Nucleic Acids (Edexcel A Level Biology (A) SNAB): Flashcards
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Define nucleotide.
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Define nucleotide.
A nucleotide is the monomer of nucleic acids, made of a pentose sugar, a phosphate group and a nitrogenous base.
What are the three components of a mononucleotide?
A pentose sugar (5-carbon sugar).
A phosphate group.
A nitrogen-containing organic base.
Which pentose sugar and bases are found in a DNA nucleotide?
The sugar is deoxyribose.
The bases are adenine, cytosine, guanine and thymine.
Which pentose sugar and bases are found in an RNA nucleotide?
The sugar is ribose.
The bases are adenine, cytosine, guanine and uracil.
Which bases are purines, and what is their structure?
Adenine and guanine are purines.
They have a double ring structure.
Which bases are pyrimidines, and what is their structure?
Cytosine, thymine and uracil are pyrimidines.
They have a single ring structure.
A pentose sugar is a sugar containing carbon atoms.
A pentose sugar is a sugar containing five carbon atoms.
Define phosphodiester bond.
A phosphodiester bond is the covalent bond that joins the phosphate group of one nucleotide to the pentose sugar of the next in a polynucleotide.
What type of reaction joins two nucleotides together?
A condensation reaction.
It occurs between the phosphate group of one nucleotide and the sugar of the next, releasing water.
What is the sugar-phosphate backbone?
The chain of alternating phosphate groups and pentose sugars running along a polynucleotide.
It is formed by many phosphodiester bonds.
True or False?
Both DNA and RNA are polymers made of nucleotides.
True.
Both are polynucleotides — long chains of many nucleotides joined by phosphodiester bonds.
Why is RNA more susceptible to hydrolysis than DNA?
RNA has a 2' hydroxyl (OH) group on its ribose sugar.
This makes it less stable, which is why DNA is the storage molecule and RNA is short-lived.
How many polynucleotide strands make up a DNA molecule?
Two strands, lying side by side.
Define antiparallel.
Antiparallel describes the two strands of DNA running in opposite directions — one 5' to 3' and the other 3' to 5'.
Which bonds hold the two strands of DNA together?
Hydrogen bonds between the complementary base pairs.
State the complementary base pairing rules in DNA.
Adenine pairs with thymine (2 hydrogen bonds).
Guanine pairs with cytosine (3 hydrogen bonds).
How many hydrogen bonds form between each base pair?
Two between adenine and thymine.
Three between guanine and cytosine.
Which bonds join the nucleotides within a single DNA strand?
Phosphodiester bonds, forming the sugar-phosphate backbone.
What is the three-dimensional shape of a DNA molecule?
A double helix.
Where are the nitrogenous bases positioned in a DNA molecule?
They project inwards from the sugar-phosphate backbones.
This places them in the interior of the double-stranded molecule, where they pair up.
The two strands of DNA are held together by bonds between complementary bases.
The two strands of DNA are held together by hydrogen bonds between complementary bases.
True or False?
In DNA, the amount of adenine equals the amount of thymine.
True.
Because of complementary base pairing, A always pairs with T, so the amount of A equals T (and G equals C).
A DNA molecule contains 30% adenine. What percentage is cytosine?
A = T = 30%, so A + T = 60%.
The remaining 40% is G + C, so cytosine = 20%.
What is complementary base pairing?
The rule that a base on one strand always pairs with a specific base on the other.
A with T, and G with C.
How many strands make up an RNA molecule?
One — RNA is single-stranded.
Which pentose sugar is found in RNA?
Ribose.
Which base replaces thymine in RNA?
Uracil (U).
Which four nitrogenous bases are found in RNA?
Adenine, guanine, cytosine and uracil.
State three structural differences between RNA and DNA.
RNA is single-stranded (DNA is double-stranded).
RNA contains ribose (DNA has deoxyribose).
RNA contains uracil instead of thymine.
How does the length of an RNA molecule compare with DNA?
RNA polynucleotide chains are relatively short compared with DNA.
Which bonds join the nucleotides in an RNA strand?
Phosphodiester bonds, forming the sugar-phosphate backbone.
Name the three types of RNA.
Messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
What is the role of messenger RNA (mRNA)?
It is a transcript copy of a gene.
It carries the code for a specific polypeptide.
Unlike DNA, RNA contains the base uracil in place of .
Unlike DNA, RNA contains the base uracil in place of thymine.
True or False?
RNA contains the base thymine.
False.
RNA never contains thymine — it contains uracil instead.
Which three bases are found in both DNA and RNA?
Adenine, guanine and cytosine.
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