Human Genome

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AS and A Level Science

Introduction

The Human Genome Project is aimed to provide complete nucleotide sequence and gene map of the genome. The feasibility of the project was discussed as early as 1985, the initial programme began in 1990 and, by June 2000, 90% of the genome had been mapped. The process was completed and the first draft published early in 2001.Using the sequence biologists are gaining a better understanding of the genome itself, identifying new genes, working out how they are controlled, and discovering what products they code for. The sequence also helps in research into the role of genes in disease and the development of new diagnostic techniques and treatments. 

How the Human Genome was sequenced

The average human gene is now known to consist of about 3000 bases, but sizes vary greatly. Since the size of human genome is enormous, so to overcome this problem,

  1. The genome is digested and the DNA cut into segments with a range of different restriction enzymes (Figure 1.1). Each restriction enzyme cuts the DNA at slightly different points and the genome is broken up in such a way that there is a degree of overlap between adjacent DNA segments.
  2. Insertion of DNA fragments into living cells such as (bacteria and yeast), stores the DNA fragments and enables more copies as the cells reproduce.
  3. This procedure allows further fragmentation of DNA into smaller segments (Figure 1.2).
  4. Once this is achieved, the sequence of the DNA can be determined (Figure 1.3) resulting in a complete ordered sequence (Figure 1.4) of the three billion nucleotides which comprise the human genome.

However, the necessity of sequencing more than three billion nucleotides, has resulted in further refinement of sequencing methods to  speed up the entire process and, therefore, to reduce the costs of the project as a whole. Further DNA chain extension is terminated by inserting specialised DNA nucleotides into DNA segments (Figure 3)

  1. Single stranded DNA recovered from viruses is divided into four separate pools.
  2. Materials are added to the pool to enable a complimentary strand of DNA to be formed (Figure 3.1).                                                        
  3. DNA chain extension is ceased by inserting a chain terminating nucleotide into DNA strands. As a result, each pool gets a terminally labelled DNA strand of different lengths with a fluorescent base of unique colour when exposed to laser.
  4. The lengths of DNA from each pool are separated (Figure 3.2) by combining the pools and loading the combined material on to capillary columns which are able to determine the size of separate DNA strands and subsequently pass the separated strands through a laser-photomultiplier detector.
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  1. Capillary separation accurately sorts the DNA strands as each terminating base manifests a different colour so as to produce a cumulative strand and hence a small section of continuous nucleotide is revealed (Figure 3.3). Computer software then matches the sequence overlaps to establish a contiguous DNA sequence.

Limitations and Ethical issues

The determination of the entire DNA sequence contained in the human genome will not enable genetic specialists to look at a person's DNA sequence and predict everything about their appearance, behaviour ...

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Human Genome
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