DNA and Protein Synthesis

DNA Does 2 Important Things in a Cell: 1)DNA is capable of replicating itself. Every time a cell divides, each DNA strand makes an exact copy of itself. 2)DNA directs the production of proteins in the cell. DNA contains the instructions on how to make proteins.

The Importance of Proteins Proteins are the workhorses of the cell. They build all of the important structures and carry on most of the important cellular functions. What types of proteins are made determine everything about the organism and how it functions.

Review of Protein Structure Proteins are large polymers made of long chains of amino acids. Amino Acids bind together with peptide bonds. A protein’s final shape determines its function. This shape is determined by its amino acid sequence.

Protein Structure Proteins are made by ribosomes. Amino acids are put together in a long chain which then folds and bends to determine the final shape. Remember: Shape equals function! So, it is VERY important that the amino acid sequence is correct!

What determines the correct amino acid sequence in a protein? The DNA in a cell determines the sequence of amino acids in a protein. The DNA sequence is a code that determines which amino acid is first, which is second, etc. DNA determines protein:

DNA leads from afar….. The DNA in a cell’s nucleus determines the sequence of amino acids that are put together at the ribosome. Yet, the DNA never leaves the nucleus! How does the DNA do this?

Chromosomes to Genes

Messenger RNA (mRNA) DNA is able to “tell” the ribosome how to build the protein by sending a messenger…. This comes in the form of an RNA molecule. Therefore, this is called messenger RNA, or mRNA.

From DNA to Protein Protein Synthesis has 2 main processes: 1) Transcription (“to write”) –making a mRNA molecule 2) Translation – (“to read”) making the protein, using the mRNA as a guide. Together, these are called Gene Expression

RNA: A Major Player Three types of RNA are used in protein building mRNA=messenger RNA (made in nucleus— carries the genetic code) tRNA=transfer RNA (brings amino acids to the ribosome) rRNA=ribosomal RNA (what ribosomes are made of) Ribonucleic Acid

RNA Vs. DNA RNA differs from DNA in 3 ways: RNA is single- stranded; DNA is double-stranded RNA has Ribose; DNA has Deoxyribose RNA has Uracil; DNA has Thymine Uracil is found in RNA only

RNA Vs. DNA

Transcription: Reading the Gene During transcription, the information in a particular segment of DNA (a gene) is copied into mRNA Step 1: RNA polymerase binds to a specific sequence of bases on the DNA called the Promoter. This is the start code.

Transcription: Reading the Gene Step 2: Helicases unwind the DNA strand in front of the promoter region. Step 3: RNA polymerase adds and links complementary RNA bases on one side of the DNA, following base pairing rules. (Exception: Adenine in DNA bonds to Uracil)

Transcription The RNA polymerase eventually reaches a “stop” location in the DNA. This stop signal is a sequence of bases that marks the end of each gene. The result is a single strand of RNA

The Genetic Code To make a correct protein, the amino acids must be put in the proper sequence. Each segment of 3 nucleotides in DNA codes for 1 amino acid. Each triplet of DNA bases makes a triplet of mRNA bases—called a Codon.

Codons of mRNA What amino acid comes from the code AUC? GGG? CGA?

Translation: RNA to Proteins Translation is the process of converting the “language” of RNA into the “language” of proteins (amino acid sequences.) Translation involves mRNA, tRNA, and rRNA.

tRNA: Transfer RNA Each tRNA is folded into a compact shape. There are 2 important places on each tRNA: Terminal end—has 3 bases which match up to mRNA. This is the Anticodon. Amino Acid attachment site

Translation, Step 1: Start Codon After leaving the nucleus, the mRNA joins with a ribosome and tRNA. The mRNA start codon, AUG, signals the beginning of a protein. The tRNA carrying Methionine binds to the start codon.

Translation Step 2:A new tRNA arrives. A new tRNA arrives and binds to the next codon on the mRNA. A peptide bond forms between the first amino acid and the second amino acid. The first tRNA detaches and leaves its amino acid behind.

The Ribosome Moves down the mRNA The ribosome moves one codon down the mRNA strand. Because the anticodon (on the tRNA) is attached to the codon, they move together as a unit. The amino acid chain grows as each new one binds.

Translation, Step 3: Releasing the protein The process ends when a stop codon is reached. The amino acid chain is released, the ribosome complex and mRNA strand fall apart.

Protein Synthesis Animations ar/theory/translation/trans1.swf ar/theory/translation/trans1.swf h20rGY&feature=related h20rGY&feature=related ature=endscreen&v=Ikq9AcBcohA ature=endscreen&v=Ikq9AcBcohA