Session 4
Transcription - 1

9/23/99

(c) Copyright 1999 by William Sofer. All Rights Reserved.

Review

Let's review what has been covered to date.

• Polymers are the most important molecules in living cells.
• The polymers that are of prime interest to molecular geneticists come in two varieties: informational and operational.
• DNA is an informational polymer; proteins are operational polymers; and some RNA's can serve as informational molecules while others can be operational.
• Proteins are polymers composed of 20 amino acids. Each amino acid carries an amino and carboxylic acid group as well as a distinctive side chain.
• DNA and RNA are polymers composed of 4 nucleotides. Each nucleotide carries a phosphate group, a five carbon sugar (called deoxyribose or ribose in DNA and RNA respectively), and a distinctive cyclic compound called a base.
• The shape of a protein is determined by the sequence of its amino acids.
• DNA is the polymer that specifies the sequence of the amino acids in the proteins in an organism.
• In turn, evolution and natural selection determine the order of nucleotides in DNA. Over billions of years, Nature tries out different sequences of DNA. Those that work well; i.e, produce proteins that work well, are retained; those that don't are discarded.

How it all works

Last time, we discussed, in excrutiating detail, the structure of amino acids, proteins, nucleotides and nucleic acids. Now I plan on answering the following big question:

How exactly does the information that is stored in the sequence of an organism's DNA get translated into the sequence of the large number of proteins that are present in that organism?

But before beginning, I need to redefine the words "chromosome" and "gene". We're going to use these terms repeatedly throughout the course, and I want to make sure that we are all in step with what they mean.

Chromosomes

The amount of DNA in a mammalian cell is enormous (there are about 3 billion nucleotides worth of DNA in a human sperm cell, twice that amount in a typical non-sex cell). Stretched out to its fullest extent, the sperm cell's DNA would be about a meter (more than 3 feet) in length. Yet most cells in an organism such as a mammal are very small, so small in fact, that they can not be seen without the aid of a microscope. In the nuclei of each of these trillions of cells, are the set of instructions for life's processes. These instructions are carried on one or more molecules called chromosomes.

DNA is the major components of the chromosomes -- assemblages of DNA and protein that lie in the nucleus. The term chromosome means "colored body", alluding to the fact that early biologists were able to stain them specifically with certain dyes. At this point in the course, we're going to consider that chromosomes are simply large pieces of DNA (later on we'll see that this is a simplification). The other constituents of the chromosomes -- some proteins -- are loosely bound to the DNA, and, among other things, help the chromosome fold properly.

The DNA in most chromosomes (certainly the chromosomes of most of the organims that you are familiar with) is double-stranded. That means that each DNA molecule consists of two polymers -- two chains -- that are hydrogen bonded to each other. The chains run in opposite directions.

Multicellular organisms, like humans, may have trillions of cells, each of which contains a nucleus. Each nucleus, in turn, contains all the chromosomes (the 22 pairs of human chromosomes and the human X and Y are shown in the figure above. An enlarged copy of one of the chromosomes -- human chromosome number two -- is shown to the right). There seems to be no rhyme or reason why DNA is divided into a certain number of chromosomes, but it is. Some organisms such as fruit flies have only four. Others such as the king crab and some ferns have hundreds. In general, every species on earth has a chromosome number that is characteristic of that species.

Cells in multicellular organisms are different from one another. For example, cells in the skin are different from cells in the heart which in turn are different from cells in the lens of the eye. However, each of these cells has a nucleus carrying the entire set of chromosomes. Amazingly, although the cells are different, each carries the same chromosomes. Thus, since the chromosomes are carriers of the DNA, all of the trillions of cells in our bodies (and in the bodies of most multicellular organisms) have the same DNA (there are a few interesting exceptions to this statement).

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