DNA and Protein Synthesis A blueprint for life

Protein Synthesis is divided into 2 parts in Eukaryotes:Transcription and Translation Transcription is the process wherein the information coded in DNA’s nucleotide sequence is copied onto a strand of Messenger RNA. The language of the information remains that of nucleotide sequencing. Transcription takes place within the nucleus. Translation is the process by which the language of the genetic information is changed from the language of nucleotides to the language of amino acids. Translation takes place in the cell’s cytoplasm using organelles called ribosomes. Translation involves all three forms of RNA: Messenger RNA, Transfer RNA, and Ribosomal RNA.

DNA strand 1 Transcription happens in the nucleus. The DNA strands separate and RNA polymerase acts on only one of the DNA strands. RNA polymerase attaches to one strand of DNA and adds corresponding RNA nucleotides until an m-RNA strand is completed. The m-RNA strand that is built in the nucleus is complementary to the active DNA strand. When the m-RNA strand is complete, it leaves the nucleus through one of the nuclear pores. DNA makes all 3 types of RNA this way. DNA strand 2 RNA nucleotides RNA polymerase

Translation needs 3 forms of RNA. In eukaryotes, the preliminary m-RNA strand is processed in the nucleus, where enzymes remove introns and assemble exons to make a completed m-RNA strand. The m-RNA strand then leaves the nucleus through a nuclear pore and moves into the cytoplasm for the next step in protein synthesis:Translation. Translation is the actual making of a polypeptide chain, which is directed by m-RNA, but also needs r-RNA and t-RNA. Translation happens on ribosomes, which are made of ribosomal RNA (r-RNA). Ribosomes link amino acids in an orderly way into polypeptide chains. Ribosomes are composed of two subunits made of r-RNA. They snap together over the m-RNA and have 2 binding sited for t-RNA. Transfer RNA (t-RNA) picks up amino acids and carries them to the ribosome, where they are attached together. m-RNA t-RNA r-RNA t-RNA site m-RNA binding site Amino acid

The genetic code determines amino acid sequences of polypeptide chains. In the genetic code, which is the same in all living things, nucleotide triplets code for certain amino acids. The sequence of these triplets in an m-RNA chain determines the order of amino acids in a polypeptide chain. It is called a codon. There are 20 amino acids that are found in the proteins of living things. These triplets code for all of them. There are many repeats that have appeared in the code over the billions of years that living things have evolved. The dictionary of the genetic code is usually shown as a series of m- RNA codons. Use the genetic code shown in your textbook to determine which amino acids are specified by the following m-RNA codons. 1) AUG 2) CUU 3) GCA 4) UAU 5) GAC Use the genetic code to identify the three “stop” codons.

There are many types of t-RNA. There are 64 kinds of t-RNA. They all have the same basic structure: a coiled chain of nucleotides with a triplet of exposed nitrogen bases at one end and an attachment site for an amino acid at the other end. The triplet of exposed nitrogen bases is called an anticodon. When the anticodon binds with a messenger RNA codon inside of a ribosome, it brings the amino acid into place so that it can join with other amino acids to build the polypeptide chain. Check your work from the last slide using the t-RNAs to the right. AUG CUU GCA UAUGAC MetAlaTyrLeuAsp Stop codons: UAA UAG UGA

Translation happens in the cytoplasm. First, the two r-RNA subunits join together around the end of a m- RNA strand to form a functional ribosome. As the strand of m-RNA moves through the ribosome, t-RNAs bind in the other two binding sites if their anticodons match the m-RNA codons. t-RNA site m-RNA binding site Gly Met Tyr Ala Leu

Translation continues in the cytoplasm. The t-RNAs bring amino acids into position so that they can bond with each other, lengthening the amino acid chain. As the amino acid is attached, the ribosome moves over one codon, the leading t-RNA is released to get another amino acid, and another t- RNA moves into the free site. Ala Leu Tyr Met Ser Ala Gly t-RNA site m-RNA binding site Trp Lys

Translation concludes. This movement of the ribosome, attachment of t-RNAs, and lengthening of the amino acid chain continues until a stop codon is reached. The stop anticodon has no amino acid bound to it. As it moves through the ribosome, the t-RNAs detach from the ribosome and the polypeptide chain bends and folds into a 3-D shape depending on its amino acid sequence. t-RNA site m-RNA binding site Tyr Leu Ile Ser Ala Gly Trp Lys