Structure & Function of DNA

By C. Kohn, Waterford WI

What are genes?

You now know that genes encode for specific traits like eye color, ear lobes, and milk production.

A gene is simply a section of DNA that creates the proteins responsible for a specific trait.

Genes are found in DNA; chromosomes are made of DNA

Structure of DNA

DNA has several key components A Phosphate Molecule A Sugar Molecule A Nitrogenous Base (A,T,G,C)

Classification

The sugar and phosphate molecules comprise the ‘skeleton’ or ‘backbone’ of DNA

The nitrogenous base is used to encode the actual information on the gene needed to create the protein (a base is the C,G, T, or A)

Nucleotides

A nucleotide is a subunit (or building block) of DNA consisting of a base, a phosphate, and a ribose sugar.

Nucleotide

Bases

DNA has four nitrogenous bases Adenine (A)

Thymine (T)

Guanine (G)

Cytosine (C)

All information encoded in DNA exists through different combinationsof these four letters.

Bases (cont)

The DNA bases always exist in the same kinds of combinations A always pairs with T G always pairs with C “Great Combinations, Always Together”

Bases (cont)

A-T and G-C combos must occur for two main reasons 1. This is the only way they will fit inside

the framework of the DNA molecule 2. This is the only way that their binding

sites will match up

Pyrimidines vs. Purines

The bases are grouped into two categories Pyrimidines

Purines

Pyrimidines vs. Purines

Two pryrimidines would be too small to fit inside the structure of DNA

Two purines would be too big to fit inside the structure of DNA

Too small

Too big

Base Bonding

C-G and T-A are also necessary because of binding sites T and A have 2 binding sites C and G have 3 binding sites▪ They wouldn’t match up any other way

Specific Combos

Because of size, G and A would be too big together, and C and T would be too small together

Because of binding sites, G only matches up with C and T only matches with A

Why does this matter?

Knowing these facts are HUGE! This feature enables the structure of

DNA to enable its function In other words, because of

the G-C, T-A combination, DNA can be read and replicated.

Review

DNA has 3 main components A phosphate molecule A ribose sugar A Base (C,T,G,or A)

A phosphate, sugar, and base together is called a nucleotide, the building block of DNA

C-G and A-T are only possible because… This is the only way they fit inside DNA This is the only way their bonding sites match

up

To make a protein…

To make a protein, we have to make a copy strand of DNA and send it to a ribosome The copy strand is called mRNA

RNA vs. DNA

While our genetic information is encoded in double-stranded DNA, copies of this information are encoded in single-stranded RNA.

RNA is a primitive version of DNA. DNA and RNA are very similar; the key

differences are that… 1. RNA can be single stranded 2. RNA replaces a T with a U (uracil) 3. Also, the sugar is slightly different (extra -

OH molecule)

Transcription

The process of creating an mRNA copy of DNA is called Transcription. Think of “transcript” of a TV program – it’s just a copy

Transcription has three stages: 1. Initiation – DNA is unwound by helicase enzyme and a

polymerase enzyme binds to the DNA strand 2. Elongation – nucleotides are added by polymerase to the

developing mRNA strand 3. Termination – polymerase and mRNA are released from

the DNA strand; the strand is re-closed

Transcription involves two key enzymes: Helicase: the enzyme the opens the DNA strand Polymerase: the enzyme that creates the mRNA copy

Transcription Animation

C TG A C TG A C TG A

G C T A G C T A G C T A

C UG A C G A C G AU U

Step 1: Helicase opens and unwinds the DNA strandStep 2: Polymerase adds a complementary base for each nucleotideStep 3: The newly created mRNA strand goes to a ribosome to be readStep 4: The DNA strand is closed and re-wound

Key Note: 5 3

Transcription always occurs in a 5 > 3 direction.

The sugar molecule has 5 carbon atoms The 5th carbon atom is

‘inside’ the nucleotide, while the 3rd carbon atom is at the ‘lower’ edge▪ NOTE – there is no top or

down in DNA, so use these terms carefully!

Just remember: 5 > 3

5

3

Translation

Once an mRNA copy has been made, the next step is Translation.

Translation is when the information in the mRNA is ‘translated’ into the creation of a protein by a ribosome, or rRNA.

How Translation Works

The mRNA copy strand’s base letters are read in groups of three

E.g. if our mRNA strand was AUGGCAAAGGACCAU

it would be read as AUG GCA AAG GAC CAU

Each group of three is called a codon. i.e. AUG is a codon; GCA is a codon; etc.

1 Codon = 1 Amino Acid

Each codon codes for a specific amino acid. An amino acid is the building block of a

protein For example, GGG codes for Glycine

AUA codes for Serine CUA codes for Leucine

Each codon will specific which amino acid is added next in order to create a protein

Translation Animation

C UG A C G A C G AU U

Arginine

Serine

IsoleucineAsparagine

Arg

Ser

Iso

Asp

Protein

tRNA

Amino acids are determined by the strand of mRNA and brought to the ribosome by tRNA tRNA will only bind to a complementary

codon; e.g. ACG will bind the UGC–form of tRNA.

Amino Acids Proteins

A protein is a long string of amino acids.

The type of amino acids in a protein, and their order, determine the function of the protein

For example, insulin is shown here at the right

As you can see, it is simply a long chain of amino acids

The order and type of amino

acids is the primary

structure.

The arrangement of amino acids will create either a

helix spring or a pleated sheet.

The combination of springs and sheets is the

tertiary structure of a protein.

The final functional protein is

the quarternary structure.

Transcription occurs in the

nucleus. Translation

occurs in the ribosomes.

DNA and mRNA are a part of

transcription. mRNA, rRNA,

and tRNA are a part of

translation.

Transcription involves making the mRNA copy of

DNA.

Translation involves using

the mRNA copy to make a functional

protein out of amino acids in the ribosome.

DNA –> RNA –> Protein -> Traits