II II X
Figure 4.2 The human karyotype. There are 22 autosomal chromosomes or autosomes. The autosomal chromosomes are numbered from 1 to 22. Each of the chromosomes can be recognized by its size, shape, staining pattern, and the position of the centromere (the constriction at which sister chromatids are anchored prior to cell division). The largest autosome is number 1, and the smallest is number 21. Historically, the second smallest chromosome has been designated number 22. Y chromosome is about the same size as chromosome 22 and the X chromosome is larger than the Y chromosome. Reprinted from US National Library of Medicine. Handbook: Help Me Understand Genetics. Bethesda, MD, USA: US National Library of Medicine. Available from: http://ghr.nlm.nih.gov/handbook/ basics/howmanychromosomes.
(Figure 4.3). In addition, an entire complement of exons encoded in a given gene need not be expressed, resulting in different species of RNA. These RNA species, generated by a process termed alternative splicing, can lead to different forms of proteins with associated functions.
Transcripts are threaded into ribosomes and are read in groups of three nucleotides termed codons. These codons interact with a tRNA that bears a complementary anti-codon to which is tethered an amino acid. Each codon is read in turn and the corresponding amino acid is added to the growing polypeptide chain (Figure 4.1). Though there are 64 possible codons (43 - four nucleotides read in groups of three), only 20 amino acids serve as the building blocks of proteins (Figure 4.4).
Gene expression is a dynamic and exquisitely regulated process. In each cell, a subset of genes is expressed at a basal level or is modulated while others are not expressed. Extracellular signals such as hormones, neuro-transmitters, nutrients, and proteins can modulate gene expression. Differentiation of cell structure and function results from, and is influenced by, regulation of gene expression at the transcriptional, translational, and post-translational levels. Both innate and acquired changes (e.g. gene mutations, variations) in transcription factors, cofactors, signaling molecules, promoters, or coding regions of a gene can result in susceptibility to disease.
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