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

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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

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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

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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

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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

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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

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aa

tRNA

GGU

aatRNA

U A C

aatRNA

GA C

tRNA

aa

A GU

ribosome

aa

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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

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transcription

cytoplasm

nucleus

translation

trait

From gene to protein

transcriptiontranscription

translationtranslation

proteinprotein