DNA and RNA Chapter 12
History of DNA Late 1800 s scientists discovered that DNA is in the nucleus of the cell 1902 Walter Sutton proposed that hereditary material resided in the chromosomes in the nucleus 1928 Frederick Griffith found out that hereditary material was transmitted somehow from one organism to another 1952 Hershey and Chase found that DNA was the hereditary substance as opposed to a protein
Eukaryotic DNA Structure 1950 s Erwin Chargaff came up with Chargaff s Rules: A-T are present in same amounts C-G are present in same amounts Remember this rhyme: A to the T and C to the G Franklin and Wilkins produced an x-ray crystallography of a DNA molecule 1953 Watson and Crick proposed that DNA resembles a twisted ladder, and named it a double helix.
Eukaryotic DNA Structure DNA consists of long strands of nucleotides. A nucleotide contains the following: A sugar (deoxyribose) A phosphate group A nitrogen base (adenine, thymine, cytosine, guanine) Nucleotide
DNA in the Nucleus Scientists estimate the if you were to stretch the DNA out in each cell into one line, it would measure 3 meters in length! So how is so much DNA packed into one cell? It is supercoiled! The ladder is first twisted, then it winds around histones (proteins), then it coils again until it forms the familiar X shaped chromosomes.
DNA Replication When a cell divides, how is more DNA made? DNA makes copies of itself through a process called replication: First, the DNA helix unwinds with the help of an enzyme called DNA helicase. Next, enzymes (called DNA polymerase) break the hydrogen bonds that hold the base pairs together, sort of like taking a chainsaw and slicing down through the middle of the ladder s rungs. Then, each strand serves as a template for a new strand. Another enzyme moves along the separated DNA strands, and matches bases from the parent strand to the new complementary strand. Last, hydrogen bonds form between bases, and you have 2 new DNA molecules!
The Genetic Code What is a gene? A region of DNA on a chromosome that controls the production of a protein, of which we have many in our bodies. What does a protein do? Proteins are used in various body functions, and each protein has a specific job in our bodies: Can be enzymes which assist chemical reactions Can transport substances from one place to another Are part of our structural support Can be hormones Can be part of the body s defense against disease Many more! Therefore, our bodies must produce many different types of proteins!
Protein Synthesis Proteins are manufactured by our bodies in a process called protein synthesis. It is a two part process that involves RNA and DNA.
RNA vs. DNA Before we begin this process, let s review the differences between RNA and DNA: Number of Strands Type of Sugar Bases Present DNA 2 Deoxyribose A, T, C, G RNA 1 Ribose A, U, C, G
Types of RNA Also before we begin, let s review the three types of RNA: Messenger RNA (mrna) carries the coded instructions for protein synthesis from the DNA in the nucleus to the ribosome in the cytoplasm Transfer RNA (trna) brings the amino acids to the ribosome in the correct order so that they can be built into the new protein Ribosomal RNA (rrna) works with several proteins to make up the structure of the ribosomes
What is a Codon? One last quick review before we delve into protein synthesis: each amino acid is coded for by a sequence of 3 bases called a codon. Each codon produces a specific amino acid, depending on the sequence of the bases. We can figure out which amino acid will be produced by looking at the codon wheel. Once you have a bunch of different amino acids produced, they are joined together to form a protein!
Protein Synthesis: The Process There are two main parts to the process of protein synthesis: Transcription the process of transferring information from a strand of DNA to a strand of RNA in the nucleus. Translation the process where ribosomes synthesize proteins with the help of other molecules in the cytoplasm.
Step 1 - Transcription Occurs in the nucleus Here are the 5 steps involved: The DNA strand in nucleus unwinds and separates. The ½ of the strand that contains the gene for a protein acts as the template. An enzyme (called RNA polynerase) matches RNA base pairs with their complementary DNA base pair. The nucleotides of the RNA are bonded together to form a strand of mrna, which contains the complete genetic code! mrna leaves the nucleus and moves into the cytoplasm for the second step.
Step 2 - Translation Occurs in the cytoplasm. Here are the steps involved: The first codon of the mrna attaches to a ribosome Then, trna molecules, each carrying a specific amino acid, approaches the ribosome. The trna with the complementary anticodon pairs with the mrna codon, joining together. Often, the first codon to be translated is the start codon, which tells the whole process to begin. The mrna then slides along the ribosome to the next codon, and the process is repeated until a stop codon is reached. Each amino acid produced is joined with the next one, until you have a long string of amino acids (polypeptide). This is a protein!
Prokaryotic DNA So now we have talked about eukaryotic DNA. Let s now concentrate on prokaryotic DNA. Remember that in bacteria, DNA exists in two forms: Chromosome is a doublehelix of DNA in a closed loop Plasmid a circular piece of DNA separate from the chromosome
Chromosome Replication Bacterial chromosomes replicate themselves in a process called binary fission. Here s how it happens:
Changes in the Genome There are many kinds of mutations that can happen in your genes: Point mutations Frameshift mutations Chromosomal mutations
Point Mutations Point mutations are mutations that occur in one point in the DNA.
Frameshift Mutations Frameshift mutations occur when a nucleotide is added or deleted, causing all nucleotides behind them to be different.
Chromosomal Mutations Chromosomal mutations are changes in the number or structure of chromosomes.
Why Are Mutations Important? Some mutations have no effect at all. Some, however, can cause too much or not enough proteins to be formed. Harmful mutations can cause cancer. Mutations are what cause changes in people s DNA over time.
Gene Regulation Not all genes we have are expressed, or turned on, all of the time. Example: genes that code for liver enzymes are NOT turned on in nerve cells. Each gene is controlled individually. A cell knows to turn a gene on or off by certain regulatory sequences in the DNA that signal the beginning of transcription.
The End!