey and Chase Experiment1. 1952: Hershey & Chase used radioactive markers on viruses a. showed that virus only injects nucleic acid into bacteria b. bacteria take up nucleic acid and can express the new genes c. demonstrated that DNA is the genetic material of the cell

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Adenine Guanine Cytosine Thymine

Phosphate group

Deoxyribose

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Components of DNA

Nucleotide has 3 parts: 1. A phosphate group 2. A sugar (called deoxyribose) 3. Nitrogen base (4 different

bases)– Adenine “A”– Thymine “T”– Cytosine “C”– Guanine “G”

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DNA is made up small, repeating units (monomers) called nucleotides.

1.1940’s: Erwin Chargaff discovered that the % of base “A” was almost equal to the % of base “T” & the percent of base “C” was nearly equal to the % of base “G”– didn’t know why

2.Also discovered that composition of DNA (% of A, T, C, & G) varies from species to species

3.Both discoveries gave support that DNA carries the genetic code!

Chargaff: Discovered Ratios of Nitrogen Bases

Chargaff

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Percentage of Bases in Four Organisms

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Source of DNA A T G CSource of DNA A T G C

Streptococcus 29.8 31.6 20.5 18.0

Yeast 31.3 32.9 18.7 17.1

Herring 27.8 27.5 22.2 22.6

Human 30.9 29.4 19.9 19.8

Streptococcus 29.8 31.6 20.5 18.0

Yeast 31.3 32.9 18.7 17.1

Herring 27.8 27.5 22.2 22.6

Human 30.9 29.4 19.9 19.8

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Rosalind Franklin: The Famous Picture Taker

1.1952: Rosalind Franklin: an expert in x-ray crystallography.

2.She took the x-ray picture of DNA -- shows DNA is a double helix.

3.1958: Franklin died (37 years old)

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James Watson & Francis Crick: Puzzle Solvers

1.1953: Discovered the structure of DNA2.Described DNA as a double helix (twisted

ladder)• Sides of ladder are made up of sugar &

phosphate groups• Steps of ladder are made up of nitrogen

base pairs (A-T & C-G)-- Applied Chargaff’s rules

• Base pairs (steps) are held together by weak hydrogen bonds

• Sequence (order) of nitrogen bases determines the genetic instructions / “genetic code” of organism.

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James Watson

Francis Crick

DRILL #12Quote: Whatever you are, be a good one!

Abraham LincolnAgenda: Homework check

DNA structure cont.Cell Division notes

Warm-up:1. Which of the following best describes cellular respiration?

a. External breathingb. Breakdown of sugar to release energy in cellsc. Movement of water from outside the cell to inside the cell.d. Removal of water from a cell

2. Describe the structure and components of a DNA nucleotide.

Knowing the Structure Tells how DNA Replicates

1.DNA Replication: makes 2 identical DNA strands by copying the original model

2.Each new strand contains one old (parent) strand & one new (daughter) strand

3.DNA replication occurs during cell division – inside the nucleus

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Parent strand: gray

Daughter strand: red

Steps in DNA Replication1.DNA unwinds (DNA Helicase)

2.DNA polymerase breaks hydrogen bonds that hold nitrogen bases together

3.DNA ligase hydrogen bond new nitrogen bases onto DNA parent strand• Chargaff’s rules applied: A-T & C-G• Nucleotides added in 5’ to 3’ direction

4.Sugar-phosphate groups bond to nitrogen bases to complete daughter strand

5.Result: 2 identical DNA strands each contains 1 parent strand & 1 daughter strand

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Steps in DNA Replication

Parent strand

Daughter strand

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Strands are antiparallel

When Does Replication Occur?

1. The cell replicates (makes a copy) its DNA right before cell division.

2. When two new cells are produced in cell division, each new cell needs its own copy of the DNA (identical copy!)

3. After DNA replication, the cell will divide in half (cell division) and give each new cell a copy of the DNA

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Chromosomes1.Fertilized human egg cell has 46

chromosomes• 23 chromosomes in egg cell• 23 chromosomes in sperm cell• Humans have 46 chromosomes

in each body cell.

2. Each cell has millions of base pairs storing our genetic code.

3. After DNA Replication, each cell has 92 “sister chromatids”• After mitosis, each cell ends up

with 46 chromosomes

Karyotype of Human

chromosomes

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Chromosomes: More than Just DNA

1.Chromosome: coiled DNA & protein “spacers” called histones.

2.Histones keep the chromosome from getting tangled.

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