Other DNA

Protein-coding sequences (specifically, coding exons) comprise less than 1.5% of the human genome.^[3] Aside from genes and known regulatory sequences, the human genome contains vast regions of DNA the function of which, if any, remains unknown. These regions in fact comprise the vast majority, by some estimates 97%, of the human genome size. Much of this is composed of:

repeat elements

• Tandem repeats
  • Satellite DNA
  • Minisatellite
  • Microsatellite
• Interspersed repeats
  • SINEs
  • LINEs

transposons

• Retrotransposons
  • LTR
    • Ty1-copia
    • Ty3-gypsy
  • Non-LTR
    • SINEs
    • LINEs
• DNA Transposons

pseudogenes

However, there is also a large amount of sequence that does not fall under any known classification.

Much of this sequence may be an evolutionary artifact that serves no present-day purpose, and these regions are sometimes collectively referred to as "junk" DNA. There are, however, a variety of emerging indications that many sequences within are likely to function in ways that are not fully understood. Recent experiments using microarrays have revealed that a substantial fraction of non-genic DNA is in fact transcribed into RNA,^[7] which leads to the possibility that the resulting transcripts may have some unknown function. Also, the evolutionary conservation across the mammalian genomes of much more sequence than can be explained by protein-coding regions indicates that many, and perhaps most, functional elements in the genome remain unknown.^[8] The investigation of the vast quantity of sequence information in the human genome whose function remains unknown is currently a major avenue of scientific inquiry.^[9]

http://en.wikipedia.org/wiki/Human_genome

What is genetic mapping?

Developing new and better tools to make gene hunts faster, cheaper and practical for any scientist was a primary goal of the Human Genome Project (HGP).

One of these tools is genetic mapping, the first step in isolating a gene. Genetic mapping - also called linkage mapping - can offer firm evidence that a disease transmitted from parent to child is linked to one or more genes. It also provides clues about which chromosome contains the gene and precisely where it lies on that chromosome.

Genetic maps have been used successfully to find the single gene responsible for relatively rare inherited disorders, like cystic fibrosis and muscular dystrophy. Maps have also become useful in guiding scientists to the many genes that are believed to interact to bring about more common disorders, such as asthma, heart disease, diabetes, cancer and psychiatric conditions.

http://www.genome.gov/10000715

Chromosomes and Genes

Chromosomes

The human genome is composed of 23 pairs of chromosomes (46 in total), each of which contain hundreds of genes separated by intergenic regions. Intergenic regions may contain regulatory sequences and non-coding DNA.

There are 24 distinct human chromosomes: 22 autosomal chromosomes, plus the sex-determining X and Y chromosomes. Chromosomes 1–22 are numbered roughly in order of decreasing size. Somatic cells usually have 23 chromosome pairs: one copy of chromosomes 1–22 from each parent, plus an X chromosome from the mother, and either an X or Y chromosome from the father, for a total of 46.

Genes

There are estimated 20,000–25,000 human protein-coding genes.^[1]

Surprisingly, the number of human genes seems to be less than a factor of two greater than that of many much simpler organisms, such as the roundworm and the fruit fly. However, human cells make extensive use of alternative splicing to produce several different proteins from a single gene, and the human proteome is thought to be much larger than those of the aforementioned organisms.

Most human genes have multiple exons, and human introns are frequently much longer than the flanking exons.

Human genes are distributed unevenly across the chromosomes. Each chromosome contains various gene-rich and gene-poor regions, which seem to be correlated with chromosome bands and GC-content. The significance of these nonrandom patterns of gene density is not well understood. In addition to protein coding genes, the human genome contains thousands of RNA genes, including tRNA, ribosomal RNA, microRNA, and other non-coding RNA genes.

Regulatory sequences

The human genome has many different regulatory sequences which are crucial to controlling gene expression. These are typically short sequences that appear near or within genes. A systematic understanding of these regulatory sequences and how they together act as a gene regulatory network is only beginning to emerge from computational, high-throughput expression and comparative genomics studies.

Identification of regulatory sequences relies in part on evolutionary conservation. The evolutionary branch between the human and mouse, for example, occurred 70–90 million years ago.^[4] So computer comparisons of gene sequences that identify conserved non-coding sequences will be an indication of their importance in duties such as gene regulation.^[5]

Another comparative genomic approach to locating regulatory sequences in humans is the gene sequencing of the puffer fish. These vertebrates have essentially the same genes and regulatory gene sequences as humans, but with only one-eighth the "junk" DNA. The compact DNA sequence of the puffer fish makes it much easier to locate the regulatory genes.^[6]

http://en.wikipedia.org/wiki/Human_genome

Human genome

The human genome is the genome of Homo sapiens, which is stored on 24 distinct chromosomes (22 autosomal + X + Y) containing an estimated 20,000–25,000 genes^[1]. The entire human genome occupies a total of just over 3 billion DNA base pairs, and has a data size of approximately 750 Megabytes^[2], which is slightly larger than the capacity of a standard Compact Disc.

The Human Genome Project has produced a reference sequence of the euchromatic human genome, which is used worldwide in biomedical sciences. The human genome had fewer genes than expected, with only about 1.5% coding for proteins, and the rest comprised by RNA genes, regulatory sequences, introns and controversially so-called junk DNA.^[3]

A graphical representation of the normal human karyotype.

http://en.wikipedia.org/wiki/Human_genome

Gene Mapping

Gene mapping involves determining the locations of genes within specific chromosomes. Two methods of gene mapping are used:

• genetic mapping
• physical mapping

Genetic mapping is used to determine the relative position of genes within a chromosome. This is measured by whether or not two genes are "linked". If both genes are inherited together they are considered linked. By determining which genes are linked, the relative positions of genes can be worked out.

Physical mapping involves determining the exact position of a specific gene within a chromosome. There are multiple techniques for accomplishing this, including creating cell hybrids for mapping out DNA in specific chromosomes.

http://www.genesolutions.com/page6.html