Protein Synthesis (Gene Expression)

Review Nucleotide sequence in DNA is used to make proteins that are the key regulators of cell functions. Proteins  polymers of amino acids The sequence of nucleotides in DNA contains information for assembling the string of amino acids that make up a single protein.

RNA-Ribonucleic Acid A. Differences between DNA and RNA 1. RNA is single-stranded (it looks like ½ of a zipper. 2. The sugar in RNA is ribose 3. RNA contains URACIL instead of thymine. Uracil pairs with adenine.

RNA-Ribonucleic Acid B. Role of RNA 1. “Worker” for protein synthesis while DNA is the “Commander-in-Chief” RNA takes the instructions on how a protein should be built and then assembles the protein, amino acid by amino acid.

RNA-Ribonucleic Acid C. Types of RNA 1. M ESSENGER RNA (mRNA)- Bring information from the DNA in the nucleus to the cytoplasm 2. R IBOSOMAL RNA (rRNA)-Subunit of ribosomes which clamps onto the mRNA and use its information to assemble the amino acids in the correct sequence. 3. T RANSFER RNA (tRNA)-Supplier of amino acids to the ribosome

Protein Production and the Genetic Code A. Transcription Location=nucleus The information found in a gene in DNA is transcribed into an mRNA molecule Steps: 1. RNA polymerase binds to the gene’s “promoter” or “start” signal 2. RNA polymerase unwinds and separates the two strands of DNA 3. RNA polymerase adds the complementary RNA nucleotides as it “reads” the gene (U=A; G=C) 4. Transcription proceeds until the RNA polymerase reaches a “stop” signal. At this point, it detaches and the mRNA molecule detaches from the DNA strand. The DNA strand will twist back up and the bonds will be restored.

Transcription RNA DNA RNA polymerase Adenine (DNA and RNA) Cystosine (DNA and RNA) Guanine(DNA and RNA) Thymine (DNA only) Uracil (RNA only)

Protein Synthesis: Transcription

Protein Production and the Genetic Code B. The nucleotide sequence transcribed from DNA to a strand of mRNA acts as a genetic message. This message is written in a language that uses nitrogen bases as its “alphabet”. The language of proteins uses an “alphabet” of amino acids. A code is needed to convert the language of mRNA into the language of proteins.

Protein Production and the Genetic Code B. continued… There are 20 different amino acids, but only 4 types of N bases in mRNA. How can these bases form a code for proteins?

Protein Production and the Genetic Code A group of three nucleotides codes for one amino acid. Each set of three N bases that codes for an amino acid is called a codon. The order of nitrogen bases in the mRNA will determine the type and order of amino acids in a protein 64 combinations are possible when a sequence of 3 bases is used. Thus, 64 different mRNA codons are in the genetic code.

Protein Synthesis: Translation

SUGAR-PHOSPHATE BACKBONE B A S E S H P O O HO O O CH 2 NH 2 N NH N N HOH P O O HO O O CH 2 NH 2 N N N N H P O OH HO O O CH 2 NH 2 N N N N O A Codon Guanine Adenine Arginine

The Genetic Code

Protein Production and the Genetic Code Some codons do not code for amino acids; they provide instructions for assembling the protein. UAA is a stop codon indicating that protein production should stop at that point. AUG is a start codon as well as being the codon for the amino acid methionine.

Protein Production and the Genetic Code As you can see from the genetic code chart, more than one codon can code for the same amino acid. However, for any one codon, there can only be one amino acid. The genetic code is nearly universal-the same codon can code for the same amino acid in many different organisms

Protein Synthesis: Translation

Protein Production and the Genetic Code C. Translation Location=cytoplasm 1. In eukaryotic cells, mRNA leaves the nucleus through an opening in the nuclear membrane and travels to the cytoplasm. 2. When the strands of RNA arrive, ribosomes attach to them like clothespins clamped onto a clothesline. 3. The process of converting the information in a sequence of amino acids that make up a protein is known as translation

DNA Cytoplasm Nucleus Eukaryotic Translation Export G AAAAAA RNA Transcription Nuclear pores G AAAAAA RNA Processing mRNA Ribosome

Protein Production and the Genetic Code 1. Role of Transfer RNA (tRNA) For proteins to be built, the 20 different amino acids dissolved in the cytoplasm must be brought to the ribosomes -> this is the job of tRNA. Each tRNA molecule attaches to only one type of amino acid Correct translation of the mRNA message depends on upon the joining of each mRNA codon with the correct tRNA molecule On the opposite side of the tRNA molecule from the amino acid attachment site, there is a sequence of 3 nucleotides that are the complement of the nucleotides in the codon. These 3 nucleotides are called an anticodon because they bond to the codon on the mRNA by the process of base pairing

Protein Production and the Genetic Code 2. Translating the mRNA code a. As translation begins, a tRNA molecule brings the first amino acid to the mRNA strand that is attached to the ribosome. b. The anticodon forms a temporary bond with the codon of the mRNA strand. This places the amino acid in the correct position for forming a bond with the next amino acid.

Protein Production and the Genetic Code c. The ribosome slides down the mRNA chain to the next codon and a new tRNA molecule brings another amino acid. d. The amino acids form peptide bonds, the first tRNA releases its amino acid and detaches from the mRNA. This tRNA molecule is now free to pick up and deliver another molecule of its specific amino acid to a ribosome.

Messenger RNA Messenger RNA is transcribed in the nucleus. Transfer RNA The mRNA then enters the cytoplasm and attaches to a ribosome. Translation begins at AUG, the start codon. Each transfer RNA has an anticodon whose bases are complementary to a codon on the mRNA strand. The ribosome positions the start codon to attract its anticodon, which is part of the tRNA that binds methionine. The ribosome also binds the next codon and its anticodon. mRNA Start codon Ribosome Methionine Phenylalanine tRNA Lysine Nucleus mRNA Translation

Protein Production and the Genetic Code e. A chain of amino acids (polypeptide chain) continues to form f. When a stop codon is reached, translation ends, and the amino acid strand is released from the ribosome. g. The amino acid chains then twist and curl into complex three-dimensional shapes and become proteins.

Forming the Polypeptide Chain The Polypeptide “Assembly Line” The ribosome joins the two amino acids— methionine and phenylalanine—and breaks the bond between methionine and its tRNA. The tRNA floats away, allowing the ribosome to bind to another tRNA. The ribosome moves along the mRNA, binding new tRNA molecules and amino acids. mRNA Ribosome Translation direction Lysine tRNA Ribosome Growing polypeptide chain mRNA Completing the Polypeptide The process continues until the ribosome reaches one of the three stop codons. The result is a growing polypeptide chain.

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