RNA and Protein Synthesis

Section 12-3

Objectives for this section Compare and contrast DNA and RNA Name the 3 main types of RNA Describe transcription and the editing of RNA Identify the genetic code Summarize translation Explain the relationship between genes and

proteins

Introduction The structure of DNA explains how it can be

copied, but it does not tell how a gene works. Genes—coded DNA instructions that control

the production of proteins within the cell

Introduction We now know that the central dogma of

biology is DNA to RNA to protein The first step in decoding the genetic message is

to copy part of the DNA nucleotide sequence into RNA (ribonucleic acid)

These RNA molecules contain the coded instructions for making proteins

The Central Dogma of Bilogy

The Structure of RNA RNA, like DNA, is a long chain of

nucleotides RNA consists of the same components as

DNA A 5-carbon sugar A phosphate group A nitrogenous base

The Structure of RNA There are 3 main differences between DNA

and RNA The sugar in RNA is ribose instead of the

deoxyribose in DNA RNA is generally single-stranded RNA contains uracil instead of thymine

The Structure of RNA RNA is like a disposable copy of a segment

of DNA In many cases, RNA is a copy of a single

gene—the ability to copy a single DNA sequence into RNA makes it possible for a single gene to produce hundreds or even thousands of RNA molecules

Types of RNA RNA molecules have many functions, but in

the majority of cells most RNA molecules are involved in just one job—protein synthesis

There are 3 main types of RNA

Types of RNA Messenger RNA (mRNA)—carry copies of

instructions for assembling amino acids into proteins; serve as “messengers” from DNA to the rest of the cell

Types of RNA Ribosomal RNA (rRNA)—a component of

ribosomes

Types of RNA Transfer RNA (tRNA)—works during the

construction of a protein; transfers each amino acid to the ribosome as it is specified by the coded messages in the mRNA

Types of RNA

Transcription Transcription—when RNA molecules are

produced by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA

Transcription requires the enzyme RNA polymerase—binds to DNA and separates the DNA strands. Then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA

Transcription How does the RNA polymerase know where

to start and stop making the RNA copy? The enzyme will only bind to regions of DNA

known as promoters, which have specific base sequences

Similar signals tell the RNA polymerase where to stop

Transcription

RNA Editing The first molecule of mRNA (known as the

pre-mRNA) produced by copying the DNA sequence is like a rough draft and it requires editing

DNA contains sequences of nucleotides called introns, which are not involved in coding for proteins

RNA Editing The DNA sequences that code for proteins are

called exons, because they are expressed in the synthesis of proteins

When an RNA molecule is formed, it contains both introns and exons The introns are cut out of the RNA molecule

while it is still in the nucleus The remaining exons are spliced back together

and form the final pre-mRNA molecule

RNA Editing

The Genetic Code Remember that proteins are made by joining

amino acids into long chains called polypeptides Each polypeptide contains a combination of any

or all of the 20 different amino acids The properties of proteins are determined by

the order in which different amino acids are joined together to produce polypeptides

The Genetic Code The “language” of mRNA instructions is

called the genetic code RNA contains 4 different nitrogenous bases (U,

C, G, A) How can a code with only 4 letters translate into

20 different amino acids? The genetic code is read 3 letters at a time, so

each “word” of the coded message is 3 bases long

The Genetic Code Each 3-letter “word” in mRNA is known as a

codon A codon consists of 3 consecutive nucleotides

that specify a single amino acid that is to be added to the polypeptide

Example: UCGCACGGU would be read as UCG-CAC-GGU. These codons represent 3 amino acids: serine-histidine-glycine

The Genetic Code Because there are 4 different bases, there are

64 possible 3-base codons. Note that some amino acids can be specified by

more than one codon There are also “start” and “stop” codons

Start codons (AUG) tell where protein synthesis is to begin

Stop condons (3 different ones) tell where the end of the polypeptide is

The Genetic Code

Translation The mRNA molecule has been transcribed

and serves as instructions, but we need something to read the instructions and put them to use In the cell, the ribosome takes care of this

Translation—the decoding of an mRNA message into a polypeptide chain (protein)

Translation Steps in translation Begins when an mRNA molecule in the

cytoplasm attaches to a ribosome Each codon of the mRNA moves through the

ribosome and the proper amino acid is brought to the ribosome by the tRNA

Translation Each tRNA carries only one kind of amino

acid and picks it up based on the anitcodon it is carrying Example: if the anticodon is UUU, the tRNA

would pick up the amino acid with the codon AAA (they are opposites)

What codon would these anticodons pick up??

ACG UGG CAG GGG GCC CAA

Translation The ribosome forms a peptide bond between

the first and second amino acids At the same time, it breaks the bond with the

tRNA molecule and releases it The ribosome moves on the third amino codon,

where a tRNA molecule brings it the amino acid specified by the third codon

Translation The polypeptide chain continues to grow until

the ribosome reaches a stop codon on the mRNA molecule When the stop codon is reached, it releases the

newly formed polypepetide and mRNA molecule, completing the process of translation

Genes and Proteins What does protein synthesis have to do with the

color of a flower, eye color, or height? Remember that many proteins are enzymes, which

catalyze and regulate chemical reactions. A gene that codes for an enzyme to produces pigment

controls flower color. Proteins are specific tools that build or operate

components of living cells.