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Biology Biology
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12-3 RNA and Protein Synthesis12–3 RNA and Protein Synthesis
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12–3 RNA and Protein Synthesis
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12–3 RNA and Protein Synthesis
Genes are coded DNA instructions that control the production of proteins.
Genetic messages can be decoded by copying part of the nucleotide sequence from DNA into RNA.
RNA contains coded information for making proteins.
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The Structure of RNA
The Structure of RNA
RNA consists of a long chain of nucleotides.
Each nucleotide is made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base.
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The Structure of RNA
There are three main differences between RNA and DNA:
• The sugar in RNA is ribose instead of deoxyribose.
• RNA is generally single-stranded.
• RNA contains uracil in place of thymine.
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Types of RNA
What are the three main types of RNA?
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Types of RNA
Types of RNA
There are three main types of RNA:
• messenger RNA
• ribosomal RNA
• transfer RNA
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Types of RNA
Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into proteins.
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Types of RNA
Ribosomes are made up of proteins and ribosomal RNA (rRNA).
Ribosome
Ribosomal RNA
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Types of RNA
During protein construction, transfer RNA (tRNA) transfers each amino acid to the ribosome.
Amino acid
Transfer RNA
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Transcription
What is transcription?
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Transcription
Transcription
RNA molecules are produced by copying part of a nucleotide sequence of DNA into a complementary sequence in RNA. This process is called transcription.
Transcription requires the enzyme RNA polymerase.
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Transcription
During transcription, RNA polymerase binds to DNA and separates the DNA strands.
RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA.
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Transcription
RNA polymerase binds only to regions of DNA known as promoters.
Promoters are signals in DNA that indicate to the enzyme where to bind to make RNA.
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Transcription
RNA
RNA polymerase
DNA
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RNA Editing
RNA Editing
The DNA of eukaryotic genes contains sequences of nucleotides, called introns, that are not involved in coding for proteins.
The DNA sequences that code for proteins are called exons.
When RNA molecules are formed, introns and exons are copied from DNA.
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RNA Editing
The introns are cut out of RNA molecules.
The exons are then spliced together to form mRNA.
Exon IntronDNA
Pre-mRNA
mRNA
Cap Tail
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The Genetic Code
The Genetic Code
The genetic code is the “language” of mRNA instructions.
The code is written using four “letters” (the bases: A, U, C, and G).
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The Genetic Code
A codon consists of three consecutive nucleotides on mRNA that specify a particular amino acid.
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The Genetic Code
Each codon specifies a particular amino acid that is to be placed on the polypeptide chain.
Some amino acids can be specified by more than one codon.
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The Genetic Code
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The Genetic Code
There is one codon AUG that can either specify the amino acid methionine or serve as a “start” codon for protein synthesis.
There are three “stop” codons that do not code for any amino acid. These “stop” codons signify the end of a polypeptide.
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Translation
What is translation?
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Translation
Translation
Translation is the decoding of an mRNA message into a polypeptide chain (protein).
Translation takes place on ribosomes.
During translation, the cell uses information from messenger RNA to produce proteins.
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Translation
Nucleus
mRNA
Messenger RNA is transcribed in the nucleus, and then enters the cytoplasm where it attaches to a ribosome.
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Translation
Translation begins when an mRNA molecule attaches to a ribosome.
As each codon of the mRNA molecule moves through the ribosome, the proper amino acid is brought into the ribosome by tRNA.
In the ribosome, the amino acid is transferred to the growing polypeptide chain.
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Translation
Each tRNA molecule carries only one kind of amino acid.
In addition to an amino acid, each tRNA molecule has three unpaired bases.
These bases, called the anticodon, are complementary to one mRNA codon.
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Translation
LysinetRNAPhenylalanine
Methionine
Ribosome
mRNAStart codon
The ribosome binds new tRNA molecules and amino acids as it moves along the mRNA.
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Translation
Protein Synthesis
tRNA
Ribosome
mRNA
Lysine
Translation direction
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Translation
The process continues until the ribosome reaches a stop codon.
Polypeptide
Ribosome
tRNA
mRNA
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The Roles of RNA and DNA
The Roles of RNA and DNA
The cell uses the DNA “master plan” to prepare RNA “blueprints.” The DNA stays in the nucleus.
The RNA molecules go to the protein building sites in the cytoplasm—the ribosomes.
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Genes and Proteins
Genes and Proteins
Genes contain instructions for assembling proteins.
Many proteins are enzymes, which catalyze and regulate chemical reactions.
Proteins are each specifically designed to build or operate a component of a living cell.
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Genes and Proteins
The sequence of bases in DNA is used as a template for mRNA.
The codons of mRNA specify the sequence of amino acids in a protein.
CodonCodon Codon
Codon Codon Codon
mRNA
Alanine Arginine Leucine
Amino acids within a polypeptide
Single strand of DNA
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12–3
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12–3
The role of a master plan in a building is similar to the role of which molecule?
a. messenger RNA
b. DNA
c. transfer RNA
d. ribosomal RNA
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A base that is present in RNA but NOT in DNA is
a. thymine.
b. uracil.
c. cytosine.
d. adenine.
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The nucleic acid responsible for bringing individual amino acids to the ribosome is
a. transfer RNA.
b. DNA.
c. messenger RNA.
d. ribosomal RNA.
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12–3
A region of a DNA molecule that indicates to an enzyme where to bind to make RNA is the
a. intron.
b. exon.
c. promoter.
d. codon.
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12–3
A codon typically carries sufficient information to specify a(an)
a. single base pair in RNA.
b. single amino acid.
c. entire protein.
d. single base pair in DNA.
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