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Unformatted text preview: What DNA, genes, and chromosomes are... You are composed of ORGANS, which are composed of TISSUES, which are composed of CELLS, which contain fluids & surfaces where the reactions of life take place. At the center of each cell, there is a NUCLEUS, which contains CHROMOSOMES, which are long molecules of spooled-up DNA. spooled~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DNA encodes PROTEINS which are assembled on RIBOSOMES, and catalyze all the reactions that take place in CELLS, directing CELLS to aggregate into TISSUES, and TISSUES to assemble into ORGANS, etc. DNA is an incredibly simple molecule. It is composed of four nucleotides: adenine, guanine, thymine, and cytosine (abbreviated A, G, T, C). These nucleotides are arrayed in mindmindbogglingly long strings, and the bases can be in any order, e.g., GATTACA. There are two long strands of nucleotides in a DNA molecule. The two strands are ersatz "mirror images" of one another... sort of. images" another... A & T can only bond with one another, and G & C can only bond with one another. So, our molecule looks like: 1 Attached to the "outside" of the bases are a outside" sugar, deoxyribose, and a phosphate which deoxyribose, provide structural support to the molecule. The sugars bond to one another in a manner called: 5'-to-3'. This constrains the direction 5' toin which a strand is assembled and "read". read" The shapes of the individual nucleotides cause the "ladder" ladder" geometry to twist into a "helix". helix" Since there are two strands, it is called a "double helix". helix" "Semiconservative" replication of DNA is Semiconservative" the secret to storing & transmitting information from parent to offspring. DNA can only be synthesized 5' to 3', so, 5' 3' one strand is assembled "backwards" backwards" (Figure 14.17 of text). Several other proteins help this process along (Figure 14.16 of text). Most of your DNA is repetitive "junk". junk" When you read along a strand, special sequences called "promoters" let you promoters" know that there is a gene ahead. Genes are segments of DNA that code for proteins. A copy of the gene's DNA gene' sequence (an RNA transcript) is sent to the cell's protein-making machinery. cell' protein- 2 Proteins are strings of amino acids. Schematic diagram of the structure of a typical gene in your DNA. There are 20 amino acids. They have single letter abbrevations: A for alanine, abbrevations: alanine, R for arginine, etc. , arginine The amino acid sequence of a protein is encoded in 3-nucleotide units called 3"codons", or "triplets". codons" triplets" (There are 64 possible codons.) codons.) Each codon encodes a particular amino acid. E.g., alanine is encoded, "GCT", GCT" "GCC", "GCA", or "GCG". GCC" GCA" GCG" There are even "start" (ATG) and start" "stop" (TAA, TAG, TGA) codons. stop" codons. The "code" by which this occurs is the code" same throughout nearly all of life. Schematic diagram of the structure of a typical gene in your DNA. A final complication: "exons" & "introns". exons" introns" Enzymes make RNA copies of your DNA ("transcription") and carry them to transcription" ribosomes. RNA is like DNA, but uses ribosomes. uracil instead of thymine. On the ribosome, transfer RNA's bring RNA' their amino acids to the blueprint and the protein is assembled ("translation"). (" translation" Together, these processes constitute "gene expression". expression" Exon-intron structure allows for Exon"alternative splicing" of RNA. splicing" 3 GenBank is the database for all DNA and protein sequence data. "Genomics" & "proteomics" show Genomics" proteomics" similarity of DNA binding inhibitor 3 in rats & mice versus humans...
Homo: Rattus: Mus: piqtaelapelvisndkrsf piqtaeltpelviskdkrsf piqtaeltpelviskdkrsf When the cell is expressing genes, chromosomes in the nucleus are unwound (below, right); this is not acceptable for cell division. They must be condensed (left). DNA is spooled around histones, and histones, twisted up relentlessly until it supercoils. supercoils. These individual DNA molecules & associated proteins are called chromosomes. You have 23 pairs. Each member of a pair (homolog) has the same loci on it (except X & Y). Schematic anatomy of a chromosome. The (supercoiled) chromosomes are the (supercoiled) vehicles of inheritance. 4 ...
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- Spring '08