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Biology, Section 3
Notes
Date Submitted: 3/2/12
Chapter 10 Section 4 Notes
Flow of Genetic Information
A. Gene: segment of DNA located on a
chromosome + codes for hereditary character
B. Genes direct the making of proteins through
an intermediate: RNA
C. Transcription: DNA acts as template for
RNA synthesis
D. Translation: RNA directs assembly of
proteins: protein synthesis aka gene
expression
E. DNA  RNA  protein
RNA Structure and Function

RNA is a nucleic acid but differs from DNA
in 4 basic ways:
RNA
Sugar: ribose
Nitrogenous base: uracil
Single-stranded
 Some regions fold to form short doublestranded sections where G-C, U-A
Much shorter in length than DNA
B. 3 Major types of RNA
DNA
Sugar: deoxyribose
Nitrogenous base: thymine
Double-Stranded
Double helix
mRNA
(messenger RNA)


Carries instructions from a gene
to make protein
In eukaryotic cells, mRNA carries
genetic “message” from DNA in
nucleus to ribosomes in cytosol
rRNA
(ribosomal RNA)

part of structure of ribosomes
tRNA
(transfer RNA)

transfers amino acids to ribosome
to make protein
Transcription
A. Transcription is the process by which genetic instructions in a specific gene are transcribed or
“rewritten” into an RNA molecule
B. Takes place in nucleus (eukaryotic cells) or DNA-containing region in cytoplasm (prokaryotic cells)
C. Steps:
a. RNA polymerase, an enzyme that catalyzes the formation of RNA on a DNA template,
binds to a promoter. A promoter is a specific nucleotide sequence of DNA where RNA
polymerase binds and initiates transcription. After RNA polymerase binds to promoter, DNA
strands unwind and separate.
b. RNA polymerase adds free RNA nucleotides that are complementary to nucleotides on one
DNA strand  RNA molecule. (ATCGAC on DNA  UAGCUG on RNA) Transcription
uses only a gene on one DNA strand to serve as template.
c. As RNA polymerase moves past, separated DNA strands rewind.
d. RNA polymerase reaches a termination signal, (a specific sequence of nucleotides that
marks the end of a gene)  RNA polymerase releases both the DNA & newly formed RNA
e. Newly made RNA can perform its job in the cell, and RNA polymerase can transcribe
another gene
The Genetic Code
A. During translation, amino acids are assembled based on instructions encoded in sequence of
nucleotides in mRNA
B. Genetic code = rules that relate how a sequence of nitrogenous bases corresponds to a particular
amino acid
a. 3 adjacent nucleotides (letters) in mRNA code for an amino acid (word) in a polypeptide
b. Each 3-nucleotide sequence is called a codon
c. Nearly universal to all life on Earth
d. Supports idea that all organisms share an ancient common ancestor
C. 64 mRNA codons & the amino acids they encode in most organisms
D. Some a.a are coded for by 2+ codons that usually differ by 1 nucleotide
E. No codon encodes more than one amino acid.
F. AUG = start codon = methionine (specific sequence of nucleotides in mRNA that indicates where
translation begins)
G. UAA, UGA, UAG = stop codons (do not code for a.a, but instead signal for translation to end)
Translation
A. Instructions for making a protein are copied from DNA to mRNA, but all 3 types of RNA are
involved in translation
B. Translation = decoding of the genetic instructions to form a polypeptide
C. Protein structure
a. Made of 1 or more polypeptides
b. Polypeptides = chains of amino acids linked by peptide bonds
c. 20 diff. amino acids found in proteins of living things
d. The amino acid sequence determines how the polypeptides will twist/fold into 3D structure
of protein
e. Shape of protein = critical to function
D. Steps of Translation
1) 2 ribosomal subunits + tRNA + mRNA join together
a. Enzymes attach a specific amino acid to one end of each tRNA according to genetic code
b. The other end of each tRNA contains the anticodon (3 nucleotides that are
complementary to sequence of codon in mRNA)
c. A tRNA carrying start codon AUG at one end & anticodon UAC at other end pairs with
AUG on the mRNA
d. Methionine is the first amino acid in nearly all polypeptides
2) Polypeptide chain is put together
a. A tRNA carrying the appropriate amino acid pairs its anticodon with the second codon in
mRNA
b. The ribosome detaches methionine from the first tRNA  peptide bond forms between
methionine and second amino acid
c. First tRNA exits ribosome
d. Ribosome moves a distance of 1 codon along the mRNA
3) The polypeptide chain continues to grow as mRNA moves along ribosome
a. A new tRNA moves in, carrying an amino acid for the next mRNA codon
b. Growing polypeptide chain moves from one tRNA to the amino acid attached to the next
tRNA
4) Polypeptide grows one amino acid at a time until stop codon  polypeptide falls off
5) Components of translation come apart
a. The last tRNA leaves the ribosome
b. Ribosome moves away from the mRNA
E. B/c a new ribosome begins translating mRNA almost as soon as the preceding ribosome has moved
aside, several ribosomes can translate the same mRNA simultaneously
F. Prokaryotes lack a nuclear envelope separating their DNA from ribosomes in cytosol translation
can begin on an mRNA even before transcription of mRNA has finished
G. In eukaryotes, translation can only happen after transcription is finished
The Human Genome
A.
B.
C.
D.
E.
F.
The entire gene sequence of the humane genome (complete genetic content) is known
Biologists deciphered the order of the 3.2 billion base pairs in the 23 human chromosomes
New challenge: Learn what information the DNA sequences encode
New field, bioinformatics, uses PCs to compare different DNA sequences
Scientists use computers to predict where genes lie along the DNA
Learning which gene sequences control particular biological functions may help
diagnose/treat/prevent genetic disorders/cancer/infectious diseases