Chapter 11

Quiz #8: February 13 th You will distinguish between the famous scientists and their contributions towards DNA You will demonstrate replication, transcription, and translation from a sample strand of DNA You will identify mutations by type and result

When trying to figure out what our genes were made of, most scientists always assumed they were proteins. Proteins were discovered in 1838; Nucleic acids were discovered in 1871. The function of proteins was discovered in 1926. Nucleic acids not until 1952. There are 20 different amino acids that make proteins. There are only 4 different nucleotides that make up nucleic acids. There s more of a variety of proteins and we ve known about them longer.

Alfred Hershey and Martha Chase devised an experiment to answer this question in 1952. Proteins contain sulfur, but nucleic acids don t. Nucleic acids contain phosphorus, but proteins don t. First the two scientists put radioactive isotopes of both phosphorus and sulfur into a virus. The radiation would show up under a special light, and will shine a different color for phosphorus and sulfur. After this, they let the virus infect an E. coli cell, which reproduced over and over again.

After the E. coli reproduced and grew, Hershey and Chase shined radioactive detectors over the cells. They found that the color for phosphorus appeared, but not for sulfur. This proved that when the virus reproduced, they passed on phosphorus, not sulfur. Since only nucleic acids have phosphorus, nucleic acids must have been what was passed on. Conclusion: Genes were made of nucleic acids, not proteins. All of a sudden, people were desperate to learn more about our genetic material: nucleic acids.

There are only 5 different types of nucleic acids. Adenine, Thymine, Guanine, Cytosine, and Uracil Each nucleotide contains three parts A 5-carbon ribose sugar, which is the structural backbone of nucleic acids A phosphate molecule, which links nucleotides together A nitrogenous base, which is the genetic code. The different nitrogenous bases give each nucleotide it s specific name.

Around the same time as Hershey and Chase, Erwin Chargoff noticed an interesting trend with nucleotides. Chargoff separated all the DNA in a chromosome into individual nucleotides. He then weighed each nucleotide to see how much of each was in a chromosome. Though the numbers were always different, the amount of adenine was always similar to the amount of thymine, and the amount of guanine was similar to the amount of cytosine

Species A T G C Bacillus Subtillus (Bacillus bacteria) 28.4 29.0 21.0 21.6 Escherichia coli (E. coli) 24.6 24.3 25.5 25.6 Neurospora crassa (Bread mold) 23.0 23.3 27.1 26.6 Zea mays (Corn) 25.6 25.3 24.5 24.6 Drosophila melanogaster (Fruit fly) 27.3 27.6 22.5 22.5 Homo Sapiens (Human) 31.0 31.5 19.1 18.4

One year after Hershey and Chase, James Watson and Francis Crick made what is considered one of the greatest discoveries in science: they decoded the structure of DNA. Using photographs from their colleague, Rosalind Franklin, the amount of each nucleotide from Chargoff, and tinker toys, they built large models showing how to build a DNA strand.

Watson and Crick proved that even though there are only four nucleotides, they could be rearranged and linked billions of times to create long sequences. Thus, genes could be created by linking hundreds, thousands, or tens of thousands of nucleotides together.

There are three processes we will discuss in this chapter. The first is DNA replication. DNA replication is how cells copy their DNA These copies will be used by each cell during mitosis and meiosis. To learn replication, you must first learn the structure of DNA chains

DNA is described as a double helix, because it is two sequences of DNA that wrap around each other. These sequences attach to each other at the nucleotides using hydrogen bonds. Adenines always attach to Thymines Guanines always attach to Cytosines. (This explains Chargoff s rule) Because these pairs always attach to each other, each strand of DNA can be used as a template, or guide, for building a new strand

The first step in DNA replication is that an enzyme has to break the hydrogen bonds between each strand. After breaking the first hydrogen bond, the enzyme continues down the strand like a zipper. Meanwhile, the endoplasmic reticulum has been building new nucleotides. These nucleotides are carried by enzymes to the nucleus to be attached to the unzipped strand.

While the strands unzip, enzymes attach new nucleotides to each strand. The enzymes know which nucleotides to use because each nucleotide can only pair with it s partner no one else. The result: each new DNA strand has one old copy of DNA and one new copy of DNA. This process of using an old strand to build a new is called semi-conservative replication.

Structurally, not much is different between DNA and RNA. DNA stands for DeoxyriboNucleic Acid RNA stands for RiboNucleic Acid The only difference in structure is that DNA is missing an oxygen atom

There are other differences with how DNA and RNA is used DNA is double stranded, RNA is single stranded (the nucleotides do not have to pair up) RNA does not contain Thymine. Instead, it contains a unique nucleotide called Uracil. DNA s only form is double helix. RNA comes in many forms. You will learn about two of these forms: messenger RNA, and transfer RNA The DNA is the main blueprint. RNA not only are copies of the blueprint, but do most of the constructing as well.

Last chapter we talked about how genes are passed on. This chapter, we will talk about how our body reads genes and turns them into our traits. To do this, we will introduce the second process of this chapter: DNA Transcription. Transcription is the process of creating an RNA sequence using a DNA template. This RNA sequence will then be carried out of the nucleus and to a ribosome to build a protein.

Transcription begins similar to Replication. An enzyme breaks the hydrogen bonds between the DNA strands and begins to unzip a section of DNA This section contains only the necessary genetic information for making a specific protein. As it unzips, enzymes attach new nucleotides to the DNA segment Guanine pairs with Cytosine (same as replication) Adenine pairs with URACIL (not Thymine this time)

When the section of nucleotides is completed, the RNA strand breaks off of the DNA strand. This section of RNA is called Messenger RNA (or mrna) because it will be the messenger carrying the genetic information to the ribosome. Another enzyme reattaches the DNA strand, and the process of transcription is complete.

Before moving on, the mrna strand first gets modified by the cell Sections of the mrna are removed by an enzyme and returned to the nucleus. The sections that are removed are called introns. The remaining sections, called exons, are what get expressed (read) by ribosomes

Why introns and exons? The short answer is who knows? Introns create redundancy, which helps reduce the likelihood that a mutation will cause a problem If 100% of the nucleotides are turned into a protein, then a mutation will cause a problem 100% of the time. If only 100 nucleotides out of 10,000 code for a gene, the likelihood of a mutation hitting those specific 100 nucleotides is very small. Introns allow for more variety of gene sequences Take the word hearth. From this word, you get the words he, ear, art, heart, and earth, depending on which letters you cut out.

The final process to learn this chapter is translation. Translation is the process of converting a strand of mrna into an amino acid sequence for a protein Translation occurs at ribosomes Remember chapter 7? What is the role of ribosomes? The first step involves the second form of RNA: Transfer RNA (trna)

The trna have two parts. On one end, they have what is called an anticodon. A codon is a sequence of three nucleotides in mrna An anticodon is a sequence that pairs with a codon On the other end, they are holding on to an amino acid.

The beginning of the sequence of mrna is read by multiple trna s until one is found whose anticodon matches the mrna s codon Once the right trna has latched onto the mrna, another trna will latch onto the next 3-nucleotide codon sequence. This process will continue until the entire strand has been covered

While trna s match their anticodon s to the mrna s codons, the amino acids on the other ends of the trna s are lining up too. Another enzyme will remove the amino acids from the trna and attach them to each other. The type of bond that holds the amino acids together is called a peptide bond Once the amino acid sequence is completed, the amino acids will fold together and form a protein

How does the mrna know that the amino acid sequence it is coding for is the right one? Each trna has a specific anticodon. That anticodon will ALWAYS go with a specific amino acid Each three-nucleotide sequence on the mrna can therefore only code for a specific amino acid. One sequence always indicates the *Start* position, and three sequences indicate the *Stop* positions

The four possible nucleotides for RNA are Uracil (U) Cytosine (C) Adenine (A) Guanine (G) Name some possible 3-nucleotide codons that we will test to see what amino acid they code for.

The human genome contains between 20,000-40,000 genes. The sequence of DNA to make these genes is around 6.6 billion nucleotides. Amazingly, most of us have survived this long without a single, lethal mistake. But errors will occur. An error that causes a change in a DNA/RNA sequence is called a mutation. In this chapter, we will talk about 4 types of mutations

Mutations can take place in gametes, and affect the offspring, or take place in another cell in the body and only affect that organism Mutations can have both positive or negative results. They can also have no affect Mutations can be problems with DNA replication/transcription/translation, or come from environmental factors

A point mutation is when only one nucleotide is incorrect. Even though it is only 1 nucleotide out of 3.3 billion, this one mistake has the potential to completely change an organism s health. Example: I have a pet cat. This could be I gave a pet cat; I wave a pet cat; I have a wet cat. I have a pet rat. Sickle cell anemia is a disease caused by a single point mutation that tells the cell to replace a glutamine with a valine

Frameshift mutations are when at least one nucleotide is added or deleted from the DNA or RNA sequence The correct sequence of amino acids is dependent on maintaining the 3-nucleotide codon pattern. If one nucleotide is added or deleted, the codon pattern does not start at the right spot This results in an entire sequence of amino acids being incorrect.

Chromosome mutations are when the chromosome itself is damaged before the DNA is able to be replicated or transcribed Chromosome mutations tend to occur during mitosis or meiosis, and usually result in the death of the cell It is possible, though rare, for healthy zygotes to grow with chromosome mutations. If this happens, the organism will most likely be sterile

There are four types of chromosome mutations 1) Deletion. When a section of a chromosome is deleted 2) Insertion. When part of a chromatid breaks off and attaches to it s sister chromatid, resulting in a duplicate of a chromosome 3) Inversion. When part of a chromosome breaks but is reattached backwards. 4) Translocation. When part of one chromosome breaks and attaches to a different chromosome

Mutagens are environmental factors that cause changes in DNA. 1) Radiation. X-rays, ultraviolet light, cosmic rays, nuclear radiation all cause radiation by breaking apart and/or changing DNA 2) Chemicals. Asbestos, benzenes, formaldehyde react with chemicals in the DNA 3) High temperatures. High temperatures can break hydrogen bonds and cause the DNA strands to fall apart

With all these possibilities, and with 3.3 billion nucleotides, how come mutations don t happen more often? The enzymes that build DNA and RNA strands have the ability to check for mistakes. Mistakes happen constantly (probably 1-10 every second). But whenever they do, enzymes repair the mistake. Mutations only occur when a mistake occurs but the enzymes don t notice OR when an organism is subjected to multiple mutagens.

This question is worth an extra 5 pts on the biweekly quiz. You may check your answers with me ahead of time for a yes or no response as many times as you like. Under ideal conditions E. coli can finish mitosis in only 36 minutes. But it takes 40 minutes to finish replication. How is that possible?