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Biology II—Ch 17 Notes—From Genes to Proteins
-one gene produces one polypeptide chain
-one gene codes for one polypeptide chain
-polypeptide chains may combine to form 1 protein
-Hemoglobin made from 2 kinds of polypeptides—thus
2 genes code for this protein
-ok to say one gene codes for one protein
-DNA contains instructions for making proteins, RNA actually
Makes the proteins
-3 main structural differences between DNA and RNA
1. DNA has 2 strands; RNA has 1 strand
2. DNA has the sugar deoxyribose; RNA has the sugar ribose
3. DNA has the nitrogen bases A,T,G,C; RNA replaces T with
Uracil
-2 main steps to get from DNA to a protein
1. Transcription: synthesis of RNA under direction of DNA
-carries genetic info from nucleus to the
cytoplasm
2. Translation: actual synthesis of a protein (or polypeptide)
that occurs under the direction of mRNA
-Translation (cont)
-3 types of RNA work together
1. mRNA carries instructions from nucleus to cytoplasm
-codon: group of 3 nucleotides in mRNA that code
for one of 20 amino acids
-each codon codes for a specific amino acid
-may be more than one codon for one amino acid
2. tRNA brings amino acids from the cytoplasm to the
ribosome to be assembled into a protein
-anticodon: group of 3 nucleotides
complimentary to the codon—ensures that amino acids are
added in the correct order
3. rRNA found in ribosomes
-ribosomes consist of 2 subunits--small and
large
-constructed of proteins and rRNA
-large and small subunits join to form a functional
unit only when they attach to an mRNA molecule
-about 60% of the weight of each ribosome is
rRNA
-ribosomes bring mRNA and tRNA together
-ribosomes have a binding site for mRNA and 3 binding
sites for tRNA
-P site: holds tRNA carrying the growing
polypeptide chain
-A site: holds tRNA carrying the next
amino acid
-E site: discharges tRNAs after they deliver
the amino acid
-act like a vise and holds the tRNA and mRNA
close together and positions the new amino acid
for addition to the growing protein
-Transcription (cont)
-mRNA first synthesized needs to be modified before it can leave
the nucleus
-RNA splicing
-primary RNA transcript is about 8000 nucleotides
long
-only takes about 1200 nucleotides to code for an
average size protein of about 400 amino acids
-most genes and their RNA transcripts have
long noncoding stretches of nucleotides—regions
not translated
-most of these noncoding sections are
interspersed between coding segments of a
gene
-Introns: noncoding segments of nucleic
acid that lie between coding regions
-Exons: coding regions that are eventually
expressed
-both introns and exons are originally transcribed
-but, introns are cut out and exons are
spliced together to form an mRNA molecule
with a continuous coding sequence
-this leaves the nucleus and enters the cytoplasm
Mutations
-Mutation—any change in the DNA sequence
-Point Mutations—chemical changes in just one or a few base pairs
In a single gene
-3 main types
1. Substitution: change in a single base pair
-may have no affect if resulting codon codes for the
same amino acid
2. Insertion: addition of a single base pair
-affects every amino acid after the mutation
-The dog bit the cat
-The dob gbi tth eca t
3. Deletion: removal of a base pair
-affects every amino acid after the mutaion
-The dog bit the cat
-The dob itt hec at
-Mutagens: any agent that interacts with the DNA and causes a
Mutation
-high energy radiation: UV light, X-rays, nuclear radiation
-chemicals: formaldehyde, benzenes, asbestos
-high temperatures