DNA Transcription & Translation

*How do genes work?

We’ve seen how DNA is replicated, but still haven’t learned exactly

how genes work!

The first step in understanding how genes work is to know how to

“decode” the DNA.

Decoding DNA

• The decoder: RNA (Ribonucleic acid)

• Long stranded like DNA, except:– Ribose instead of

Deoxyribose– Uracil instead of Thymine– Single stranded instead of

double stranded

Types of RNA• The main job of all RNA types is protein

synthesis (creation)• 3 types: (*You will learn what each does later)

Messenger RNA Transfer RNA Ribosomal RNA

mRNA tRNA rRNA

Protein Creation

• DNA holds the instructions for making proteins– Proteins do the cell’s work

• Two steps required for protein synthesis (creation)

1. TRANSCRIPTION2. TRANSLATION

Transcription

• TRANSCRIPTION is the process by which a DNA gene is “rewritten” into RNA

Transcription• A gene is only a segment of DNA

– Only the gene is transcribed, not all of the DNA (like replication)

TranscriptionDNA

mRNA

Transcription

3 phases:1. Initiation (“the beginning”)

• Enzymes recognize appropriate gene in DNA

2. Elongation (“the middle”)

• Enzymes copy gene into mRNA

3. Termination (“the end”)

• Enzymes unbind from DNA

Why re-write the instructions?

• RNA is like an inexpensive, disposable copy of the master plan

• Used at the work site (the ribosomes)

• DNA is like the “master plan” for the construction of a house

• Too important to bring to the work site and risk being damaged or lost

TranscriptionINITIATION

• RNA POLYMERASE (RP) recognizes a promoter on the DNA sequence

– PROMOTER: base sequence that signals the start of a gene

• DNA is split by RP and one strand is used as a template

Animation #1

Animation #2

Transcription

• ELONGATION:– RP makes an mRNA copy of the gene

Transcription

TERMINATION• RP reaches TERMINATOR SEQUENCE

– Signals it to unbind and stop transcription

Transcription

• After transcription is done you end with one mRNA copy of a gene

• Some of the mRNA must be “spliced out” (removed) before we translate it

mRNA Processing

• Pieces of the mRNA that don’t contain genes (called INTRONS) are removed by a SPLICEOSOME

mRNA Processing

• Pieces of DNA that do code for genes (called EXONS) are left and pieced together

The Genetic Code• The mRNA strand is then used to make

proteins in the process of TRANSLATION

The Genetic Code• The “language” of

mRNA is known as the genetic code

• The 4 “letters” (A-G-C-U) are used to write “words” that correspond to different amino acids

The Genetic Code

• “Words”are written using 3 letters

• Every 3 letters = 1 codon

• Every 1 codon = 1 amino acid

The Genetic Code

• Because there are four different bases, there are 64 (43) different codon combinations

• Certain codons code for the same amino acid

Translation

• Shortly after mRNA is transcribed, translation begins.

• The sequence of bases in mRNA serves as instructions for the order in which amino acids should be joined

Translation• Translation: The

decoding of an mRNA message and creation of a polypeptide chain (protein)

• RIBOSOMES are the factories that assemble the proteins

Translation

INITIATION• mRNA in the

cytoplasm attaches to a ribosome

ELONGATION• The ribosome

“reads” the codons and attaches the proper amino acids

Start codon

Stop codon

Transfer RNA• Transfer RNA brings the

correct amino acids for the ribosome to use

• Has an ANTI – CODON on one end, and an amino acid on the other

• Anti-codon has a sequence complementary to an mRNA codon

TranslationTERMINATION

• The ribosome continues along until it reaches a stop codon

• The amino acid chain is then released and allowed to fold into a protein

• Protein then performs its job within the cell

Translation

Translation can occur at many different spots along the mRNA strand, creating many proteins at one time.

Let’s Try it ourselves

• Transcribe and translate the following DNA sequence:

TAC GCA TGG AAT

AUG CGU ACC UUA

Translation AnimationsTranslation

Transcribe and Translate a Gene

Why do fireflies glow?

Let’s try it ourselves

• Transcribe and translate the following DNA sequence:

TAC GCA TGG AAT

AUG CGU ACC UUA

Met – Arg – Thr – Leu

Mutations

• Every now and then cells make mistakes.– Inserting incorrect bases– Deleting bases– Adding extra bases

• Mistakes that cause a change in genetic information are called mutations.

Point Mutations

• Point mutations are mutations that affect one nucleotide.

• 3 different types:– Substitution– Insertion– Deletion

Substitution• Occurs when the wrong base is added to

a growing nucleotide chain

Original DNA: TAC GCA TGG AATOriginal mRNA: AUG CGU ACC UUA

Original Protein: Met – Arg – Thr - Leu

SubstitutionOriginal DNA: TAC GTA TGG AAT

Original mRNA: AUG CAU ACC UUA

Original Protein: Met – His – Thr - Leu

Insertion• Occurs when an extra base is added to

a growing nucleotide chain

Original DNA: TAC GCA TGG AATOriginal mRNA: AUG CGU ACC UUA

Original Protein: Met – Arg – Thr - Leu

Insertion

Original DNA: TAT CGC ATG GAA T

Original mRNA: AUA GCG UAC CUU A

Original Protein: Ile – Ala – Tyr – Leu

Deletion

• When a base is deleted from a growing nucleotide chain

THE FAT CAT ATE THE RAT

TEF ATC ATA TET HER AT

Frameshift Mutations

• Mutations that cause a shift in the reading frame– Insertion– Deletion

• More than one amino acid gets changed downstream

TACGCATGGAAT

TATCGTATGGAAT

Silent Mutations

• DNA changes that do not cause a change in the amino acid chain

Original DNA: TAC GCA TGG AATMutated DNA: TAC GCC TGG AAT

mRNA: AUG CGG ACC UUA Protein: Met – Arg – Thr - Leu