DNA and RNA Unit 6, Part 1

The Structure of DNA DNA stands for deoxyribonucleic acid. As you know, it’s the primary source of genetic information that is passed down from generation to generation. DNA also carries the instructions, or blueprints, for each cell’s activities. No two creatures have the same DNA, except in one case. What is it?

A Model of DNA DNA was first discovered in a lab in 1869, though its structure was not determined until many years later. In the 1920s, P.A. Levine showed that DNA can be broken down into a sugar (deoxyribose), a phosphate group, and four nitrogen-containing bases. These bases are called adenine (A), guanine (G), thymine (T), and cytosine (C.)

DNA - The Double Helix DNA is described as a double helix, or a twisted ladder. The steps are made up of nitrogen bases that are held together by hydrogen bonds, and the sides are sugars and phosphate groups.

Nucleotides and Base Pairing Levine called each group of sugar, phosphate, and nitrogen base a nucleotide. Each nucleotide is named for the base it’s paired with. For example, an adenine nucleotide contains adenine. The bases pair up with other bases in the structure of DNA. A always pairs with T and C always pairs with G to form the steps of the DNA ladder.

Watson and Crick Two scientists, James Watson and Francis Crick, came up with the model of DNA that we use today, the double helix. They were the first to determine the arrangement of the components. They also figured that A and G belong to a group of nitrogen compounds called purines and C and T belong to a group called pyrimidines. The structure of a purine will only allow it to bond with a pyrimidine. Never will two purines bond, or two pyrimidines bond.

Replication of DNA So, how are genes passed on the cells when new ones are produced during mitosis and meiosis? Through a process called DNA replication, DNA makes exact copies of itself (remember interphase?) as a protein binds to a section of DNA called the origin. Once this happens, an enzyme begins breaking the hydrogen bonds between base pairs, “unzipping” the helix. When a section unzips, that piece is able to go through replication. The result is a complementary section of DNA. If the parent chain had A, the new piece will have T.

DNA Replication

The Role of DNA As stated before, DNA acts as a blueprint for all the cells in an organism. It tells them what their functions are, what organelles they should have, where they should be located, and what they should look like. DNA also includes the instructions for making proteins and RNA, two important cell functions.

The DNA Code An important function of DNA is that it codes for proteins. We know that proteins are made of amino acids and that there are 20 amino acids that we know of. We also know that DNA is made of 4 bases, and that these 4 bases combine in different orders to code for each of the amino acids.

Transcription Before proteins can be made, DNA must transcribe itself into RNA. This process is called transcription. RNA stands for ribonucleic acid, and it carries the instructions for protein synthesis. RNA is found in ribosomes of the cytoplasm and the nucleus, and is made up of a sugar, a phosphate group, and four bases. It differs from DNA because (1)it is single-stranded, (2)its sugar is ribose instead of deoxyribose, and (3)instead of thymine as a base, it contains uracil (U).

DNA to RNA In order for RNA to be produced from DNA, a process much like replication occurs. DNA “unzips” and a specific sequence of amino acids codes for the start of transcription. When this happens, only one side of the DNA ladder is involved, while the other half waits for the process to stop. After the process is completed, the half of DNA involved in transcription reattaches itself to the original strand.

RNA Transcription

Types of RNA There are three types of RNA, based on their function and location. Messenger RNA, or mRNA, is transcribed from DNA and codes for a polypeptide. Transfer RNA, or tRNA, functions to bring amino acids to the ribosomes for protein synthesis. Ribosomal RNA, or rRNA, is specifically coded by DNA to make up ribosomes, where proteins are made.

Translation The synthesis of a polypeptide (protein chain) by mRNA molecule is called translation. The role of tRNA becomes important here: amino acids must be transported to the ribosomes (the site of protein synthesis) in order to be changed into proteins.

Translation

Codons A codon is a sequence of three bases in a row, and each codon stands for a specific amino acid. Codons are also called triplets. The table on the following slide shows which three bases code for each of the amino acids.

The Path From Genes to Proteins To summarize the path from genes to proteins: DNA has the instructions for protein-building. It gives the instructions to mRNA. The instructions are read by tRNA, which gets the right amino acids in the right order. tRNA takes the amino acids to the ribosomes, which are made by rRNA, where the proteins are put together.

DNA and RNA: Sources of Error Unit 6, Part 2

Sources of Variation There are many places along the way of DNA replication, RNA transcription, and protein translation in which problems can arise. In many cases, this can result in variations that can be detrimental to the cell, and possibly, to the organism.

Mutation A gene mutation occurs when a daughter cell contains different information than the parent cell. Some gene mutations are not harmful, while others could cause the cell to die. These mutations can arise from changes to individual genes or changes in the arrangement of genes in a chromosome.

Deletion Deletion, which is one type of gene mutation, occurs when a nucleotide is left out. Leaving out a nucleotide shifts the reading of the DNA message over by one nucleotide, resulting in synthesis of a polypeptide with an altered amino acid sequence. For example, an original DNA strand reads: ACTGTA, while its replicate might read: TGAAT.

Insertion Insertion occurs when an extra nucleotide is added to a chain during replication. For example, an original DNA strand might read: ACTGTA, while its replicate reads: TGACCAT. Deletion and insertion mutations result in amino acid sequences that are very different from those coded for by normal genes because all the amino acids in the protein beyond the point of mutation will be affected.

Inversion Inversion occurs when an entire codon is flipped or moved to the opposite end of the DNA. For example, an original DNA strand might read: TGCACTGTA, while its replicate reads CATACGTGA.

Point Mutation During point mutation, a base substitution occurs, and one nucleotide is substituted for another. This type of gene mutation affects only one amino acid in the polypeptide. With such a small change in its structure, the polypeptide may be able to function normally or nearly normally. For example, an original DNA strand might read: ACTGTA, while its replicate reads: TGACGT.

Mutagens Mutagens are external agents that cause mutations. They can work by breaking the chemical bonds of DNA or by causing unusual bonds to occur. Radiation, high temperature, environmental factors, and a variety of chemicals are examples.

Chromosome Mutations There are many types of chromosome mutations. Parts of chromosomes can be broken off or lost during mitosis or meiosis. Sometimes, these broken parts rejoin the chromosome from which they came, but attach backwards or at the wrong end. They also might reattach to a different chromosome. Any of these changes can result in abnormal information in the cell’s genetic make-up.

Gene Duplication During meiosis, homologous chromosomes may fail to segregate properly, causing both chromosomes to move to the same pole. The offspring cell would have duplicates of all the genes on that chromosome, which would be too many genes.

Cancer Cancer is a rapid, uncontrolled growth of cells. It creates cells with an abnormal shape, color, or size, which produce more cells that look like the cancerous ones. Cancer can be caused by oncogenes or carcinogens. Oncogenes are genes that cause cancer, while carcinogens are external agents that cause cancer.

Variation through Technology An example of recombinant DNA is when bacterial DNA is altered by the insertion of a foreign gene. This is useful because when a bacterial cell absorbs a gene from another cell, it then exhibits the phenotype of the cell that donated the DNA. This can be used to make new cells with the genes desired.