DNA YUSRON SUGIARTO, STP, MP, MSc

STRUCTURE OF DNA

DNA REPLICATION

4

DNA is wrapped tightly around histones and coiled tightly to form chromosomes

DNA

• DNA is often called the blueprint of life.

• In simple terms, DNA contains the instructions for making proteins within the cell.

DNA 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 m or 93 million miles from the sun.

Why do we study DNA? We study DNA for many

reasons, e.g.,

• its central importance to all life on Earth,

• medical benefits such as cures for diseases,

• better food crops.

Chromosomes and DNA

• Our genes are on our chromosomes.

• Chromosomes are made up of a chemical called DNA.

DNA Structure DNA consists of two molecules that are arranged

into a ladder-like structure called a Double Helix.

A molecule of DNA is made up of millions of tiny subunits called Nucleotides.

Each nucleotide consists of: 1. Phosphate group 2. Pentose sugar 3. Nitrogenous base

One Strand of DNA • The backbone of the

molecule is alternating phosphate and deoxyribose, a sugar, parts.

• The teeth are nitrogenous bases.

phosphate

deoxyribose

bases

Nucleotides Phosphate

Pentose Sugar

Nitrogenous Base

Nucleotides The phosphate and sugar form the backbone

of the DNA molecule, whereas the bases form the “rungs”.

There are four types of nitrogenous bases.

Nucleotides

A

Adenine

T

Thymine

G

Guanine

C

Cytosine

Nucleotides Each base will only bond with one other specific

base.

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)

Form a base pair.

Form a base pair.

Four nitrogenous bases

• Cytosine C

• Thymine T

• Adenine A

• Guanine G

DNA has four different bases:

Two Kinds of Bases in DNA

• Pyrimidines are single ring bases.

• Purines are double

ring bases.

C

C

C

C

N

N

O

N

C

C

C

C

N

N

N

N

N

C

Thymine and Cytosine are pyrimidines

• Thymine and cytosine each have one ring of carbon and nitrogen atoms.

C

C

C

C

N

N

O

N

cytosine

C

C

C

C

N

N

O

O

thymine

C

Adenine and Guanine are purines

• Adenine and guanine each have two rings of carbon and nitrogen atoms.

C

C

C

C

N

N

N

Adenine N

N

C

C

C

C

C

N

N

O

N

Guanine N

N

C

Two Stranded DNA • Remember, DNA has

two strands that fit together something like a zipper.

• The teeth are the nitrogenous bases but why do they stick together?

C

C

C

C

N

N

O

N

C

C C

C

N

N

O

N

N

N

C

Hydrogen Bonds • The bases attract each other

because of hydrogen bonds. • Hydrogen bonds are weak

but there are millions and millions of them in a single molecule of DNA.

• (The bonds between cytosine and guanine are shown here.)

Hydrogen Bonds, cont. • When making

hydrogen bonds, cytosine always pairs up with guanine,

• And adenine always pairs up with thymine.

• (Adenine and thymine are shown here.)

C

C

C

C

N

N

O

O

C

Important:

• Adenine and Thymine always join together

• A T

• Cytosine and Guanine always join together

• C G

DNA Structure

Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand.

G

G

A

T

T

A

A

C

T

G

C

A

T

C

DNA Structure

To crack the genetic code found in DNA we need to look at the sequence of bases.

The bases are arranged in triplets called codons.

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

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

DNA Structure A gene is a section of DNA that codes for a protein.

Each unique gene has a unique sequence of bases.

This unique sequence of bases will code for the

production of a unique protein.

It is these proteins and combination of proteins that give us a unique phenotype.

STRUCTURE OF DNA

DNA REPLICATION

DNA

DNA Replication • DNA must be copied

• The DNA molecule produces 2

IDENTICAL new complementary strands following the rules of base pairing:

A-T, G-C

•Each strand of the original DNA serves as a template for the new strand

DNA Replication • Complementary base pairs form new strands.

DNA Replication

DNA Replication Models

DNA Replication

• Semiconservative Model:

Watson and Crick showed: the two strands of the separate, and each functions as a for synthesis of a new complementary strand. Parental DNA

DNA Template

New DNA

Leading strand

lagging strand

Replication fork

Replication fork

DNA

1. Why is replication necessary?

2. When does replication occur? 3. Describe how replication works. 4. Use the complementary rule to create the complementary

strand:

A---? G---? C---? T---? A---? G---? A---? G---? C---? A---? G---? T---?

Replication Quiz

1. Why is replication necessary? So both new cells will have the correct

DNA 2. When does replication occur? During interphase (S phase). 3. Describe how replication works. Enzymes unzip DNA and complementary

nucleotides join each original strand. 4. Use the complementary rule to create the complementary strand:

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

Replication Quiz

Satu tim besar yang terdiri dari enzim dan protein lain menjadi pelaksana replikasi DNA

Protein-protein yang berperan dalam replikasi DNA

2. Protein pengikat untai tunggal: menjaga agar untai-untai tetap terpisah selama bertindak sebagai cetakan dalam sintesis untai-untai komplementer yang baru.

1. Helikase: enzim yang berfungsi membuka heliks ganda di cabang replikasi, memisahkan untai lama.

3. Primase: membentuk primer

Satu tim besar yang terdiri dari enzim dan protein lain menjadi pelaksana replikasi DNA

Protein-protein yang berperan dalam replikasi DNA

4. DNA polimerase: pemanjangan untai DNA baru

5. Ligase: menggabungkan rantai DNA

Enzymology of DNA replication DNA strand separation

Helicases: unwind double strand DNA Single-strand DNA binding proteins (SSBs): participate in DNA strand separation but do not catalyze the strand separation process. They bind to single strand DNA as soon as it forms and coat it so that it cannot anneal to reform a double helix. Topoisomerases: introduce transient single or double stranded breaks into DNA and thereby allow it to change its form, or topology.

DNA polymerases Three DNA polymerases (I, II, III) DNA polymerase I (102 KD): has three different enzymatic

activities 1) DNA polymerase 2) 3’-5’ exonuclease activity 3) 5’-3’ exonulcease activity

(proof reading to increase fidelity)

3’-5’ exo

(remove RNA primers or damage DNA on its path)

Pol III holoenzyme 10 subunits Move at an extremely rapid rate (1000 nts/second)

Proof reading

Eukaryotes have multiple DNA polymerases

THANK YOU YUSRON SUGIARTO, STP, MP, MSc