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.

What are Genes• You learned earlier that proteins are polymers of

amino acids.

• The sequence of nucleotides in your DNA is a gene that contains the information for assembling the string of amino acids that make up a single protein. (instructions to make a protein)

• Proteins control an organism:• Enzymes, Steroids, Structural Proteins etc.. Are all made from

the sequence of letters in your DNA

• Proteins are polymers made of amino acid monomers

• Where are proteins made?• Ribosomes!!

Cell organization• Remember:– DNA is in the nucleus• DNA contains genes = instructions for

making proteins– Cells want to keep DNA in the nucleus where it

is protected• “locked in the vault”

– How does the code for a protein get to a ribosome if the DNA can’t leave the nucleus?

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.

• RNA Function: takes from DNA the instructions on how the protein should be assembled, then—amino acid by amino acid—RNA’s assemble the protein.

Types of 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 (Ribosome).

• 2. Ribosomal RNA (rRNA), globular form, makes up the ribosome –the construction site of proteins (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. Has a specific anticodon that is complementary to the sequence on the mRNA.

From gene to protein

transcriptiontranscription

translationtranslation

proteinprotein

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 -> Protein• process of converting the information in a sequence

of nitrogenous bases in mRNA into a sequence of amino acids in protein

Transcription

• Making mRNA from DNA• DNA strand is the

template (pattern)– match bases

• U : A• G : C

• Enzyme– RNA polymerase- splits the DNA, then attaches a

complementary strand of RNA. Afterwards the DNA reattaches

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• RNA polymerase attaches at a promoter

which is a region of DNA that signals the start of a gene.

A G GGGGGT T A C A C T T T T TC C C CA A

Matching bases of DNA & RNA• Match complimentary RNA

bases to DNA bases on one of the DNA strands

(T is replaced with U)

U

A G GGGGGT T A C A C T T T T TC C C CA A

U

G

G

A

A

A C CRNA polymerase

C

C

U

U

UC

C

C

G

G

G

G

A

A

A

AA

Matching bases of DNA & RNA• U instead of T is matched to A

TACGCACATTTACGTACGCGGDNA

AUGCGUGUAAAUGCAUGCGCCmRNA

RNA Processing• Not all the nucleotides in the DNA of eukaryotic cells carry

instructions—or code—for making proteins.

• Genes usually contain many long non-coding 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)

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 Ribosome the construction site where the protein is

made3. Transfer RNA (tRNA) = the molecule delivering the proper amino

acid to the site at the right time

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)- groups of 3– 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

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

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 AlaAmino acids/protein

?

How can you code for 20 amino acids withonly 4 DNA bases (A,U,G,C)?

AUGCGUGUAAAUGCAUGCGCCmRNA

mRNA codes for proteins in triplets called codonsTACGCACATTTACGTACGCGGDNA

AUGCGUGUAAAUGCAUGCGCCmRNA

Met Arg Val Asn Ala Cys Alaprotein

?

Codon = block of 3 mRNA bases

codon

• For ALL life!– support for a common

origin for all life

• Code has duplicates– several codons for each

amino acid– This “wiggle room” is

mutation insurance!

Start codon AUG methionine

Stop codons UGA, UAA, UAG

The Genetic code

How are the codons matched to amino acids?

• The tRNA molecule has a complementary codon to the mRNA called an anticodon

• There is a specific tRNA for each amino acid

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

From gene to protein

TranscriptionDNA-> RNATranscriptionDNA-> RNA

TranslationRNA-> proteinTranslationRNA-> protein

proteinprotein

• DNA – TAC CAA GGA AGT GCG ATA CAT CGT AGC GGT

• mRNA-

• A.A. –