T RANSCRIPTION & T RANSLATION

C ENTRAL D OGMA Information flows in one direction from DNA to RNA to proteins. This is known as the central dogma.

RNA RNA (ribose nucleic acid) is similar to DNA in that it consists of chains of nucleotides which contain a phosphate group, a sugar and a nitrogen-containing base.

D IFFERENCE BETWEEN RNA AND DNA There are three specific differences: RNA contains ribose instead of deoxyribose RNA has uracil instead of thymine RNA is single stranded not double stranded

T RANSCRIPTION Transcription is the process of copying a strand of DNA to make a complementary RNA strand. During transcription, a gene is transcribed into an RNA message. RNA polymerases catalyse the reactions.

S TEPS OF TRANSCRIPTION RNA polymerases recognize the start site of a gene. A complex of RNA polymerases and proteins assemble on the DNA strand and begin to unwind the DNA strands RNA polymerases use one strand of DNA to make a complementary strand of RNA Uracil pairs with adenine, while cytosine pairs with guanine

RNA hangs freely while DNA zips back together RNA detaches from DNA once the entire gene has been transcribed

T HREE TYPES OF RNA 1) mRNA (messenger RNA): is an intermediate message that is used to translate to proteins. 2) rRNA (ribosomal RNA): forms part of ribosome. 3) tRNA (Transfer RNA): brings amino acids from the cytoplasm to the ribosome to help make growing proteins.

Transcription allows for a cell to adjust to demands by making certain quantities of proteins, by producing more RNA.

T RANSLATION Translation is the process which converts mRNA to polypeptides (which make up proteins). The four nucleotides code for all 20 amino acids.

A CODON is a three letter sequence that codes for a specific amino acid.

Many amino acids coded for by more than one codon. There are also start and stop codons.

R IBOSOMES Ribosomes are the sites of proteins synthesis. Ribosomes consist of two subunits: a large and a small subunit. tRNA binds to the large subunit. mRNA binds to the small subunit.

tRNA acts as an adaptor between mRNA and amino acids. tRNA is needed to carry free floating amino acids from the cytoplasm to the ribosome. tRNA folds into an L shape. One end is attached to an amino acid, while the other end (known as the anticodon) recognizes specific codons.

An anticodon is a set of three nucleotides that are complementary to an mRNA codon. Translation occurs in the cytoplasm. Before translation can occur, an mRNA strand must bind to the small subunit of the ribosome. Next, tRNA must bind with methionine to the start codon. This signals the large subunit, with three binding sites for tRNA, to join the ribosome.

The ribosome pulls the mRNA strand through one codon at a time. As the strand moves, the start codon and its complementary tRNA move into the second site of the large subunit. This leave the first site open for the next mRNA codon.