Chapter 4Section 1: What does

DNA Look Like?

DNA• DNA stands for: Deoxyribonucleic

acid.• DNA stores and passes on genetic

information from one generation to the next.

Discovering DNA• In the 1950’s Rosalind Franklin &

Maurice Wilkins took x-ray pictures of DNA, which suggested that it had a spiral shape.

• In 1953, James Watson and Francis Crick used the x-ray pictures to determine that the shape of DNA is a double helix.

Discovering DNA

Examples: Double Helix

A staircase in the shape of a double

helix, in the Vatican Museum

Calling DNA a double helix is

the same as saying it’s a

twisted ladder.

The sides of the “ladder” are made up of millions of alternating sugars & phosphatesphosphates.

Structure of DNA

• The sugar is named deoxyribose.

• The sides of the ladder are held together by rungs (steps) attached to the sugars.

• The rungs (steps)of the DNA ladder are made up of two bases that contain nitrogen, called nitrogenous bases.

• The 2 bases of each rung are held together by a hydrogen bond.

• There are 4 bases to choose from:

Adenine Thymine

Guanine Cytosine

Structure of DNA

Erwin Chargraff discovered that:

He concluded that a rung can have: A and T or it can have C & G •This is called complementary base pairing.

Erwin Chargraff

• The amount of adenine (A) is always equal the amount of thymine (T)

• The amount of cytosine (C) is always equal the amount of guanine (G).

• States that only these bases pair together:• A always pairs with T• T always pairs with A• G always pairs with C• C always pairs with G.

If you know one base pair of the DNA, you will be able to figure out the other!

Complementary Base Pairing

Try it: Name the nitrogenous base that’s

missing from each

spot.

1

2

3

4

5

6

7

8

NucleotideNucleotide is a subunit (small piece) of DNA which contains:

• 1 base• 1 sugar• 1 phosphate

Lots of nucleotides connect to form a large DNA molecule.

• Human genome: has 3 billion base pairs

• Largest genome: South American marbled lungfish has 133 billion base pairs.

• Smallest genome: Symbiotic insect bacteria, Carsonella ruddii has159,662 base pairs.

Length of DNA

South American marbled lungfish Symbiotic insect bacteria, Carsonella ruddii

DNA ReplicationDNA Replication - process in which DNA makes an exact copy of itself.

When does DNA replication occur?•During interphase before mitosis begins.

DNA Replication is semi-conservative.

half

saved

Semi-conservative - each new DNA is made of ½ new and ½ old nucleotides. The old nucleotides were from the original strand (saved).

DNA Replication

1. DNA unzips at the hydrogen bonds.

2. New complementary nucleotides move in to match both halves of the DNA ladder.

3. They form hydrogen bonds with the old nucleotides.

4. Two identical DNA molecules are formed!

Steps of DNA Replication

Identical base sequences

DNA Replication

Why does DNA replication occur?•So when a cell splits, each cell has identical DNA in each cell.

How is it possible that new DNA and original DNA are identical?•The bases are complementary so they always pair with the same exact base.

DNA Replication

DNA replication animation

Recap Questions1.) What does DNA stand for?

Deoxyribonucleic acid.

2.) In the 1950’s these two scientists took x-ray pictures of DNA, which suggested that it had a spiral shape.

Rosalind Franklin & Maurice Wilkins

3.) In 1953, these two scientists used the x-ray pictures to determine that the shape of DNA is a double helix.

James Watson & Francis Crick

Recap Questions4.) What are the sides of a DNA molecule made up of?

Alternating sugars & phosphates

5.) What is the name for the sugar on a DNA molecule?

Deoxyribose

6.) What are the steps attached to on a DNA molecule?

Sugar

Recap Questions7.) The rungs (steps)of the DNA ladder are called what?

Nitrogenous bases

8.) How many nitrogenous bases attach together on each step (rung) of a DNA molecule?

Two

9.) What are the two bases of DNA held together by?

Hydrogen Bond

Recap Questions10.) What bases always pair together?Adenine/Thymine & Guanine/Cytosine

11.) What scientists discovered that the amount (A) = the amount of (T) and the amount of (C) = the amount of (G).

Erwin Chargraff

12.) Determine the complementary base pairs for the sequence: ATG CCT AGC

TAC GGA TCG

Recap Questions13.) A subunit of DNA that contains 1 base, 1 sugar and 1 phosphate is called?

A nucleotide14.) What do lots of nucleotides together form? DNA molecule

15.) What is the process in which DNA makes an exact copy of itself?

DNA replication

16.) When does DNA replication take place?Interphase (before mitosis)

Recap Questions17.) What is it called when each new DNA is

made of ½ new and ½ old nucleotides(which were from the original strand)?

Semi-Conservative

18.) Why does DNA replication occur?So when a cell splits, each cell has

identical DNA in each cell.

19.) How is it possible that new DNA and original DNA are identical?The bases are complementary so they always

pair with the same exact base.

Recap Questions20.) Put the steps of DNA replication in order:

A.) New complementary nucleotides move in to match both halves of the DNA ladder.

B.) Two identical DNA molecules are formed!

C.) They form hydrogen bonds with the old nucleotides.

D.) DNA unzips at the hydrogen bonds.

D, A, C, B

Chapter 4Section 2: How DNA

works

DNA• DNA is like the blueprint for a house.

• DNA contains instructions that determines the traits an organism inherits and needs to live.

No, not that GENE.

DNA is made up of

smaller segments

called genes.

No, not those jeans!

DNA molecule

How genes work•A gene consists of a string of nucleotides that give the cell instructions about how to make a specific trait.

•Genes do not do the work themselves, they carry the code for making specific proteins.

What are Proteins?•Found throughout cells

•Control cellular functions

•Determine physical traits such as:•Skin Color•Eye color•How tall you grow•What colors you can see•Whether your hair is curly or straight

DNA, Genes, & Proteins• DNA has instructions how to make all traits

for an organism.

• Gene carries instructions for how to make one specific trait.

• DNA and genes NEVER leave the nucleus so a copy of the gene is needed to make the protein.

• Proteins are made from a copy of one specific gene. Proteins do all the work! Yet, DNA gets all the credit!

How genes work•When a particular protein is needed, that part of the DNA (the gene) is copied.

•The copy of the gene is called mRNA or messenger ribonucleic acid.

How genes work• When a particular protein is needed it goes

through a process called protein synthesis.

•Two parts of protein synthesis:1. Transcription – gene is

copied in nucleus (because DNA never leaves the nucleus)

2. Translation – protein is made at the ribosome

• Transcription is like taking the blueprints and copying a single section about one specific part of the house.

Ex. Copying the blueprint only for the kitchen.

Transcription

5 Steps of Transcription

1.) The cell gets a message that a protein is needed.

2.) DNA unzips at hydrogen bonds exposing only one gene.

“Help, a protein is needed!”

ONLY one gene being

exposed

Steps of Transcription3.) Complementary nucleotides move into one side of the gene forming mRNA.•mRNA (messenger ribonucleic acid) – the copy of the gene.

4.) mRNA leaves the nucleus to go into the cytoplasm.

5.) DNA rezips at the hydrogen bonds.

A U G G G C U U A A A G C A G

1.) mRNA goes through the cytoplasm to meet the ribosome.Steps of Translation

• Codons – 3 bases in a row on an mRNA molecule.

ribosome

A U G G G C U U A A A G C A G U G C A C G U U A

2.) mRNA goes through the ribosome in groups of 3 bases at a time called codons

A U G G G C U U A A A G C A G U G C A C G U U A

ribosome

codon codon codon codon codon codon codon codon

mRNA

Steps of Translation

3.) The arrangement of bases in a codon creates a specific amino acid. •Amino acid - The combination of the 3 letters on a codon. ( ex. AGG, CAG, AGC)

• Example: AGG brings a different amino acid than CAG, and AGC, etc.)

A U G G G C U U A A A G C A G U G C A C G U U A

codon codon codon codon codon codon codon codon

Amino acid that the codon codes for

Steps of Translation

A U G G G C U U A A A G C A G U G C A C G U U A

4.) Amino Acids get connected by a peptide bond.

A U G G G C U U A A A G C A G U G C A C G U U A

Peptide bond

Amino acid Amino acid

• Peptide Bond = bond that connects 2 amino acids

5.) Many amino acids connected together makes a protein.

Steps of Translation

protein

Protein

Differences between mRNA & DNADNA mRNA

•Longer •shorter•Thousands/millions of genes

•1 gene

•Double stranded (helix) •Single stranded (helix)•Bases - ATGC •Bases – AGCU (U = Uracil)•Stays in the nucleus •Goes from nucleus to

cytoplasm and ribosome•Produced in DNA Replication

•Produced in Transcription

•Deoxyribose Sugar •Ribose Sugar

Mutation- changes or mistakes that take place in the DNA or mRNA.•Happens during DNA replication or transcription.•Can occur randomly or caused by mutagens such as:

– High energy radiation (from x-rays or sunlight)– Asbestos– Chemicals in cigarette smoke

• If mutations occur in sex cells, they can be passed on to offspring.

• Cells make proteins that can detect errors, so they are usually fixed.

Mutations

• Mutations can be good, neutral, or bad.

There are 3 types of mutation:

1. Deletion - one pair of bases is removed.

2. Insertion- one pair of bases is added.

3. Substitution - one pair of bases is replaced with another pair

Mutations

Deletion• When one pair of bases is removed.

Insertion• When one pair of bases is added.

SubstitutionSubstitution• When one pair of bases is replaced with

another pair of bases.

Examples using the following sequence

“THE DOG BIT THE CAT”

•Deletion: “THE DOB ITT HEC AT”

•Substitution: “THE DOT BIT THE CAT”

•Insertions: “THE DAO GBI TTH ECA T”