Transcription and Translation

The Central Dogma of Molecular Biology:

DNA --> RNA --> Protein

Protein synthesis requires two steps: transcription and

translation.

Before we get into transcription and translation, let’s review some vocabulary

gene - coded DNA instructions that control the production ofproteins in cellsDNA - deoxyribonucleic acid, the genetic material that tells yourcells how to make proteinsWhat are the four nucleotide bases in the DNA code?RNA - ribonucleic acid. It carries out the instructions coded inDNA. What are the four nucleotide bases in RNA?mRNA - messenger RNARNA polymerase - an enzyme that attaches to DNA at specificsequences to start transcriptionribosomes - small particles made of protein that are found on the rough endoplasmic reticulum. They bind mRNA and tRNA duringtranslation. codon - a three base sequence in DNA or RNA that codes for oneamino acid

DNA Contains Codes

Three bases in DNA code for one amino acid. The DNA code is copied to produce mRNA. The order of amino acids in the polypeptide (the protein) is determined by the sequence of 3-letter codes (codons) in mRNA.

TranscriptionTo transcribe something is to copy it. In transcription,

DNA is copied into mRNA. RNA molecules are

produced by copying part of the DNA sequence into a

complementary RNA sequence.

TranscriptionTo transcribe DNA to mRNA, we need an enzyme called

RNA polymerase. It is similar to DNA polymerase.

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 mRNA.

Where does transcription take place?

How does RNA polymerase know what part of the DNA to bind to? How does

it know when to start and stop?

There are regions of DNA called promoters, which have specific sequences. Promoters act as signals in the DNA to tell RNA polymerase where to bind. There is also a similar sequence in the DNA that signals RNA polymerase to stop transcription.

RNA EditingLike a rough draft of a paper, many RNA molecules need to be edited before they are ready for translation. The DNA of many eukaryotes contains sequences called introns that do not code for any proteins and must be cut out of the RNA sequence. The remaining sequences that do code for proteins are called exons.

To help you remember, think of it this way – INtrons go IN the trash

RNA EditingWhat is the point of this? Nobody knows for sure. RNA editing may play a role in evolution. Mutations in introns have no effect on protein synthesis, but mutations in exons will probably have an effect and change the protein that will be produced.

Once the RNA has been edited, it is released to thecytoplasm and attaches to a ribosome on the rough endoplasmic reticulum so that translation can begin

TranslationTo translate something is to express it in another language. mRNA is translated into amino acids, which form chains called polypeptides, which get folded and shaped into proteins.

More Vocabulary!

tRNA - transfer RNA, it transfers an amino acid to the growing polypeptide chain. Each tRNA molecule carries one particular amino acid

codon - a sequence of three nucleotides that together form a unit of genetic code in a DNA or RNA molecule. Each codon codes for one amino acid

amino acids - the subunits that make up polypeptides and proteins

polypeptide - a chain of amino acidsprotein - a finished polypeptide that has been folded

into shape

TranslationTranslation begins when an mRNA moleculein the cytoplasm attaches to the ribosome. As eachcodon of the mRNA molecule moves through theribosome, the proper amino acid is brought to thebrought to the ribosome by tRNA. In the ribosome,the amino acid is transferred to the growingpolypeptide chain

Video! During the video, write down at least one question you have about transcription and/or translation.

QuickTime™ and a decompressor

are needed to see this picture.

How do we find out what the fox says?

In this activity, you will examine the DNA sequence of a fictitious

fox. These foxes say different things that are determined by their

genes. Our fake foxes have one gene that determines what

phrases they say. Your job is to analyze the genes of its DNA and

determine what the fox says.

The gene sequences we will be using are much

smaller than -real-gene sequences found

in living organisms. Each gene has two

versions that result in a different trait

(the phrase that the fox says)

being expressed in the fox.

How do we find out what the fox says?

Each of the DNA samples on your worksheets was taken

from volunteer foxes. The DNA was then transcribed to its

complementary mRNA strand. Your job is to analyze the

mRNA sample and determine what the fox says based on the

sequence. You will choose one sample (the sequences from

one fox) to analyze.

No foxes were harmed in the making of this presentation.

You will also determine four other traits that the fox has

The first gene determines what the fox

says. The rest of the genes determine

the fox’s fur color (brown or red), eye

color (brown or black), tail length (short or

long), and fur length (short or long).

The Codon ChartUse this codon chart to determine whichamino acids are in your fox’s sequence.

ExampleThe mRNA of gene 1 in the Ylvis Fox isGUC AGC AAA

Looking at the codon chart, GUC = val, AGC = ser, and AAA = lys

Looking at the table, the amino acid sequence(the polypeptide or protein) val-ser-lys results inthe Ylvis Fox’s first phrase being “ring-ding-dingeringeding”

Continue this process until you have decodedall five genes, and you will know what the foxsays and what it looks like!

Example

Once you have

determined all five of

your fox’s traits, draw

a picture of your fox

and give it a text

bubble showing what

it says

Just in case you were curious…

QuickTime™ and a decompressor

are needed to see this picture.