6.C: Students will…

explain the purpose and process of

transcription and translation using

models of DNA and RNA

DNA mRNA Protein

DNA is found in the nucleus, but making a protein occurs at the ribosome (either floating in the cytoplasm or attached to the rough ER)

The DNA code must be copied and moved from the nucleus out to the cytoplasm (this is called transcription)

Once in the cytoplasm, the code must be read so amino acids (building blocks of protein) can be assembled (this is called translation)

DNA cannot fit through the nuclear pores,

and therefore cannot leave the nucleus to

go to the ribosomes

In order to get the DNA code out of the

nucleus and into the cytoplasm, it must be

turned into another nucleic acid called RNA

RNA stands for ribonucleic acid

RNA is different from DNA because it

contains the sugar ribose (instead of

deoxyribose)

RNA does not contain the base thymine

– instead it has the base uracil

› A goes with U

› C goes with G

The RNA molecule

is single-stranded

Messenger RNA (mRNA) – copies the

DNA code for a gene and carries that

information to the ribosomes outside the

nucleus

Transfer RNA (tRNA) – transfers (carries)

amino acids to the ribosomes, where

proteins are made

Long, straight chain of nucleotides

Made in the nucleus – copies the DNA code

for a gene and then exits out the nuclear

pores and goes to a ribosome

Contains the nitrogen bases A, G, C, and U

Carries information to build a specific protein

A sequence of 3 bases is called a codon

Transfers (carries) amino acids to the

ribosomes, where proteins are made

Single-stranded molecule

with an attachment site at

one end for an amino acid

The other end has three

nucleotide bases called

the anti-codon

(complementary to the

codon)

Transfer = taxi

A codon designates a specific amino acid

There are 20 amino acids created from

the 64 possible codons

One codon tells the ribosome to start

translating (AUG), and three tell the

ribosome to stop translating (UAA, UAG, or

UGA)

The production of a polypeptide chain

› Polypeptide chain = protein

› A string of amino acids held together with

peptide bonds

DNA mRNA Protein

› The mRNA is read in sets of 3 bases (codon)

Two steps in protein synthesis – transcription

and translation

The process of copying the sequence of

one strand of DNA (the template strand)

and turning that information into mRNA

Requires the enzyme RNA polymerase

Practice: what would be the

complementary RNA strand for the

following DNA sequence?

DNA 5’ – GCGTATG – 3’

Practice: what would be the

complementary RNA strand for the

following DNA sequence?

DNA 5’ – GCGTATG – 3’

RNA 3’ – CGCAUAC – 5’

During transcription, RNA polymerase binds

to DNA and separates the DNA strands

RNA polymerase then uses one strand of

DNA as a template to assemble

nucleotides into RNA

› Promoters are regions on DNA that show where

RNA polymerase must bind to begin transcription

› Specific base sequences act as signals to stop

transcription (termination signal)

Also known as RNA splicing

After DNA is transcribed into RNA, it must

be edited to make it functional

› Introns – non-functional segments of RNA

(they are taken out of the chain)

› Exons – coding segments of RNA (they are

left in)

mRNA leaves the nucleus through the

nuclear pores and goes to the ribosome

The process of decoding the mRNA and

turning it into a polypeptide chain

(protein)

Ribosomes read

mRNA 3 bases

(one codon) at

a time

Step 1 – Initiation

› The mRNA start codon (AUG) attaches to the

ribosome

› The first tRNA molecule (with the first amino

acid attached) moves into the P site

› A second tRNA molecule enters the A site

Step 2 – Elongation› Peptide bonds attach the amino acids together

› Once complete, the tRNA in the P site moves to

the E site (exit site) and detaches

› The ribosome moves down the mRNA,

continuing to attach amino acids with peptide

bonds

Step 3 – Termination› When the ribosome reaches one of the three

stop codons, translation stops

› The ribosome releases the amino acid chain

(polypeptide)

End Product

› The end product of protein synthesis is a

protein (primary structure)

› A sequence of amino acids bonded

together with peptide bonds (also called a

polypeptide)

aa1

aa2 aa3 aa4aa5

aa200

aa199

Once it has been released, the polypeptide

folds into its appropriate shape

There are 20 amino acids and 64 codons

This means several different codons can specify the same amino acid

There are several cases in which the third base of the codon can be changed, but the same amino acid will be coded for (silent mutation)› Means the point mutation won’t affect the

final protein

› Ex: mRNA GCU, GCA, GCG, GCC all code for the amino acid alanine

A U G G G C U C C A U C G G C G C A U A AmRNA

start

codon

codon 2 codon 3 codon 4 codon 5 codon 6 codon 7codon 1

methionine glycine serine isoleucine glycine alanine stop

codonprotein

Primary structure of a protein

aa1 aa2 aa3 aa4 aa5 aa6

peptide bonds

T A C C C G A G G T A G C C G C G T A T TDNA