Human Chromosomes

Human Chromosomes arranged to form a karyotype
Human Chromosomes arranged to form a karyotype
The chromosomes in the photo to the left, are from a typical human body cell, which contains 46 chromosomes. They are numbered roughly in order decreasing in size. Every human began life when a haploid sperm cell fertilized a haploid egg both containing 23 chromosomes. The result is a diploid zygote (fertilized egg) that contained the full complement of 46 chromosomes.

The Human Genome is stored onto 23 chromosome pairs. 22 out of the 23 chromosomes are autosomal chromosome pairs, the rest of them are sex determintive. There are just a little over 3 billion copies of base pairs from the haploid human genome. The haploid has over 23,000 protien coding genes. 1.5% are human genome protien codes, the others are non coding RNA genes.

Nucleotides serve as the alphabet for the language of life and are represented by just 4 letters: A,C,G and T corresponding to, adenine, cytosine, guanine, and thymine..external image genome.jpg
Combonations of three nucleotides indicate one of twenty possible amino acids, so sets of nucleotide triplets from the instructions that cells use to build proteins.external image Amino-Acid-Chart.gif These proteins perform the work of the cells from development throughout life, contributing to both our physical features and many of our less tangible features such as behavior and learning. A gene is a a segment of a DNA molecule that codes for one complete protein.

Genes

The human genome, categorized by function of each gene product, given both as number of genes and percentage of all genes
The human genome, categorized by function of each gene product, given both as number of genes and percentage of all genes


The human genome, categorized by function of each gene product, given both as number of genes
and percentage of all genes.

There are estimated to be between 10,000 and 25,000 human protein-coding genes. The estimate of the number of human genes has been repeatedly revised down as genome sequence quality and gene finding methods have improved. In the late 1960s, predictions estimated that human cells had as many as 2,000,000 genes.

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, a larger proportion of human genes are related to the central nervous system and especially brain development.

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.

Variation


DNA molecule 1 differs from DNA molecule 2 at a single base-pair location.
DNA molecule 1 differs from DNA molecule 2 at a single base-pair location.

Most studies of human genetic variation have focused on single-nucleotide polymorphisms (SNPs), which are substitutions in individual bases along a chromosome. Most analyses estimate that SNPs occur 1 in 1000 base pairs, on average, in the euchromatic human genome, although they do not occur at a uniform density. Thus follows the popular statement that "we are all, regardless of race, genetically 99.9% the same",[36] although this would be somewhat qualified by most geneticists. For example, a much larger fraction of the genome is now thought to be involved in copy number variation.[37] A large-scale collaborative effort to catalog SNP variations in the human genome is being undertaken by the International HapMap Project.

The genomic loci and length of certain types of small repetitive sequences are highly variable from person to person, which is the basis of DNA fingerprinting and DNA paternity testing technologies. The heterochromatic portions of the human genome, which total several hundred million base pairs, are also thought to be quite variable within the human population (they are so repetitive and so long that they cannot be accurately sequenced with current technology). These regions contain few genes, and it is unclear whether any significant phenotypic effect results from typical variation in repeats or heterochromatin.

Most gross genomic mutations in gamete germ cells probably result in inviable embryos; however, a number of human diseases are related to large-scale genomic abnormalities. Down syndrome, Turner Syndrome, and a number of other diseases result from nondisjunction of entire chromosomes. Cancer cells frequently have aneuploidy of chromosomes and chromosome arms, although a cause and effect relationship between aneuploidy and cancer has not been established.


Code For Life: The Human Genome