Biochemistry – Lecture Section II

DNA is a complex molecule (polymer) found in all living things.

The primary function of DNA is to store and transmit genetic information that tells cells which proteins to make and when to make them.

Polymers are complex molecules composed of repeating subunits. The repeating subunits in DNA and RNA are called nucleotides.

In 1953, James Watson and Francis Crick proposed a model for the DNA molecule that consisted of two nucleotide chains that wrap around each other to form a double spiral. This shape is called a double helix. The picture at the top of this page represents a double helix.

Each nucleotide is composed of:

A Sugar:

• Deoxyribose - a 5-carbon sugar in DNA
• Ribose - a 5-carbon sugar in RNA. This sugar has more oxygen than the sugar in DNA.

One of these four nitrogen bases:

• Purines - have a double ring of carbon and nitrogen atoms.
• Adenine
• Guanine
• Pyrimidines - have a single ring of carbon and nitrogen atoms.
• Thymine
• Thymine is replaced by uracil in RNA.
• Cytosine

A phosphate group - PO₄

Base-pairing rules: these rules describe the behavior of the bases.

1. Cytosine always bonds with guanine by forming three hydrogen bonds.
2. Adenine always bonds with thymine by forming two hydrogen bonds.

A pair of bases that always bond together is known as a complementary base pair.

Replication the process of duplicating the DNA molecule.

Chromosome: a coiled, double rod-shaped form of condensed DNA that forms during cell division.

DNA is normally found in long strands in the cell nucleus. The coiled structures we call chromosomes are found in cells only during cell division. DNA is duplicated prior to cell division. When the chromosomes form, one side of each chromosome consists of the original cell DNA while the other side consists of the duplicated DNA. One half of each chromosome goes to both cells during cell division. This insures that the new cell will have exactly the same genetic make-up as the original cell.

Every species has a distinct number of chromosomes. Humans have 46 chromosomes (23 pairs).

RiboNucleic Acid

RNA is responsible for the movement of genetic information from the DNA in the cell nucleus to the site of protein synthesis in the cytoplasm. RNA's sugar molecule is ribose instead of the deoxyribos sugar in DNA.

Types of RNA:

1. Messenger RNA - mRNA - a single uncoiled strand that transmits information from DNA to the ribosomes during protein synthesis.
2. Transfer RNA - tRNA - a single folded strand that bonds with a specific amino acid.
3. Ribosomal RNA - rRNA - a globular form that is the major constituent of the ribosomes.

Transcription - the process of forming a mRNA strand from a DNA strand.

• The DNA double strands are separated and a mRNA strand is formed by RNA bases pairing up with the exposed DNA sequence and each base linking to the next.
• The mRNA that is formed carries the genetic information that will be translated into a protein.
• The rate of transcription is about 40 nucleotides per second.

Protein synthesis is the formation of proteins using information coded on DNA and carried out by RNA.

Translation is the assembling of protein molecules from information encoded in mRNA.

The central points to note are:

• DNA duplicates itself in replication.
• DNA produces RNA in transcription.
• RNA produces proteins in translation.