RNA and Protein Synthesis

The Function of DNA The DNA molecule contains all of your hereditary information in the form of genes. Genes are portions of the DNA molecule that code for the production of specific types of proteins. However, DNA is confined to the nucleus, while proteins are made by ribosomes in the cytoplasm. Thus, a messenger molecule is needed. http://www.accessexcellence.org/RC/VL/GG/images/genes.gif

Comparing DNA and RNA RNA is the nucleic acid that acts as a messenger between DNA and the ribosomes. The RNA produced during transcription is structurally different from DNA in 3 basic ways: 1. The sugar in RNA is ribose whereas the sugar in DNA is deoxyribose. 2. RNA is single stranded while DNA is double stranded. 3. RNA contains a base called uracil instead of thymine. http://www.dkimages.com/discover/previews/769/85011519.JPG

The Role of RNA RNA molecules have many functions, but in the majority of cells, most RNA molecules are involved in protein synthesis. Protein synthesis is the assembly of amino acids into proteins.

Types of RNA The RNA produced during transcription is modified into 3 basic types: 1. messenger RNA (mRNA) copies one segment (or gene)on one strand of DNA in the nucleus and carries that information to the ribosome. 2. ribosomal RNA (rRNA) makes up a large part of the ribosome and is responsible for reading and decoding mRNA. 3. transfer RNA (tRNA) carries amino acids to the ribosome where they are joined to form proteins. http://images.encarta.msn.com/xrefmedia/zencmed/targets/illus/ilt/T068340A.gif

Protein Synthesis During protein synthesis, genes copied onto RNA are expressed by the production of specific types of proteins. Protein synthesis involves two processes: 1. Transcription – process in which a RNA copy of a segment (gene) of DNA is made. 2. Translation – the decoding of mRNA message into a polypeptide chain (protein).

Transcription Transcription occurs on the DNA in the nucleus. RNA Polymerase binds to DNA at a promoter and separates the DNA strands. RNA Polymerase uses one strand of DNA as a template to assemble complementary nucleotides into a strand of RNA. Transcribed mRNA leaves the nucleus and goes to the ribosome.

Transcription Practice DNA: AACTGGTACGTA mRNA: UUGACCAUGCAU DNA: ATGTTCACTGGA mRNA: UACAAGUGACCU DNA: GTTCATGCATAT mRNA: CAAGUACGUAUA DNA: TGCCATCGATTC mRNA: ACGGUAGCUAAG DNA: CTGATGCTGATA mRNA: GACUACGACUAU

Decoding mRNA The sequence of bases in an mRNA molecule serves as instructions for the order in which amino acids are joined to produce a polypeptide. Ribosomes decode these instructions by using codons, sets of 3 bases that each code for one amino acid. Each codon is matched to an anticodon on the tRNA to determine the order of the amino acids. http://www.gwu.edu/~darwin/BiSc150/One/codon.gif

Translation-occurs in the cytoplasm at the ribosomes. mRNA attaches to a ribosome Translation begins at AUG, the start codon tRNA brings an amino acid to the ribosome The tRNA binds to the mRNA by matching its anticodon to the codon the mRNA The ribosome moves along the mRNA as each tRNA drops of its amino acid

The ribosome joins amino acids with peptide bonds and the tRNAs leave the ribosome The process continues until the ribosome reaches the STOP codon The result is a polypeptide. (protein)

Using a Codon Chart A codon chart can be used to determine the sequence of the amino acids in the polypeptide. The mRNA bases or CODONS are used to find the amino acid. http://www.safarikscience.org/biologyhome/7_dna/codon_qu estion.png

CODON CHART

Decoding Practice For the following examples, give the appropriate mRNA sequence and amino acid sequence. (Remember: U replaces T in mRNA.) Example 1: DNA: TAC GCA TGG AAT mRNA: AUG CGU ACC UUA Amino Acids: Met Arg Thr Leu Example 2: DNA: CGT GGA GAT ATT mRNA: GCA CCU CUA UAA tRNA: CGU GGA GAU AUU Amino Acids: Ala Pro Leu stop