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CH. 12.3 : DNA, RNA, and Protein
Section Objectives:•Relate the concept of the gene to the
sequence of nucleotides in DNA.
•Sequence the steps involved in protein synthesis.
•Explain the different types of RNA involved in protein synthesis
Genes and Proteins
• The sequence of nucleotides in DNA contain information.
• This information is put to work through the production of proteins.
• Proteins fold into complex, three- dimensional shapes to become key cell structures and regulators of cell functions.
• Thus, by encoding the instructions for making proteins, DNA controls cells.
Genes and Proteins
• You learned earlier that proteins are polymers of amino acids.
• The sequence of nucleotides in each gene contains information for assembling the
string of amino acids that make up a single protein.
DNA has the information to build proteins genes
DNA Proteins Cells Bodies
proteinscells
bodiesDNA gets all the glory,Proteins do all the work
cytoplasm
nucleus
Cell organization• DNA– DNA is in the nucleus• genes = instructions for making proteins
– want to keep it there = protected• “locked in the vault”
Cell organization• Proteins– chains of amino acids– made by a “protein factory” in cytoplasm– protein factory = ribosome
nucleus
cytoplasm
ribosome
aa
aa
aa
aa
aa
aa
aa
aaaa
aa
buildproteins
Passing on DNA information: need RNA
• RNA like DNA, is a nucleic acid
• RNA structure differs from DNA structure in three ways.– 1. Has ribose sugar instead
of deoxyribose (DNA)
– 2. Replaces thymine (T) with uracil (U)
– 3. Single stranded as opposed to double stranded DNA
Sugar(ribose)
Phosphategroup
Uracil (U)
Nitrogenous base(A, G, C, or U)
RNA
• RNA has a different function than DNA• Whereas DNA provides the instructions for
protein synthesis, RNA does the actual work of protein synthesis.
• RNAs take from DNA the instructions on how the protein should be assembled, then—amino acid by amino acid—RNAs assemble the protein.
RNA
• 3 types of RNA– 1. Messenger RNA (mRNA), single, uncoiled strand
which brings instructions from DNA in the nucleus to the site of protein synthesis.
– 2. Ribosomal RNA (rRNA), globular form, makes up the ribosome –the construction site of proteins binds (site of protein synthesis); binds to the mRNA and uses the instructions to assemble the amino acids in the correct order.
– 3. Transfer RNA (tRNA) single, folded strand that delivers the proper amino acid to the site at the right time
Passing on DNA information• Need to get DNA gene information
from nucleus to cytoplasm– need a copy of DNA– messenger RNA
nucleus
cytoplasm
ribosome
mRNA
buildproteins
aa
aa
aa
aa
aa
aa
aa
aaaa
aa
Protein Synthesis: 2 step process1. Transcription 2. translation
1.Transcription: DNA -> mRNA
• In the nucleus, enzymes make an RNA copy of a portion of a DNA strand The main difference between transcription and DNA replication is that transcription results in the formation of one single-stranded RNA molecule rather than a double-stranded DNA molecule.
2. Translation: mRNA -> Proteinprocess of converting the information in a sequence of
nitrogenous bases in mRNA into a sequence of amino acids in protein
mRNA
From nucleus to cytoplasm
DNAtranscription
nucleus cytoplasm
translation
trait
aa
aa
aa
aa
aa
aa
aa
aaaa
aa
protein
Transcription
• Making mRNA from DNA• DNA strand is the
template (pattern)– match bases• U : A• G : C
• Enzyme– RNA polymerase
Matching bases of DNA & RNA• Double stranded DNA unzips
A G GGGGGT T A C A C T T T T TC C C CA A
Matching bases of DNA & RNA• Double stranded DNA unzips
A G GGGGGT T A C A C T T T T TC C C CA A
Matching bases of DNA & RNA• Match RNA bases to DNA
bases on one of the DNA strands
U
A G GGGGGT T A C A C T T T T TC C C CA A
U
UU
U
U
G
G
A
A
A C CRNA polymerase
C
C
C
C
C
G
G
G
G
A
A
A
AA
Matching bases of DNA & RNA• U instead of T is matched to A
TACGCACATTTACGTACGCGGDNA
AUGCGUGUAAAUGCAUGCGCCmRNAaa
aa
aa
aa
aa
aa
aa
aaaa
aa
U C CCCCCA A U G U G A A A A AG G G GU Uribosome
RNA Processing• Not all the nucleotides in the DNA of eukaryotic cells carry
instructions—or code—for making proteins.
• Genes usually contain many long noncoding nucleotide sequences, called introns, that are scattered among the coding sequences.
• Regions that contain information are called exons because they are expressed.
• When mRNA is transcribed from DNA, both introns and exons are copied.
• The introns must be removed from the mRNA before it can function to make a protein.
• Enzymes in the nucleus cut out the intron segments and paste the mRNA back together.
• The mRNA then leaves the nucleus and travels to the ribosome.
RNA Processing:simplified
• Noncoding segments called introns are spliced out ( coding segment = exons)
Genetic information written in codons is translated into amino acid sequences
• Transfer of DNA to mRNA uses “language” of nucleotides– Letters: nitrogen bases of nucleotides (A,T,G,C)– Words: codons ~triplets of bases ( ex. AGC)– Sentences: polypeptide chain– The codons in a gene specify the amino acid
sequence of a polypeptide
The Genetic Code• The nucleotide
sequence transcribed from DNA to a strand of messenger RNA acts as a genetic message, the complete information for the building of a protein..
• Virtually all organisms share the same genetic code
Translation: From mRNA to Protein
• takes place at the ribosomes in the cytoplasm.Involves 3 types of RNA1. Messenger RNA (mRNA) =carries the blueprint for construction of a protein2. Ribosomal RNA (rRNA) = the construction site where the protein is
made3. Transfer RNA (tRNA) = the truck delivering the proper amino acid to
the site at the right time
Transfer RNA molecules serve as interpreters during translation
• In the cytoplasm, a ribosome attaches to the mRNA and translates its message into a polypeptide
• The process is aided by transfer RNAs
• Each tRNA molecule has a triplet anticodon on one end and an amino acid attachment site on the other
• Anticodon base pairs with codon of mRNA
aa
aa
aa
aa
aa
aa
aa
aaaa
aa
protein
cytoplasm
nucleus
traitU C CCCCCA A U G U G A A A A AG G G GU U
ribosome
How does mRNA code for proteins• mRNA leaves nucleus• mRNA goes to ribosomes in cytoplasm• Proteins built from instructions on mRNA
aa aa aa aa aa aa aa aa
How?
mRNA
U C CCCCCA A U G U G A A A A AG G G GU U
How does mRNA code for proteins?TACGCACATTTACGTACGCGGDNA
AUGCGUGUAAAUGCAUGCGCCmRNA
Met Arg Val Asn Ala Cys Alaprotein?
How can you code for 20 amino acids withonly 4 DNA bases (A,U,G,C)?
ribosome
aa aa aa aa aa aa aa aa
AUGCGUGUAAAUGCAUGCGCCmRNA
mRNA codes for proteins in triplets
TACGCACATTTACGTACGCGGDNA
AUGCGUGUAAAUGCAUGCGCCmRNA
Met Arg Val Asn Ala Cys Alaprotein
?
Codon = block of 3 mRNA bases
codon
ribosome
• For ALL life!– strongest support for a
common origin for all life
• Code has duplicates– several codons for each
amino acid– mutation insurance!
Start codon AUG methionine
Stop codons UGA, UAA, UAG
The Genetic code
How are the codons matched to amino acids?
TACGCACATTTACGTACGCGGDNA
AUGCGUGUAAAUGCAUGCGCCmRNA
anti-codon
codon
tRNAUAC
MetGCA
ArgCAU
Val Anti-codon = block of 3 tRNA bases
aminoacid
mRNA to protein = Translation• The working instructions mRNA• The reader ribosome• The transporter transfer RNA (tRNA)
mRNAU C CCCCCA A U G U G A A A A AG G G GU U
aaaa
aa
tRNA
GGU
aatRNA
U A C
aatRNA
GA C
tRNA
aa
A GU
ribosome
aa
aa
aaaa
aa
aa
aa
mRNA
From gene to protein
DNAtranscription
nucleus cytoplasm
proteintranslation
trait
U C CCCCCA A U G U G A A A A AG G G GU Uribosome
tRNAaa
protein
aa
aa
aa
aa
aa
aa
aa
aaaa
aa
aa
transcription
cytoplasm
nucleus
translation
trait
From gene to protein
transcriptiontranscription
translationtranslation
proteinprotein