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Chromosomes and DNAChromosomes and DNA

• Our genes are on our chromosomes.

• Chromosomes are made up of a chemical called DNA.

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DNADNA• DNA DNA is “the

blueprint of life”

• DNA contains the instructions for making proteins within the cell.

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What does DNA look like?

Watson & Crick’s Model

• Double Helix

• Twisted Ladder

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Made of nucleotides:3 Components

• Sugar (deoxyribose)

• Phosphate Group

• Nitrogenous Base

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The two sides of the ladderS

S

S

P

P

S

S

S

P

P

The sugar and phosphate make the sides of the ladder

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The rungs are the bases

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There are 4 bases

•Adenine Thymine

•Guanine Cytosine

• A always pairs with T (sticks)• C always pairs with G (curves)

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A-T and C-G

A

T

C

T

A

G

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Complimentary Base Pairing

A

T

C

Hydrogen bonds hold bases together

T

A

G

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Why do we study DNADNA?

• medical reasons

• better food crops

• forensics

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DNA by the NumbersDNA by the Numbers• Each cell has about 2

m of DNA.• The average human

has 75 trillion cells.• The average human

has enough DNA to go from the earth to the sun more than 400 times.

• DNA has a diameter of only 0.000000002 m.

The earth is 150 billion mor 93 million miles from the sun.

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Chargaff’s RuleChargaff’s Rule

• AdenineAdenine must pair with ThymineThymine

• GuanineGuanine must pair with CytosineCytosine

• Their amounts in a given DNA molecule will be about the sameabout the same.

G CT A

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Question:Question:

• If there is 30% AdenineAdenine, how much CytosineCytosine is present?

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Question:Question:

• When and where does When and where does DNA DNA Replication Replication take place?take place?

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Synthesis Phase (S Synthesis Phase (S phase)phase)

• S phase in interphase of the cell cycle.

• Nucleus of eukaryotes

Mitosis-prophase-metaphase-anaphase-telophase

G1 G2

Sphase

interphase

DNA replication takesDNA replication takesplace in the S phase.place in the S phase.

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Replication

• DNA copies itself before division• Steps:

– Unwinds– Unzips– Copies using the old strands as templates

• Uses complementary base pairing

• Semiconservative: end up with one old strand and one new strand

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DNA Replication

Replication ForksReplication Forks: hundredshundreds of Y-Y-shapedshaped regions of replicating DNA replicating DNA moleculesmolecules where new strands are growing.

ReplicationReplicationForkFork

Parental DNA MoleculeParental DNA Molecule

3’

5’

3’

5’

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DNA Replication

Replication BubblesReplication Bubbles:

Bubbles Bubbles

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DNA ReplicationDNA Replication

• Semiconservative Model:Semiconservative Model:

Old strands serve as templates

New molecules have one old, one new strand

Parental DNA

DNA Template

New DNA

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Question:

• What would be the complementary DNA strand for the following DNA sequence?

DNA 5’-GCGTATG-DNA 5’-GCGTATG-3’3’

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Answer:Answer:

DNA 5’-GCGTATG-DNA 5’-GCGTATG-3’3’

DNA 3’-CGCATAC-DNA 3’-CGCATAC-5’5’

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You Try it!

• http://www.pbs.org/wgbh/aso/tryit/dna/shockwave-nojs.html