Chapter 5 - Summary

Information is contained in the DNA and converted into proteins

Cells arise by division. They then need to be developed and maintained. The work is carried out by proteins. Proteins are synthesised on particles called ribosomes from a blueprint contained in mRNA. mRNA itself is copied from portions of the huge molecules of DNA in the nucleus of the cell. The DNA, in effect, stores all the information for making the proteins that the cell requires. Some people think of it as being like a library.

There are two sections to this chapter:

1)Cells arise by division (MITOSIS)

2) DNA transcribed to RNA, modified and exported from nucleus

Cells arise by division (MITOSIS)

All cells arise by division. A cell swells, its crucial DNA information-store is replicated and then the cell divides, driven by movements of the fibrous cytoskeleton. Each of the daughter cells therefore receives, more-or-less, half of everything a mature cell contains, plus a full set of instructions on how to build, maintain and operate a cell.

Translating the RNA / DNA code into protein

When the messenger RNA reaches the cytoplasm of the cell, the sequence of the 4 nucleotides is used to assemble a sequence of a different kind of molecule, the amino acid. Chains of amino acids are called proteins and are the most useful molecules in the cell - it is in this way that the information in the DNA comes to have its effect.

There are, however, 20 different amino acids, so these are represented by groups of 3 (of each of the 4 possible) nucleotides, called codons.

Amino acids and nucleotide codons do not naturally stick together: in order to match the nucleotide triplets with amino acids the cell needs a key. It turns out that nucleotides have a special property: A's are attracted to T's and G's are attracted to C's. So the cell uses for the key small loops of RNA called tRNA (transfer RNA; about 70-80 base pairs long). tRNA molecules are floating free in the cell and will bind both to specific nucleotide triplets (codons) AND to specific amino acids. There are different tRNA loops to pair every codon and amino acid. The process, called translation, is shown in the following diagram.

Chapter 6 - Summary

The Cell's energy: where does it come from?

Cells are 80% water. Apart from the big molecules we've looked at, they contain a number of small molecules and ions. There is one more class of big molecules, however: the carbohydrates.

Carbohydrates are mostly used for the storage of energy. Mitochondria are the organelles where that energy is converted into a useable form. In plants, there are also organelles called chloroplasts that convert light energy (e.g. from the sun), carbon dioxide (a gas in the air) and water (from the ground) into carbohydrate. It is therefore plants that are the ultimate source of energy for all living things on Earth.

A cell keeps are reserve of molecules called ATP in case it requires energy. When ATP is broken down to ADP plus P there is a significant release of energy.

ATP ---> ADP + P + ENERGY (-30.5 kJmol-1)

Since this is such a central and important reaction, Chemists might be curious to see what ATP actually LOOKS like.

Chapter 7 - Summary

The DNA "library" is responsible for inheritance - the fact that children are like their parents. With every generation some copying errors creep into the DNA, errors we call mutations. This means that, in a population of single-celled organisms, not every organism is the same. One consequence of this is that some varieties will survive better than others. The better surviors prosper and multiply, the less-good survivors get left behind and may even die out. So the population, as a whole, takes on the characteristics of the successful variants, becomes more and more adapted to the environment in which it lives. We call this process evolution.

Evolution can explain not only a cells adaptation to its environement but also how the first cells arose in the chemical soup that existed on earth when the planet was newly formed.

Heredity in single-celled creatures.

Remember we saw that cells arise by division? That the DNA in a cell is first duplicated - a process called replication - so that the cell has twice as many chromosomes as before, then that those chromosomes separate and the cell divides. Well, if cells divided perfectly - the DNA replicated exactly by each copying process - then every related cell should be genetically identical. They're not, for about 1 base ( A, G, C or T ) in every 1,000,000 that is copied mutates. This means that every time a strand of DNA is replicated about one in a million bases is not copied accurately.



Chapter 1-3

Chapter 4

Chapter 5-7

Chapter 8

Chapter 9-10 information on DNA Genealogy.