8.4 DNA Transcription

8.5 Translation

8.4 – DNA Transcription

KEY CONCEPT Transcription converts a gene into a single-stranded RNA molecule.

Gene: A specific region of DNA that codes for a particular protein

RNA carries DNA’s instructions.

The central dogma states that information flows in one direction, starting with DNA

Through Transcription, DNA changes into RNA

Through Translation, RNA changes into a Protein.

• The central dogma includes three processes.

• DNA contains the instructions to make proteins. RNA is a link between DNA and proteins.

replication

transcription

translation

– Replication– Transcription– Translation

• RNA differs from DNA in three major ways.

1. DNA has a deoxyribose sugar, RNA has a ribose sugar.

2. RNA has uracil instead of thymine (found in DNA)

1. A pairs with U

3. DNA is a double stranded molecule, RNA is single-stranded.

Transcription makes three types of RNA.

Transcription copies a piece of DNA (a gene) to make a strand of RNA.

• 1. Transcription is catalyzed (run) by RNA polymerase (an enzyme).

RNA polymerase and other proteins form a transcription complex. The transcription complex recognizes the start of a gene and unwinds a segment.

start site

5. nucleotides

transcription complex

RNA polymerase bonds the nucleotides together.

The DNA helix winds again as the gene is transcribed.

2. ONE strand of DNA serves as a template Nucleotides pair with one strand of the DNA.

4. DNA

7. RNA polymerase moves along the DNA

3. The RNA strand detaches from the DNA once the gene is transcribed.

6. RNA

• Transcription makes three types of RNA.

– Messenger RNA (mRNA) carries the message that will be translated to form a protein.

– Ribosomal RNA (rRNA) forms part of ribosomes where proteins are made.– Transfer RNA (tRNA) brings amino acids (protein building blocks) from the

cytoplasm to a ribosome to build the protein.

The transcription process is similar to replication. Transcription and replication both involve complex

enzymes and complementary base pairing. Both processes take place in the nucleus.

The two processes have different end results. Replication copies

all the DNA;transcription copiesa gene.

Replication makesone copy;transcription canmake many copies.

growing RNA strands

DNA

onegene

8.5 – TranslationKEY CONCEPT

Translation converts an mRNA message into a polypeptide, or protein.

Amino acids (protein building blocks) are coded for by mRNA base sequences.

A codon is a sequence of three nucleotides that codes for an amino acid.

codon formethionine (Met)

codon forleucine (Leu)

• The genetic code matches each codon to its amino acid or function.

– one start codon (AUG), codes for methionine and to start translation

– three stop codons (UAA, UAG, UGA) signal the end of a chain of amino acids.

The genetic code matches each RNA codon with its amino acid or function.

• Reading frame: multiple codons that code for a chain of amino acids• A change in the order in which codons are read changes the resulting

protein – this is why having a clear “start” and “stop” is important

• Common language: Regardless of the organism, codons code for the same amino acid.

Amino acids are linked to become a protein. An anticodon is a set of three nucleotides that is

complementary to an mRNA codon. An anticodon is carried by a tRNA. tRNA carries amino

acids from cytoplasm to the ribosome to become part of the growing protein.

EXAMPLE:mRNA codon=GUUtRNA anticodon=CAAAmino acid=Valine

• Ribosomes are the site of protein synthesis. They are located in the cytoplasm & on the rough ER.

• Ribosomes are made of rRNA & proteins. They have a large & small subunit.• Helps form peptide bonds between amino acids.

Where does translation happen?

1. For translation to begin, tRNA binds to a start codon (Met in picture) and signals the ribosome to assemble.

– A complementary tRNA molecule binds to the exposed codon (Leu in picture), bringing its amino acid close to the first amino acid.

How does translation happen?

2. The ribosome helps form a polypeptide bond between the amino acids.

The ribosome pulls the mRNA strand the length of one codon.

3. The now empty tRNA molecule exits the ribosome.

– A complementary tRNA molecule binds to the next exposed codon.– Once the stop codon is reached, the ribosome releases the protein

and disassembles.