DNA

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		DNA The Information Molecule

Key to Hyperlink Symbols
ACT	Interactive activity	GA	Gray's Anatomy
ANIM	Animation	pp	PowerPoint slide
FIG	Figure	term	Define, pronounce

Structure of DNA

Nucleotide is the subunit, made of three components:

Sugar (ribose in RNA; deoxyribose in DNA)

Phosphate

Nitrogen base

Different types of nitrogen base: cytosine, guanine, adenine, thymine (DNA only), uracil (RNA only)

First letters are used as abbreviations: C, G, A, T, U

Bases may link C-G or G-C and T-A or A-T (in RNA, U-A or A-U)

This rule is complementary or obligatory "base-pairing"

Nucleotides combine to form a double helix structure

Every three bases make up one "word" or codon in genetish

Each different codon represents a different amino acid

Amino acids combine to form polypeptides or proteins

A sequence of codons containing the information needed to make one polypeptide/protein is called a gene (gene = "recipe" for one protein)

Genes are found at various locations along a DNA molecule (chromatin/chromosome)

All genes on all chromosomes is a set of information called the genome

	This table is a type of "phrasebook" for the language of "genetish" --showing the meaning of different possible codons. The small-case letters are abreviations for amino acids (e.g. leu = leucine, ala = alanine)
Click here to see Ben Fry's easier-to-read version of this classic table! Ben constructed this as a student at MIT. This version is similar to that found in your textbook.

Function of DNA

Master code for proteins made by the cell ANIM

Proteins perform functions that regulate the cell, make up parts of the cell, and regulate the synthesis (and breakdown) of other types of molecules (lipids, carbohydrates, so on)

In humans, temporary "working copies" of specific genes from DNA are in the form of RNA (review Cell Structure & Function)

rRNA - ribosomal RNA	Forms ribosomes
mRNA - messenger RNA	Unfolded strand contains gene (code for one polypeptide)
tRNA - transfer RNA	Brings specific amino acids to ribosome and places them according to code on mRNA
nuclear DNA	"Master" genetic code in the nucleus
mDNA or mtDNA - mitochondrial DNA	Additional genetic code in the mitochondrion

Replication of DNA

DNA helix "unzips" and nucleotides with bases complementary to those in each exposed DNA strand "fill in" and make a new side

Results in two identical "daughter molecules" of DNA

Semiconservative (half is new; half is old)

Facilitated by enzymes (isn't everything?)

Transcription of RNA ANIM

mRNA is a "transcribed" copy of one gene in DNA

DNA unzips at one gene, and other side "fills in" with RNA nucleotides with bases that complement the exposed bases of the DNA strand

Promoter - sequence of bases that tells the cell where to start transcribing the gene

RNA polymerase - enzyme that facilitates formation of mRNA strand

mRNA is edited

Introns - parts of sequence that are deleted ("nonsense")

Exons - parts of sequence the remain in the final mRNA molecule

A structure called the spliceosome forms on the mRNA, facilitating the splicing of transcript (removal of introns, gluing together of exons)

Other forms of RNA made in similar manner; usually fold into complex shapes

mRNA leaves nucleus via nuclear pores

Translation of RNA (protein synthesis) ANIM

Initiation - mRNA associates with rRNA of ribosome

Elongation - tRNA brings amino acids into place (anticodons on tRNA complement codons on mRNA) elongating string of amino acids

Termination - protein is released

Mutations

Chromosomal mutations - additions, deletions to chromosome

Point mutations - change in one or few nucleotides in a gene sequence

Regulation of gene expression

Chromosome level

Packing - DNA wraps around histone proteins, forming "beads" called nucleosomes

Packing can prevent certain genes from being activated (transcribed)

Transcription level

Master genes turn on a group of other genes

Enhancer genes change the rate of transcription of other genes

Regulatory chemicals, such as steroid hormones, may activate certain genes

mRNA can be edited in different ways (exons can be linked in different ways)

mRNA can be "made ahead of time" and then "masked" until needed later

This photo of James Watson (l) and Francis Crick (r) shows them in 1953 with their original model for the double helix model of the DNA molecule.

Maurice Wilkins and Rosalind Franklin have also been given credit for participating in the discovery of this vitally important molecule's structure and essential function. (Wilkins shared the 1962 Nobel Prize in Medicine or Physiology with Watson & Crick)

Researchers continue to unravel the many complexities and mysteries of what Watson called, "the most golden of all molecules."

Genomic cartography is the study of methods to map out the genome of humans an other species in a way that makes the information easy to interpret and therefore more useful to everyone. Click here for an optional discussion from genomic cartographer Ben Fry.

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