12 Biology

MOLECULAR BASIS OF INHERITANCE

HUMAN GENOME PROJECT

Goals of HGP

Some of the important goals of HGP were as follows:

  • Identify all the approximately 20,000-25,000 genes in human DNA;
  • Determine the sequences of the 3 billion chemical base pairs that make up human DNA;
  • Store this information in databases;
  • Improve tools for data analysis;
  • Transfer related technologies to other sectors, such as industries;
  • Address the ethical, legal, and social issues (ELSI) that may arise from the project.




Methodologies: The methods involved two major approaches. One approach focused on identifying all the genes that expressed as RNA (referred to as Expressed Sequence Tags (ESTs). The other took the blind approach of simply sequencing the whole set of genome that contained all the coding and non-coding sequence, and later assigning different regions in the sequence with functions (a term referred to as Sequence Annotation).

For sequencing, the total DNA from a cell is isolated and converted into random fragments of relatively smaller sizes and cloned in suitable host using specialised vectors. The cloning resulted into amplification of each piece of DNA fragment so that it subsequently could be sequenced with ease. The commonly used hosts were bacteria and yeast, and the vectors were called as BAC (bacterial artificial chromosomes), and YAC (yeast artificial chromosomes).

The fragments were sequenced using automated DNA sequencers that worked on the principle of a method developed by Frederick Sanger. These sequences were then arranged based on some overlapping regions present in them.

These sequences were subsequently annotated and were assigned to each chromosome. The sequence of chromosome 1 was completed only in May 2006 (this was the last of the 24 human chromosomes – 22 autosomes and X and Y – to be sequenced).

Another challenging task was assigning the genetic and physical maps on the genome. This was generated using information on polymorphism of restriction endonuclease recognition sites, and some repetitive DNA sequences known as microsatellites.



Salient Features of Human Genome

  • The human genome contains 3164.7 million nucleotide bases.
  • The average gene consists of 3000 bases, but sizes vary greatly, with the largest known human gene being dystrophin at 2.4 million bases.
  • The total number of genes is estimated at 30,000–much lower than previous estimates of 80,000 to 1,40,000 genes. Almost all (99.9 per cent) nucleotide bases are exactly the same in all people.
  • The functions are unknown for over 50 per cent of discovered genes.
  • Less than 2 per cent of the genome codes for proteins.
  • Repeated sequences make up very large portion of the human genome.
  • Repetitive sequences are stretches of DNA sequences that are repeated many times, sometimes hundred to thousand times. They are thought to have no direct coding functions, but they shed light on chromosome structure, dynamics and evolution.
  • Chromosome 1 has most genes (2968), and the Y has the fewest (231).
  • Scientists have identified about 1.4 million locations where single-base DNA differences (SNPs – single nucleotide polymorphism, pronounced as ‘snips’) occur in humans. This information promises to revolutionise the processes of finding chromosomal locations for disease-associated sequences and tracing human history.



DNA FINGERPRINTING

99.9 per cent of base sequence among humans is the same. Differences lie in rest of the sequence. It is these differences in sequence of DNA which make every individual unique in his/her phenotypic appearance.

DNA fingerprinting involves identifying differences in some specific regions in DNA sequence called as repetitive DNA, because in these sequences, a small stretch of DNA is repeated many times. These repetitive DNA are separated from bulk genomic DNA as different peaks during density gradient centrifugation.

DNA fingerprinting involves identifying differences in some specific regions in DNA sequence called as repetitive DNA, because in these sequences, a small stretch of DNA is repeated many times. These repetitive DNA are separated from bulk genomic DNA as different peaks during density gradient centrifugation.

These sequences normally do not code for any proteins, but they form a large portion of human genome. These sequence show high degree of polymorphism and form the basis of DNA fingerprinting. Since DNA from every tissue (such as blood, hair-follicle, skin, bone, saliva, sperm etc.), from an individual show the same degree ofpolymorphism, they become very useful identification tool in forensic applications. Further, as the polymorphisms are inheritable from parents to children, DNA fingerprinting is the basis of paternity testing, in case of disputes.

As polymorphism in DNA sequence is the basis of genetic mapping of human genome as well as of DNA fingerprinting, it is essential that we understand what DNA polymorphism means in simple terms. Polymorphism (variation at genetic level) arises due to mutations.

New mutations may arise in an individual either in somatic cells or in the germ cells. If a germ cell mutation does not seriously impair individual’s ability to have offspring who can transmit the mutation, it can spread to the other members of population

Allelic sequence variation has traditionally been described as a DNA polymorphism if more than one variant (allele) at a locus occurs in human population with a frequency greater than 0.01. In simple terms, if an inheritable mutation is observed in a population at high frequency, it is referred to as DNA polymorphism. The probability of such variation to be observed in non-coding DNA sequence would be higher as mutations in these sequences may not have any immediate effect/impact in an individual’s reproductive ability. These mutations keep on accumulating generation after generation, and form one of the bases of variability/polymorphism.

The technique of DNA Fingerprinting was initially developed by Alec Jeffreys. He used a satellite DNA as probe that shows very high degree of polymorphism. It was called as Variable Number of Tandem Repeats (VNTR). The technique, as used earlier, involved Southern blot hybridisation using radiolabelled VNTR as a probe. It included

  • isolation of DNA,
  • digestion of DNA by restriction endonucleases,
  • separation of DNA fragments by electrophoresis,
  • transferring (blotting) of separated DNA fragments to synthetic membranes, such as nitrocellulose or nylon,
  • hybridisation using labelled VNTR probe, and
  • detection of hybridised DNA fragments by autoradiography.

The VNTR belongs to a class of satellite DNA referred to as mini-satellite. A small DNA sequence is arranged tandemly in many copy numbers. The copy number varies from chromosome to chromosome in an individual. The numbers of repeat show very high degree of polymorphism. As a result the size of VNTR varies in size from 0.1 to 20 kb. Consequently, after hybridisation with VNTR probe, the autoradiogram gives many bands of differing sizes. These bands give a characteristic pattern for an individual DNA. It differs from individual to individual in a population except in the case of monozygotic (identical) twins. The sensitivity of the technique has been increased by use of polymerase chain reaction. Consequently, DNA from a single cell is enough to perform DNA fingerprinting analysis. In addition to application in forensic science, it has much wider application, such as in determining population and genetic diversities. Currently, many different probes are used to generate DNA fingerprints.



Double Helix DNA

DNA Genetic Material

Transcription

Translation

Human Genome

NCERT Solution