D) Nucleic Acids

Chromosomes found in the nucleus carry the hereditary material -- DNA

DNA (deoxyribonucleic acid)

-- DNA controls cellular activity by influencing the production of enzymes.

(Structure of DNA Molecules)

DNA is a very long chain polymer made up of thousands of repeating units called nucleotides.

Nucleotide Unit is composed of a phosphate group, a pentose sugar, and a nitrogenous base.

The Nitrogenous Bases are; adenine (A) thymine (T) guanine (G) cytosine (C)

Watson and Crick (early 1950's)

-- determined the structure of the DNA molecule

** Consists of two chains of nucleotide units in a twisted ladder-like structure. (resembles a spiral staircase)

This spiral staircase is called an alpha helix. (37 degree turn)

-- The sides of the ladder are made up of alternating deoxyribose sugar -- phosphate group units.

-- The rungs of the ladder are made of 2 nitrogenous bases per rung linked together by a weak hydrogen bond.

-- Only 2 combinations of base pairs can form the rungs of the DNA molecule.

Adenine - Thymine (A-T)

Guanine - Cytosine (C-G)

** This specific matching up of the nitrogenous bases is called complementary base pairing.

** DNA is able to replicate itself.

[How does DNA form duplicates of itself during mitosis and meiosis?]

1. The double stranded DNA molecule unwinds and

unzips between the weak hydrogen bonds between

the nitrogenous base pairs.

2. Free nucleotides present in the nucleus attach

themselves by forming new hydrogen bonds with

the exposed bases in the single chain. The only

base which can reattach is the same type of base that was originally joined to it.

A view of DNA replication

RNA (ribonucleic acid)

** RNA is also a polymer formed by a sequence of nucleotides.

(How does RNA differ in structure from DNA?)

1. The RNA molecule is a SINGLE nucleotide

strand, not a double strand as in DNA.

2. The sugar molecule in RNA is RIBOSE --

not deoxyribose as in DNA.

3. The base URACIL (U) takes the place of thymine. (T