. Translation By Jane Nyandele

The Central Dogma (of molecular biology) This is the concept that the sequence of bases on DNA defines the sequence of bases on RNA, which then defines the sequence of amino acids in proteins. Information always flows in the direction from DNA to protein. In some cases, it flows back from RNA to DNA (i.e. reverse transcription). Information however NEVER flows from protein back to RNA or DNA.

The Genetic Code The Genetic code is the way in which the nucleotide sequence in nucleic acids (DNA, RNA) specifies the amino acid sequence in proteins. Its Nature  It is a triplet code, where by groups of 3 nucleotides, called codons, specify the amino acid sequence.  There are 64 codons (4 3 =64) and 20 amino acids. Because many of the 64 codons specify the same a.a, the genetic code is degenerate (i.e. it has redundancy). Multiple codons that code for same a.a usually differ only in their third position.  Only 61 of the 64 codons code for a.a. the 3 stop codons are nonsense codons a.k.a terminator codons.  It is nearly universal. i.e. same codons code for same a.a in nearly all organisms.

UCAG U C A G G A C U G A C U G A C U G A C U UUUUUU UUCUUC UUAUUA UUGUUG CUUCUU CUCCUC CUACUA CUGCUG AUUAUU AUCAUC AUAAUA AUGAUG GUUGUU GUCGUC GUAGUA GUGGUG phe leu ile met (start) val UCUUCU UCCUCC UCAUCA UCGUCG CCUCCU CCCCCC CCACCA CCGCCG ACUACU ACCACC ACAACA ACGACG GCUGCU GCCGCC GCAGCA GCGGCG ser pro thr ala UAUUAU UACUAC UAAUAA UAGUAG CAUCAU CACCAC CAACAA CAGCAG AAUAAU AACAAC AAGAAG AAAAAA GAUGAU GACGAC GAAGAA GAGGAG tyr stop his gln asn lys asp glu UGUUGU UGCUGC UGAUGA UGGUGG CGUCGU CGCCGC CGACGA CGGCGG AGUAGU AGCAGC AGAAGA AGGAGG GGUGGU GGCGGC GGAGGA GGGGGG cys stop trp arg ser arg gly First Base Third Base Second Base Virtually all organisms share the same genetic code “unity of life”

Primary molecules involved in Translation mRNA This is the RNA molecule that carries information from DNA in the nucleus to the cytoplasm where translation is carried out. tRNA This is the adaptor molecule that delivers a.a. to the ribosomes and decodes information on mRNA. Each codon on the mRNA is recognized by a complementary triplet known as the anticodon on the end of a tRNA molecule. Ribosomes Protein synthesizers of the cell. Has 3 sites: the A site, E site and P site.

. The A site (Aminoacyl site) is most frequently occupied by aminoacyl-tRNA which acts as an acceptor for the growing protein during peptide bond formation. The P site (Peptidyl site) is most frequently occupied by peptidyl-tRNA which carries the growing peptide chain. The E site (Exit site) usually harbors decylated tRNA on transit out from the ribosome.

Translation Defn: Translation is the synthesis of a protein molecule from a mRNA template. Genes and proteins are written in different “languages”. Genes are constructed out of nucleotides, while proteins are constructed out of amino acids. In order for a biological cell to make proteins, genetic information from DNA has to be translated.

An Exercise in Translating the Genetic Code Figure 10.8B Start codon RNA Transcribed strand Stop codon Translation Transcription DNA Polypeptide

Mechanism of Translation Translation involves 3 steps: Initiation, Elongation and Termination. 1.Initiation Process begins with the binding of the small ribosomal subunit to a sequence on the mRNA located a few nucleotides from the initiation codon, AUG. once the small subunit has bound, a tRNA molecule recognizes and binds to the initiator codon AUG. Next, the large subunit binds, forming what is known as the translation initiation complex. After formation of the initiation complex, the tRNA occupies the P site (peptidyl site) of the ribosome and the A site is left empty. The entire initiation process is facilitated by extra proteins, called initiation factors.

2. Elongation After formation of the initiation complex, an aminoacyl tRNA with a complementary anticodon sequence can bind to the mRNA codon passing through the A site. The binding is aided by elongation factors. With tRNA bearing a chain of amino acids in the P site and tRNA containing a single amino acid in the A site, a peptide bond is formed between the carboxyl group in the peptide chain located in the P site and the amine group on the amino acid in the A site. The bond formation is catalyzed by enzyme peptidyl transferase.

Peptide bond formation

The result of this is that the peptide chain shifts over to the A site. The tRNA in the A site becomes peptidyl RNA, and shifts over to the P site. Meanwhile, the ribosome engages in a process called translocation. Aided by elongation factors, the ribosome moves so that a new mRNA codon is accessible in the A site. When the A site is open again, the next appropriate aminoacyl tRNA can bind there and the same reaction takes place again. This process repeats, creating a polypeptide chain in the P site of the ribosome

3. Termination When the stop codon is reached in the process, a release factor reads the triplet, and the polypeptide synthesis ends. The polypeptide is released from the tRNA, the tRNA is released from the ribosome, and the two ribosomal subunits separate from the mRNA.

Mutations A mutation is a permanent alteration to the nucleotide sequence of the genome of an organism. What causes mutations? What are the consequences of mutations?  Small-Scale Mutations 1.Substitution mutations These are often caused by chemicals or malfunction of DNA replication, and result in exchange of a single nucleotide for another Silent Mutations-Code for a similar amino acid Missense mutations- Code for a different amino acid Nonsense mutations- Code for a stop codon and stop the translation process

. 2 Insertions These types of mutations add one or more extra nucleotides into the DNA. They are caused by transposable elements as well as errors during replication. Insertions cause “frameshifts”, i.e. they alter the reading frame of the gene. e.g. the fat cat sat becomes thh efa tca tsa t 3 Deletions Deletions remove one or more nucleotides from the DNA. Like insertions, deletions alter the reading frame of the gene.  Large Scale mutations 1.Translocations Genetic material is exchanged between two or more non-homologous chromosomes. 2.