Chapter 11 DNA

What is DNA? Living things need proteins to survive. –most proteins are enzymes DNA provides the complete set of instructions for making all the proteins for an organism.

The structure of DNA DNA is very large - It is a nucleic acid that is made up of subunits called nucleotides. Nucleotides are made up of three parts: –A carbohydrate called deoxyribose –A phosphate group –A nitrogen base (4 possible types)

The structure of DNA There are 4 possible types of nitrogen bases in a nucleotide. The nitrogen bases are: –Adenine (A) –Guanine (G) –Cytosine (C) –Thymine (T) Nucleotides are named from the nitrogen base they have.

The structure of DNA DNA is composed of many nucleotides joined together. –the sequence (order) of the nucleotides is the code for making proteins The phosphate group of one nucleotide bonds to the deoxyribose sugar of an adjacent nucleotide. –This forms the skeleton of the DNA molecule called the sugar-phosphate backbone. The 4 nitrogen bases stick out from the backbone like teeth in a zipper.

The Double Helix Watson and Crick discovered that DNA was made up of two long chains of twisted nucleotides called the double helix. Think of the double helix like a twisted zipper.

The bases will only bond to its complementary base: –Adenine bonds to Thymine –Guanine bonds to Cytosine –Because of the complementary base pairing, adenine and thymine always exist in equal amounts; and guanine and cytosine always exist in equal amount. –There are not always equal amounts of A-T to G-C however.

DNA Replication Replication: The process by which a DNA molecule makes an exact copy of itself. Replication occurs during Interphase – S of the cell cycle. Replication begins by the DNA molecule unzipping. –This is done by an enzyme which cuts the bonds between bases (A-T, G-C). As the DNA unzips, another enzyme bonds free-floating nucleotides to each of the exposed nucleotides in the single strands. Another enzyme bonds the sugar phosphate backbone until each parent strand has a new complementary daughter strand. Because the new DNA molecules have one old strand (parent), and one new strand (daughter), we say that replication is a semi-conservative process.

DNA is instructions for making proteins The sequence (or order) of nucleotides determines what type of protein will be made. A gene is a segment of DNA that holds the instructions for making a single protein. One chromosome can have thousands of genes in it.

Making proteins from genes DNA doesn’t make proteins - It gets help from a molecule called RNA. RNA differs from DNA in three ways: –It is usually single stranded rather than double stranded. –It has a ribose sugar rather than a deoxyribose sugar. –It contains the base uracil in place of thymine.

Making proteins from genes There are 3 types of RNA that help to build proteins. –messenger RNA (mRNA) – is a copy of the gene that is needed to make a protein. –ribosomal RNA (rRNA or ribosome) – reads the mRNA and assembles the protein. –transfer RNA (tRNA) – brings the amino acids to the ribosome for protein assembly.

Making proteins from genes A cell doesn’t always need every protein to be made all the time (that would be too much work). So the cell tells the DNA which gene it needs copies of, in order to make the protein that it needs at a particular time. In other words, the gene is “switched on” or activated when the cell needs that protein.

Transcription Transcription – the process in which enzymes make an mRNA copy of a gene. The mRNA is then moved into the cytoplasm for translation (the next step).

Translation Translation – is the process of reading the mRNA and assembling the protein. tRNA brings the amino acids to the ribosome for assembly into a protein.

Translation How does the tRNA know which amino acid to bring to the ribosome? Each 3 bases in the mRNA (called a codon), codes for a single amino acid. A tRNA molecule has three bases on it that are complementary to the codon, called an anticodon. Each tRNA carries only the amino acid that it’s anticodon specifies. The process continues until a codon that means “stop” is reached by the ribosome. Then the ribosome releases the amino acid chain. Once the amino acid chain is released, it folds into a distinct shape and is a protein.

Mutations Mutations are changes in a DNA sequence. As a result, the protein that is coded for is also changed. Sometimes DNA mutations are harmless and have no effect on the cell. More often, however, mutations can have negative and even life-threatening results.

Mutations Mutations can occur in body cells and gametes.Mutations can occur in body cells and gametes. If the mutation occurs in the gametes, the mutation is passed on to the offspring.If the mutation occurs in the gametes, the mutation is passed on to the offspring. When a mutation occurs in a body cell, the mutation is not inherited by the offspring, but the mutation is passed to the daughter cells when the mutated cell divides during mitosis.

Cancer Some mutations in body cells affect the genes that control mitosis. When this happens, the cell divides uncontrollably and rapidly; this is cancer. The following pictures are of breast cancer and are graphic in nature.

Types of Mutations A point mutation is a change in a single base pair (A-T, G-C).A point mutation is a change in a single base pair (A-T, G-C). A point mutation causes an incorrect amino acid to be inserted into the growing amino acid chain during translation; this results in a protein that does not function properly.

Types of Mutations A frameshift mutation is the insertion or deletion of a single base (A, T, G, C). A frameshift mutation causes each codon to be out of place by one letter; as a result every amino acid from the insertion / deletion is incorrect.

Frameshift mutation

Chromosomal Mutations Much larger mutations can occur at the chromosome level. Entire pieces of chromosomes can be broken off and lost during mitosis and meiosis. Often times, the broken pieces then rejoin the chromosome incorrectly. Changes to the structure of the chromosome are called chromosomal mutations.

Chromosomal Mutations Chromosomal mutations occur in all organisms, but are most common in plants. Few chromosomal mutations are passed to offspring, because the zygote with the chromosomal mutations usually dies. In cases where the zygote lives, the mature organism with a chromosomal mutation is often sterile.

Causes of Mutations Some mutations are spontaneous; they just happen by chance during DNA replication. However, many mutations are caused by environmental factors, including: Radiation (gamma rays, UV rays, X- rays) Chemicals (formaldehyde, dioxins, benzene, cyanide, asbestos) Any agent that can cause a mutation is called a mutagen.

Repairing DNA There are enzymes that proofread DNA and replace incorrect nucleotide sequences with the correct sequences.There are enzymes that proofread DNA and replace incorrect nucleotide sequences with the correct sequences. The repair enzymes work well, but are not perfect. The greater the damage, the harder it is to repair.

TEST – NEXT CLASS! Structure of DNA (double helix; nucleotide parts; 4 nitrogen bases and how they bond; Watson, Crick, Wilkins, and Franklin; sugar-phosphate backbone)Structure of DNA (double helix; nucleotide parts; 4 nitrogen bases and how they bond; Watson, Crick, Wilkins, and Franklin; sugar-phosphate backbone) DNA replicationDNA replication (how and when it happens) Activation, Transcription, Translation (gene, differences between RNA and DNA, mRNA, rRNA, tRNA, codon, anticodon, amino acid, protein)Activation, Transcription, Translation (gene, differences between RNA and DNA, mRNA, rRNA, tRNA, codon, anticodon, amino acid, protein) Mutations (mutagen, point mutation, frameshift mutation, chromosomal mutations, DNA repair)Mutations (mutagen, point mutation, frameshift mutation, chromosomal mutations, DNA repair)