RNA AND PROTEIN SYNTHESIS

CHAPTER 13

What is the genetic material responsible for our physical traits?

DNA

What are the functions of DNA?1. Copying information2. Storing information3. Transmitting information

How does DNA store and transmit information?

Genes contain coded DNA instructions for making proteins

DNA stored in nucleus Proteins are made in the cytoplasm by

ribosomes

PROBLEM…how to get the code to the machinery? Another nucleic acid, RNA, translates the DNA

message into protein

Central Dogma of Molecular BiologyDescribes flow of information from DNA to protein

RNADNA Protein

transcription translation

RNA is the link between DNA and proteins.

Analogy for central dogma

The master plan has all the information needed to construct a building.

Analogy for central dogma

But builders never bring a valuable master plan to the building site where it might be damaged or lost.

Analogy for central dogma

Instead, builders work from blueprints, inexpensive disposable copies of the master plan.

Analogy for central dogma

Master plan = DNABuilding site = ribosomes (in cytoplasm)Blueprint copies = RNA

DNA Consists of nucleotides Double stranded Deoxyribose as sugar Bases = A, C, T and G

RNA Consists of nucleotides Single stranded Ribose as sugar Uracil (U) replaces T

Comparison of DNA and RNA

RNA Builds Protein

1. Messenger RNA – carries the instructions from DNA to cytoplasm on how to make the protein

2. Ribosomal RNA – form ribosomes in cytoplasm to help build the protein

3. Transfer RNA – delivers the amino acids needed to build the protein

Transcription (RNA synthesis)

DNA RNA Occurs in the nucleus Requires RNA polymerase

Builds RNA strand Uses one strand of DNA as template

Involves single gene Produces MANY RNA copies

Transcription (RNA Synthesis)How does RNA polymerase know where to start and stop?

Sequences mark the start (promoter) and end (terminator) of a gene. RNA polymerase recognizes these sequences.

RNA editing: non-coding sections of the mRNA transcript (introns) are cut out. Remaining pieces (exons) are spliced together.

Transcription (RNA Synthesis)

What do you think would happen if introns were not removed from the pre-mRNA?

RNAi Video

Replication vs Transcription

Replication copies the entire DNATranscription only copies one gene

Replication only makes one copyTranscription makes many copies

BOTH involve complex enzymesBOTH follow complementary base pairingBOTH occur in the nucleus

If you were given the sequence of a DNA strand, could you figure out the sequence of the mRNA strand? Remember, U instead of T in RNA!!!For example:

DNA T A C G C C C T A T T G A T

mRNA ?? A U G C G G G A UA AC U A

RIBOSOMES AND PROTEIN SYNTHESIS

13.2

Translationa.k.a. Protein Synthesis

RNA Protein

The language of protein synthesis Language of RNA = nucleotides Language of protein = amino acids

20 different amino acids found in proteins Sequence of amino acids influences the shape of

the protein which determines its function Triplet code: three mRNA bases (codon) code for

one amino acid

How is the order of bases in DNA and RNA translated into a particular order of amino acids in a protein?

Genetic Code (mRNA codon translated into amino acid)

Genetic Code:

Codon = a group of three nucleotide bases in the mRNA that codes for a particular amino acid

Genetic Code: (mRNA codon to amino acid)

Genetic Code: punctuation

START codon signals the start of translation AUG also codes for methionine

STOP codons signal stop of translation UGA, UAA, UAG Do not code for any amino acid

Genetic Code = common language UNIVERSAL – shared by all organisms REDUNDANT – more than one codon may

code for the same amino acid Allows flexibility if mistakes are made

Quick Check

A certain gene has the following base sequence: GACAAGTCCAATC

Write the sequence of the mRNA molecule transcribed from this gene

Divide you mRNA sequence into codons How many codons?

What amino acid does each codon code for? How many amino acids?

The role of ribosomes in translation…

Ribosomes use the sequence of codons in mRNA to assemble amino acids into protein chains

Each tRNA molecule carries one kind of amino acid

Anticodon on tRNA recognizes complementary codon on mRNA For example, tRNA for methionine has the

anticodon UAC which pairs with the methionine codon (AUG)

The role of transfer RNA (tRNA) in translation…

Process of Translation

1. Ribosome binds to mRNA

2. mRNA codons attract complementary tRNA anticodons

3. Ribosome forms a peptide bond between amino acids then breaks bond holding the amino acid to the tRNA

4. Empty tRNA leaves; the ribosome pulls the mRNA exposing the next codon

QUICK CHECK… Remember, U instead of T in RNA!!!For example:

DNA T A C G C C C T A T T G A T A

mRNA ??

Amino acids

tRNA

Central Dogma of Molecular BiologyDescribes flow of information from DNA to protein

RNADNA Protein

transcription translation

RNA is the link between DNA and proteins.

MUTATIONS

13.3

Mutations are changes in the DNA1. Gene mutations (single gene)2. Chromosomal mutations (multiple genes

involved)

mutated

base

1. Gene Mutations

Also known as point mutations because they occur at a single point in the DNA sequence

Occur during replication Different types

A. SubstitutionsB. Insertions and deletions

A. Substitutions

One base is changed to a different base

Only affect one amino acid Sometimes have no effect (silent) EX: changing mRNA codon from CCC

to CCA Codon still specifies proline; SILENT

EX: changing mRNA codon from CCC to ACC Replaces proline with threonine

B. Insertions and Deletions

Frameshift mutations – “shift” the reading frame

Effects are dramatic Can change every amino acid after the

mutation

FrameshiftMutations

2. Chromosome mutations Changes in number or structure of

chromosomes Occur during meiosis Four types

A. Deletion (loss of all or part of a chromosome)

B. Duplication (extra copy)C. Inversion (reverse in the direction of a

chromosome)D. Translocation (one chromosome attaches to

another)

Mutagens

Chemical or physical agents in the environment that can cause mutations in DNA

Include Pesticides, tobacco, environmental pollutants,

UV light, X-rays

Harmful and Helpful Mutations

Mutations can be harmful if… They cause drastic changes in the protein that

is produced Defective proteins can disrupt normal function Ex: sickle cell anemia, some cancer

Harmful and Helpful Mutations

Beneficial effects Variation produced by mutations can be highly

advantageous to organisms in different or changing environments

Responsible for evolution EX: pesticide resistance (bad news for

humans but good news for mosquitoes) EX: human resistance to HIV