Section 3: Nucleotides And Nucleic Acids Flashcards
What is the general nucleotide structure?
A nucleotide is a type of biological molecule. It’s made from: a penthouse sugar (that’s a sugar with five carbon atoms), a nitrogenous (nitrogen-containing) base and a phosphate group. All nucleotides contain the elements C, H, O, N and P.
What are the importance of nucleotides?
Nucleotides are really important. For a start they’re the monomers that make up DNA and RNA. DNA and RNA are both types of nucleic acid. They’re found in all living cells. DNA is used to store genetic information - the instructions an organism needs to grow and develop . RNA is used to make proteins from the instructions in DNA.
There are also special types of nucleotide, such as ADP and ATP. They’re used to store and transport energy in cells.
What is the general DNA nucleotide structure?
The nucleotides in DNA all contain the same pentose sugar called deoxyribose. (DNA stands for deoxyribonucleic acid.) Each DNA nucleotide also has the same phosphate group. The base on each nucleotide can vary though. There are four possible bases - adenine (A), thymine (T), cytosine (C) and guanine (G).
A molecule of DNA contains two polynucleotide chains - each chain is made up of lots of nucleotides joined together.
What is the general RNA nucleotide structure?
RNA (ribonuclease acid) contains nucleotides with a ribose sugar (not deoxyribose). Like DNA, an RNA nucleotide also has a phosphate group and one of four different bases. In RNA though, uracil replaces thymine as a base. An RNA molecule is made up of a single polynucleotide chain.
What are purines and pyrimidines? What is the difference between them?
There are two types of base present in DNA and RNA nucleotides - these are called purines and pyrimidines. Each of the bases present in DNA or RNA nucleotides can be classed as one of these types. Adenine and guanine are both purines. Cytosine, thymine and uracil are pyrimidines.
The difference between these types of bases is in their structures. A purine base contains two carbon-nitrogen rings joined together, whereas a pyrimidines base only has one carbon-nitrogen ring. So a pyrimidine base is smaller than a purine base.
What is the general ADP and ATP nucleotide structure?
ADP and ATP are phosphorylated nucleotides. To phosphorylase a nucleotide, you add one or more phosphate groups to it. ADP (adenosine diphosphate) contains the base adenine, the sugar ribose and two phosphate groups. ATP (adenosine triphosphate) contains the base adenine, the sugar ribose and three phosphate groups.
Describe and explain the making and using of ATP
Plant and animal cells release energy from glucose - this process is called respiration. A cell can’t get its energy directly from glucose. So, in respiration, the energy released from glucose is used to make ATP and then molecules of ATP provide energy for chemical reactions in the cell.
ATP is synthesised from ADP and inorganic phosphate (Pi). The ADP is phosphorylated to form ATP and a phosphate bond is formed.
Energy is stored in the phosphate bond. When this energy is needed by a cell, ATP is broken back down into ADP and inorganic phosphate (Pi). Energy is released from the phosphate bond and used by the cell.
Describe the polynucleotide structure.
Nucleotides join together to form polynucleotides. The nucleotides join up between the phosphate group of one nucleotide and the sugar of another via a condensation reaction. This form a phosphodiester bond (consisting of the phosphate group and two ester bonds). The chain of sugars and phosphates is known as the sugar-phosphate backbone. Polynucleotides can be broken down into nucleotides again by breaking the phosphodiester bonds using hydrolysis reactions.
Describe the DNA structure.
DNA is composed of two polynucleotide strands joined together to form a double-helix shape. The two strands join together by hydrogen bonding between the bases. Each base can only join with one particular partner - this is called complementary base pairing. Adenine always pairs with thymine (A-T) and guanine always pairs with cytosine (G-C). This means that a purine (A or G) always pairs with a pyrimidine (T or C). Two hydrogen bonds form between A and T, and three hydrogen bonds form between C and G.
The two polynucleotide strands are antiparallel - this means they run in opposite directions. Two antiparallel strands twist to form a DNA double-helix.
You can use computer modelling to investigate the structure of DNA and other nucleic acids. For example, the computer software RasMol can be used to produce graphical representations of molecules.
How do you purify DNA?
Scientists often need to extract a pure DNA ample from cells in order to analyse it, e.g. for use in forensics. DNA can be purified using a precipitation reaction. To purify DNA, you:
- Break up the cells in your sample. You can do this using a blender or a pestle and mortar.
- Make up a solution of detergent (a dilute washing-up liquid will suffice), salt (sodium chloride) and distilled water.
- Add the broken-up cells to a beaker containing the detergent solution.
- Incubate the beaker in a water bath at 60 degrees C for 15 minutes . Whilst in the water bath, the detergent in the mixture breaks down the cell membranes. The salt binds to the DNA and causes it to clump together. The temperature of the water bath should be high enough to stop enzymes in the cells from working properly and breaking down the DNA.
- Once incubated, put your beaker in an ice bath to cool the mixture down.
- When it’s cooled, filter the mixture using coffee filter paper (or gauze) and a funnel. Transfer a sample of your filtered mixture to a clean boiling tube and discard the contents of the filter paper.
- Add protease enzymes to the filtered mixture. These will break down some proteins in the mixture, e.g. proteins bound to the DNA.
- Slowly dribble some cold ethanol down the side of the tube, so that it forms a layer on top of the DNA-detergent mixture.
- If you leave the the tube for a few minutes, the DNA will form a white precipitate (solid), which you can remove from the tube using a glass rod (or a hoked instrument, like a bent paper clip).
Why does DNA replicate?
DNA copies itself before cell division so that each new cell has the full amount of DNA. This is important for making new cells and for passing genetic information from generation to generation/
How is DNA replicated?
A DNA molecule has a paired base structure which makes it easy for DNA to copy itself.
- DNA helicase (an enzyme) breaks the hydrogen bonds between the two polynucleotide DNA strands. The helix unzips to from two single strands.
- Each original single strand acts as a template for a new strand. Free-floating DNA nucleotides join to he exposed bases on each original template strand by complementary base pairing - A with T and G with C.
- The nucleotides on the new strand are joined together by the enzyme DNA polymerase. This forms the sugar-phosphate backbone. Hydrogen bonds form between the bases on the original and new strand. The strands twists to form a double-helix. Each new DNA molecule contains one strand from the original DNA molecule and one new strand.
This type of copying is called semi-conservative replication because half of the strands in each new DNA molecule are from the original piece of DNA (i.e. the new molecule contains one old strand and one new strand).
Evaluate the accuracy of DNA replication
DNA replication is really accurate - it has to be, to. Make sure genetic information is conserved (stays the same) each time the DNA in a cell is replicated. Every so often though, a random, spontaneous mutation occurs. A mutation is any change to the DNA base sequence. Mutations don’t always have an effect, but they can alter the sequence of amino acids in a protein. This can cause an abnormal protein to be produced. The abnormal protein might function better than the normal protein - or it might not work at all.
What is a gene?
DNA contains genes. A gene is a sequence of DNA nucleotides that codes for a polypeptide. The sequence of amino acids in a polypeptide forms the primary structure of a protein. Different proteins have a different number and order of amino acids. It’s the order of nucleotide bases in a gene that determines the order of amino acids in a particular protein. Each amino acid is coded for by a sequence of three bases (called a triplet) in a gene. Different sequences of bases code for different amino acids. This is the genetic code. So the sequence of bases in a section of DNA is a template that’s used to make proteins during protein synthesis.
Describe DNA, RNA and protein synthesis.
DNA molecules are found in the nucleus of the cell, but the organelles that make proteins (ribosomes) are found in the cytoplasm. DNA is too large to move out of the nucleus, so a section is copied into mRNA. This process is called transcription. The mRNA leaves the nucleus and joins with a ribosome in the cytoplasm, where it can be used to synthesise a protein. This process is called translation.
RNA is a single polynucleotide strand and it contains uracil (U) as a base instead of thymine. Uracil always pairs with adenine during protein synthesis. RNA isn’t all the same though.
