1 RNA and Protein Synthesis

2 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.

3 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.

4 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.

5 Types of RNA The RNA produced during transcription is modified into 3 basic types: –1. messenger RNA (mRNA) is responsible for copying one strand of DNA in the nucleus and carrying that information to the ribosomes in the cytoplasm. –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.

6 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 – the process where a portion of the DNA sequence is copied into a complementary RNA sequence. –2. Translation – the decoding of an mRNA message into a polypeptide chain (protein).

7 Transcription and Translation Transcription occurs on the DNA in the nucleus. Transcription Demo Translation is completed by the ribosomes located in the cell’s cytoplasm. All three types of RNA work together during translation to produce polypeptides (proteins).

8 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 1 amino acid. Each codon is matched to an anticodon, or coplementary sequence on the tRNA to determine the order of the amino acids. Translation Demo

9 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. estion.png

10 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

11 What is a gene mutation? Sometimes during replication, an error is made that causes subsequent changes in the mRNA and proteins that are made using that DNA. These errors or changes are called mutations.

12 Types of Gene Mutations Point mutation: –changes in only 1 or a few nucleotides of DNA. –Substitutions, in which one base is changes to another, are a type of point mutation. Frameshift mutation: –When one base is deleted (deletion) or added (insertion) into a DNA sequence. –This causes the entire sequence of codons to shift over by one base.

13 What are Chromosomal Mutations? Mutations involving a long segment of DNA. –These mutations can involve deletions, insertions, or inversions of sections of DNA. –In some cases, deleted sections may attach to other chromosomes, changing both the chromosomes that lost the DNA and the one that gained it, also called a chromosomal rearrangement.

14 What is the significance of mutations? Mutations in DNA cause changes in the sequence of amino acids which ultimately creates changes in proteins and their function.