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1.1.5 Nucleic acids
Terms in this set (22)
What are nucleic acids?
Ribonucleic acid (RNA) and deoxyribonucleic acid (DNA)
What is the structure of a nucleotide?
- A pentose sugar (5 carbon atoms)
- A phosphate group
- A nitrogen-containing organic base (cytosine, thymine, uracil, adenine and guanine)
How is a mononucleotide formed?
A pentose sugar, phosphate group, and nitrogenous base are joined by condensation
How is a dinucleotide formed?
Two mononucleotides joined together by condensation between the deoxyribose sugar of one mononucleotide and the phosphate group of another, forming a phosphodiester bond
What is the structure of DNA?
- Double helix made up of two, long, polynucleotide strands held together by hydrogen bonds formed between certain bases run anti-parallel to one another
- The pentose sugar is deoxyribose
- The organic bases are adenine, thymine, guanine and cytosine
What is base pairing?
Hydrogen bonds only form between base pairs A-T and base pairs G-C
What is DNA's double helix structure?
- The uprights of phosphate and deoxyribose wind around one another to form a double helix
- They form the structural backbone of the DNA molecule
What is the stability of DNA?
Why is DNA a stable molecule?
- The phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
- Hydrogen bonds link the organic base pairs forming bridges between the phosphodiester uprights. As there are 3 hydrogen bonds between cytosine and guanine, the higher the proportion of C--G pairings, the more stable the DNA molecule
- There are other interactive forces between the base pairs that hold the molecule together (base stacking)
What is the function of DNA?
- To store and transmit hereditary information
How are DNA molecules adapted to carry out its function?
- Sugar-phosphate backbone provides strength and stability
- Long molecule so can store lots of information
- Helix so compact
- Double stranded so replication can occur semi-conservatively
- Weak hydrogen bonds allow for replication
- Many hydrogen bonds provides stability
How does DNA replicate?
Why do scientists doubt that DNA carries the genetic code?
Because of its relative simplicity
What is semi-conservative replication?`
Each new DNA molecule contains one strand from the original DNA template strand and one new strand
Why is it important that DNA is replicated before a nucleus divides in cell division?
To ensure that all the daughter cells have the genetic information to produce the enzymes and other proteins what they need
Why is the process of DNA replication very precise?
Because all the new cells that are formed are more or less genetically identical to the parent cells
What are the requirements for semi-conservative replication to take place?
- The four types of nucleotide, each with their bases, must be present
- Both strands of the DNA molecule acts as a template for the attachment of these nucleotides
- DNA polymerase
- A source of chemical energy is required to drive the process
What is the process of DNA replication?
1. DNA helicase breaks the hydrogen bonds between bases on the polynucleotide DNA strands
2. The double helix separates and each strand acts as a template
3. Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand - A with T and C with G
4. DNA polymerase line up nucleotides of new strand and hydrogen bonds form between the bases on the original and new strands
What is evidence for semi-conservative replication?
Meselson and Stahl's experiment showed DNA is replicated using the semi-conservative method. Their experiment used two isotopes of nitrogen (DNA contains nitrogen) - heavy nitrogen and light nitrogen
1. Two samples of bacteria were grown for many generations - one in a nutrient broth containing light nitrogen, and one in a broth with heavy nitrogen. As the bacteria reproduced, they took up nitrogen from the broth to help make nucleotides for new DNA. So the nitrogen gradually became part of the bacteria's DNA
2. A sample of DNA was taken from each batch of bacteria and spun in a centrifuge. The DNA from the heavy nitrogen bacteria settle lower down the centrifuge tube than the DNA form the light nitrogen bacteria because it's heavier
3. Then the bacteria grown in the heavy nitrogen broth were taken out and put in a broth containing only light nitrogen. The bacteria were left for one round of DNA replication, and then another DNA sample was taken out and spun in the centrifuge
4. If replication was conservative, the original heavy DNA, which would still be together, would settle eat the bottom and the new light DNA would settle at the top
5. If replication was semi-conservative, the new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen and one strand of new DNA containing light nitrogen. So the DNA would settle out between where the light nitrogen DNA settled and where the heavy nitrogen DNA settled out
6. As it turned out, the DNA settled out in the middle, showing that the DNA molecules contained a mixture of heavy and light nitrogen. The bacterial DNA had replicated semi-conservatively in the light nitrogen
What is the structure of RNA?
- Single stranded, relatively short, polynucleotide chain
- Pentose sugar is always ribose
- Organic bases are adenine, guanine, cytosine and uracil
What is the function of RNA?
- RNA stores and transfers information from DNA to ribosomes
- Involved in protein synthesis
What are ribosomes made of?
Proteins and RNA
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