3.1.5 Nucleic Acids Structure of DNA and RNA

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1 alevelbiology.co.uk Nucleic Acids Structure of DNA and RNA SPECIFICATION Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important information-carrying molecules. In all living cells, DNA holds genetic information and RNA transfers genetic information from DNA to the ribosomes. Ribosomes are formed from RNA and proteins. Both DNA and RNA are polymers of nucleotides. Each nucleotide is formed from a pentose, a nitrogen-containing organic base and a phosphate group: The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil. A condensation reaction between two nucleotides forms a phosphodiester bond. A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs. An RNA molecule is a relatively short polynucleotide chain. Students should be able to appreciate that the relative simplicity of DNA led many scientists to doubt that it carried the genetic code. Source: AQA Spec

2 Structure of DNA and RNA DNA and RNA The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material found in all living organisms, ranging from single-celled bacteria to multicellular mammals. It is found in the nucleus of eukaryotes and in the organelles, chloroplasts, and mitochondria. In prokaryotes, the DNA is not enclosed in a membranous envelope. RNA, is mostly involved in protein synthesis. The DNA molecules never leave the nucleus but instead use an intermediary to communicate with the rest of the cell. This intermediary is the messenger RNA (mrna). Other types of RNA like rrna, trna, and microrna are involved in protein synthesis and its regulation. DNA and RNA are made up of monomers known as nucleotides. The nucleotides combine with each other to form a polynucleotide, DNA or RNA. The basic structure of a nucleotide is made up of three components: 1. a pentose sugar (made with 5 carbon atoms) 2. a nitrogenous base 3. a phosphate group

3 Structure of DNA and RNA Sugar in DNA is Called Deoxyribose The pentose sugar in a DNA nucleotide is called deoxyribose Every DNA nucleotide has the same sugar and phosphate group, but the base can vary There are four possible bases - adenine (A), thymine (T), cytosine (C), and guanine (G). Sugar in RNA is Called Ribose The sugar in an RNA nucleotide is called ribose Just like DNA, an RNA nucleotide has a phosphate group and one of four different bases In RNA, the base uracil (U) replaces thymine (T) Compare DNA to RNA Deoxyribose sugar Nucleic acid Ribose sugar Double stranded Thymine Adenine Guanine Phosphate Cytosine Uracil Single stranded

4 Structure of DNA and RNA Nucleotides Join Together and Form Polynucleotides A polynucleotide is a polymer of nucleotides. Both DNA and RNA form polynucleotides. Nucleotides join via a condensation reaction between the phosphate group of one nucleotide and the sugar of another. This forms a strong, covalent, phosphodiester bond (which consists of the phosphate group and two ester bonds). This chain of sugars and phosphates is known as the sugar-phosphate backbone. When a polynucleotide is formed, the 5 phosphate of the incoming nucleotide attaches to the 3 hydroxyl group at the end of the growing chain.

5 Structure of DNA and RNA The Structure of DNA DNA is made of two polynucleotide chains in a double-helix structure. Two DNA polynucleotide strands join together by hydrogen bonding between bases Each base can only join with one particular partner - this is called complementary base pairing Adenine always pairs with thymine (A-T) and cytosine always pairs with guanine (C-G) which means there are always equal amounts of adenine and thymine in a DNA molecule and equal amounts of cytosine and guanine Two hydrogen bonds form between A and T and three hydrogen bonds form between C and G This complementary base pairing ensures that replication of DNA is accurate (it will also ensure accurate transcription of the genes for protein synthesis) Individually these H bonds are weak and allow the DNA to be separated so replication and transcription can occur Two antiparallel polynucleotide strands twist in opposite directions to form the DNA double-helix This double strand is important as each strand can act as a template in replication, a semi conservative process, meaning that the DNA is copied accurately

6 alevelbiology.co.uk Nucleic Acids DNA replication SPECIFICATION The semi-conservative replication of DNA ensures genetic continuity between generations of cells. The process of semi-conservative replication of DNA in terms of: unwinding of the double helix breakage of hydrogen bonds between complementary bases in the polynucleotide strands the role of DNA helicase in unwinding DNA and breaking its hydrogen bonds attraction of new DNA nucleotides to exposed bases on template strands and base pairing the role of DNA polymerase in the condensation reaction that joins adjacent nucleotides. Students should be able to evaluate the work of scientists in validating the Watson Crick model of DNA replication. Source: AQA Spec

7 DNA replication The Structure of RNA RNA is a relatively short, single polynucleotide chain and not a double one like DNA. It is much shorter than most DNA polynucleotides. RNA vs DNA chain

8 DNA replication DNA Replication DNA has an incredible ability to replicate itself. It copies itself before cell division, so that each new cell has the full amount of DNA. This is called semi-conservative replication because half the strands in each new DNA molecule come from the original DNA molecule. By doing this, it ensures genetic continuity between generations of cells (the cells produced by cell division will inherit their genes from parent cells) The DNA replication process 1. The enzyme DNA helicase breaks the hydrogen bonds between the base pairs on the two polynucleotide DNA strands. This unwinds the helix to form two single strands. 2. Each original single strand acts as a template for the new strand. Complementary base pairing means that free-floating DNA nucleotides are attracted to their base pair - A with T and C with G 3. Condensation reactions join the nucleotides of the new strands together using the enzyme DNA polymerase as the catalyst. Hydrogen bonds form between bases on the original and new strands 4. Each new DNA molecule contains one strand from the original DNA molecule and one new strand

9 DNA replication How Do We Know DNA is Semi-Conservative? Watson and Crick determined the structure of DNA and came up with the theory of semi-conservative DNA replication. However, it wasn t until Meselson and Stahl s experiment years later than the theory was validated. Before their experiment, scientists were not sure if DNA replication was semi-conservative or conservative. If it was conservative, the original strands would stay together and the new DNA molecules would contain two new strands. Meselson and Stahl showed that DNA is replicated using the semi-conservative method by using two isotopes of nitrogen (DNA containing nitrogen) - heavy nitrogen ( 15 N) and light nitrogen ( 14 N). On the following page, we ll explain how the Meselson-Stahl experiment was able to confirm that DNA replicates using the semi-conservative method.

10 DNA replication The Meselson-Stahl Experiment Meselson and Stahl experimented with E. coli grown first in heavy nitrogen ( 15 N) then in 14 N DNA grown in 15 N (red band) is heavier than DNA grown in 14 N (orange band), and sediments to a lower level in cesium chloride solution in an ultracentrifuge When DNA grown in 15 N is switched to media containing 14 N, after one round of cell division the DNA sediments halfway between the 15 N and 14 N levels, indicating that it now contains fifty percent 14 N. In subsequent cell divisions, an increasing amount of DNA contains 14 N only. This data supports the semi-conservative replication model