NUCLEIC ACIDS

Structure and function

A brief history of nucleic acids

• Frederich Meischer (1844-1895)– Extracted pus from wounds and

bandages– Isolated a substance from white

blood cells which he named “nuclein” (because it was found in the nucleus of cells).

• Edmund Beecher Wilson (1856-1939)– Establishes that “mother’s” nucleus contain

the same number of chromosomes as “father’s nucleus”, and both are present in the offspring…

– Therefore half of the information is received from each parent!

• Oscar Hertwig (1849-1922)– Suggests nuclein is needed to inherit

characteristics from parents.

• Richard Altmann (1852-1900)– Determines nuclein is actually acidic and

changes the name to “nucelic acid”

• Phoebus Aaron Levene (1869-1940) – Suggested nucleic acids had

a highly repetitive sequence…(although he could not describethe repetition patterns)

• Albrecht Kossel (1853-1927)– Worked with yeast (why?)– Showed there were 2 types of nucleic acids

(and won a Nobel prize for this!)

So what are nucleic acids?

• There are 2 types:– DNA (deoxyribonucleic acid) – RNA (ribonucleic acid)

• Polymers (i.e made of many monomers joined

ogether)

More about nucleic acids• They are in charge of:

»Of genetic information

STORAGE

TRANSMISSION

USE

More on nucleic acids

• Composed of monomers called nucleotides

• Each nucleotide has:– A pentose (5 carbon) sugar – A phosphate group– A nitrogen-containing base

More on nucleic acids (remember there are 2 types: DNA and RNA)

Can be the same for DNA or RNA

DNA (deoxyribose)RNA (ribose)

DNA bases RNA bases

Thymine (T) Uracil (U)

Adenine (A) Adenine (A)

Cytosine (C) Cytosine (C)

Guanine (G) Guanine (G)

Each nucleotide must have:

1. Phosphate group

2. A single sugar

A single base

DNA’s nucleotides purines

pyrimidine

s

DNA is unique because of the nucleotide sequence

• Only 4 nitrogenous bases – thus only 4 nucleotides are found in DNA

• What does this mean?

1. DNA sequences are unique

• Nucleotides can join to each other in 2 ways:- As a sequence- By complementary base pairing (not e shown

below)

The sequence is called the “genetic code” and is UNIQUE to each individual.

A C C G T A T A G

How do nucleotides join in a sequence?• Nucleotides can join through

covalent bonds between their

sugar group AND

phosphate group

Forming the sugar-phosphate backbone

bases are not involved directly in this type of bonding.

2. Complementary base pairing

• RNA consists of a single strand, but DNA consists of a double strand.

• In a double strand, BASES bond to each other.

• This IS NOT AT RANDOM, but occurs through COMPLEMENTARY BASE PAIRING

• A purine will always bond a pyrimidine.

Complementary base-pairing

Always adenine –

thyminecytosine -

guanine

A

C G

T

Exercise: Which is the complementary strand?

A

C

G

T

C

C

T

T

A

A

G

G

G

C

• Hydrogen bonds attach nucleotide bases to each other, and determine the bases that CAN join (i.e complementary bases)

• C and G make 3 H bonds.

• A and T make 2 H bonds.

The hydrogen bonding is one of the causes leading a DNA molecule to twist (like a double helix)

• This was discovered by James Watson and Francis Crick (both of whom won a Nobel Prize for this).

The 2 strands run anti-parallel to each other

Questions:1. Physically, why can’t a guanine (G) in one

strand bond with an adenine (A) in another strand? Guanine and Adenine are

both purines. This means they are

slightly larger molecules than Thymine and Cytosine.

If they pair up, the distance between one strand and the other is larger than the average, and surrounding nucleotides could not bond.

1. Physically, why can’t a guanine (G) in one strand bond with an adenine (A) in another strand?

Guanine can make 3 hydrogen bonds, whereas Adenine can only make 2. This makes the bonding unstable as Guanine ends up with a “lose” end (non-bonded hydrogen)

Questions

2. Why does DNA “need” to have a “coiled” shape?

DNA carries ALL the information that makes up an organism.

It is present in EVERY nucleus of EVERY cell of the organism.

(If DNA was extended side by side, the DNA in our bodies would be long enough to go around the earth!)

By coiling DNA (in fact, SUPER-COILING it), DNA can actually fit into the nucleus of each cell.

DNA carries the hereditary information.

By COILING it, the “strong” part (sugar-phosphate backbone) is exposed, rather than the bases.

This confers some “protection” to the information

2. Why does DNA “need” to have a “coiled” shape?

Summary on DNA structure: DNA is a ______ helix. Each strand runs _____________ to the other. Alternating ______ and _________ molecules form a

backbone for each strand. The two strands backbone sugar and phosphate

molecules are held together by ___________ bonds. The two strands are held together by _________ bonds

between complementary base pairs. There are four nitrogenous bases: two _______, Adenine

(A) and Guanine (G) and two __________, Thymine (T) and Cytosine (C).

Adenine and ________ always bond through 2 hydrogen bonds.

Cytosine and _______ always bond through 3 hydrogen bonds.

Summary on DNA structure: DNA is a double helix. Each strand runs anti-parallel to the other. Alternating sugar and phosphate molecules form a

backbone for each strand. The two strands backbone sugar and phosphate

molecules are held together by covalent bonds. The two strands are held together by hydrogen bonds

between complementary base pairs. There are four nitrogenous bases: two purines, Adenine

(A) and Guanine (G) and two pyrimidines, Thymine (T) and Cytosine (C).

Adenine and Thymine always bond through 2 hydrogen bonds.

Cytosine and Guanine always bond through 3 hydrogen bonds.

So how does DNA

STORE

TRANSMIT

USEgenetic information?