2.3 - B - Nucleic Acids Flashcards
Name 2 nucleic acids
DNA, RNA
What are the 3 parts of a nucleotide?
Phosphate group Pentose sugar (deoxyribose in DNA, ribose in RNA) Nitrogenous base (A, T, G, C, U)
How are nucleotides joined together?
Where are they joined together?
2 nucleotides are joined together with a covalent bond between the phosphate group of 1 and the pentose sugar of the other in a
condensation reaction.
What are the 2 different types of nitrogenous bases?
Which are bigger?
List all of each type
Purine (larger) - Adenine and Guanine
Pyramidines (smaller) - Thymine (DNA), Uracil (RNA) and Cytosine
What is the backbone of a nucleic acid?
Sugar‐phosphate backbone
What is a double helix?
How is this formed?
The shape of any DNA molecule, due to coiling of the 2 sugar-phosphate backbone strands into a right-handed spiral configuration.
What is a monomer?
A molecule that, when repeated, makes up a polymer. Amino acids are the monomers of proteins. Nucleotides are the monomers of nucleic acids.
What is a nucleotide?
A molecule consisting of a 5-carbon sugar, a phosphate group and a nitrogenous base.
Nucleotides are the monomers of nucleic acids.
What is a polynucleotide?
A large molecule containing many nucleotides
What is adenine RNA the same as?
Adenosine monophosphate
What is a macromolecule?
A molecule containing a very large number of atoms, such as a protein, nucleic acid, or synthetic polymer.
What is DNA an example of? How is this so?
What does DNA consist 2 of?
What is specific about these 2 things?
What are the bonds in DNA called?
DNA molecules are long. Why is this important?
A polymer - it is made up of many repeating nucleotide monomers.
A molecule of DNA consists of 2 polynucleotide strands.
They run in opposite directions - they are antiparallel.
Phosphodiester.
They can carry a lot of encoded genetic information.
What are the 2 antiparallel DNA strands joined to each other by?
Which nitrogenous bases pair with which? Why?
How many hydrogen bonds are between them?
What do the hydrogen bonds allow for in DNA?
Weak hydrogen bonds.
Pyrimidines always pair with purines, giving equal-sized ‘rungs’ on the DNA ladder. These can then twist into the double helix coil. This gives the molecule stability.
Adenine always pairs with thymine - 2 hydrogen bonds.
Guanine always pairs with cytosine - 3 hydrogen bonds.
Allow the molecule to unzip for transcription and replication.
During which phase of the cell cycle does DNA replication occur?
S phase of interpahse.
Chromosomes double to become identical sister chromatids.
Explain how DNA is organised in eukaryotic cells
The majority of the DNA content, or the genome, is in the nucleus.
Each large molecule of DNA is tightly wound around special histone proteins into chromosomes. Each chromosome is therefore one molecule of DNA.
There is also a loop of DNA, without the histone proteins, inside mitochondria and chloroplasts.
Explain how DNA is organised in prokaryotic cells
DNA is in a loop and is within the cytoplasm, not enclosed in a nucleus.
It is not wound around histone proteins, and is described as naked.
What is DNA polymerase?
What does it do?
An enzyme that catalysts formation of DNA from activated deoxyribose nucleotides, using single-stranded DNA as a template.
Catalyses the addition of the new nucleotide bases in the 5’ to 3’ direction, to the single strands of DNA; it uses each single strand of unzipped data as a template.
What is helicase?
An enzyme that catalysed the breaking of hydrogen bonds between the nitrogenous pairs of bases in a DNA molecule.
Unzips DNA by breaking the hydrogen bonds.
What is semi-conservative replication?
How DNA replicates. This results in 2 new molecules, each of which contains one old strand and one new strand. One old strand is conserved in each new molecule.
What is a genome?
All the DNA within a cell
What are the steps in DNA replication?
Gyrase unwinds DNA.
DNA helicase unzips DNA by breaking the weak hydrogen bonds. This results in 2 single strands of DNA with exposed nucleotide bases.
DNA primers attach at the 5’ end.
DNA polymerase catalyses the addition of the new nucleotide bases in the 5’ to 3’ direction, to the single strands of DNA; it uses each single strand of unzipped data as a template.
The leading strand is synthesised continuously, whereas the lagging strand is in Okazaki fragments that are later joined by DNA ligase.
What were the three theories for how DNA molecules made copies of themselves in the 1950s?
In the 1950s there were 3 theories as to how DNA was replicated.
Conservative - original molecule acts as a template and a new molecule is made.
Dispersive - the original molecule breaks up into nucleotides, each joins to a complementary nucleotides and new ones join up again.
Semi-conservative - The new molecule consists of one original strand and one newly formed strand.
Explain Meselson and Stahl’s investigation
In 1958 they carried out an experiment which showed that DNA was semi-conservative.
They grew E. coli for 14 generations in a medium containing the heavy isotope of nitrogen, 15N. This contains an extra neutron in every atomic nucleus. After 14 gens, most of the DNA in the bacteria would be heavy.
They then transferred some of these bacteria into a medium containing the normal 14N isotope, and left them for long enough to undergo one replication.
The DNA from these bacteria after 1 division was found to be hybrid DNA. This showed that DNA does not replicate conservatively, as that would have produced 2 bands of DNA, 1 heavy and 1 light.
The bacteria were allowed to divide once more and their DNA was extracted and centrifuged. This produced 2 bands of DNA, 1 hybrid and 1 light, showing that replication is semi-conservative and not dispersive.
How often are mutations estimated to occur?
1 in 10^8 base pairs
What does gyrase do?
Unwinds DNA
What does the 3’ to 5’ mean?
Which carbon attaches to the phosphate
What do DNA primers do?
What makes DNA primers?
Attach to the open strand as a mark for where DNA polymerase should start catalysing.
Primase.
What are Okazaki fragments?
Short sequences of DNA nucleotides which are synthesized discontinuously and later linked together by the enzyme DNA ligase to create the lagging strand during DNA replication by hydrolysis reactions to create phosphodiester bonds.
What is a gene?
A length of DNA that codes for a polypeptide or for a length of RNA that is involved in regulating gene expression
What is a polypeptide?
Give an example.
A polymer made of amino acid units joined together by peptide bonds. Insulin is a polypeptide with 51 amino acids.
What is a protein?
A large polypeptide of 100+ amino acids.
How is RNA structurally different from DNA?
The sugar molecule in each nucleotide is ribose, not oxyribose.
The nitrogenous base uracil (a pyrimidine) replaces thymine.
The polynucleotide chain is usually single stranded and shorter.
What are the 3 forms of RNA?
Messenger RNA - mRMA
Transfer RNA - tRNA
Ribosomal RNA - rRNA
What happens in transcription of protein synthesis?
RNA nucleotides align against the DNA template strand and complementary base pair (A‐U and G‐C). The enzyme RNA polymerase catalyses the formation of temporary hydrogen. The nucleotides are joined together to make a strand of mRNA (it is a copy of the DNA coding strand).
What happens in translation?
The formation of a protein, at ribosomes, by assembling amino acids into a particular sequence according to the coded instructions carried from DNA to the ribosome by mRNA.
The mRNA is taken to the ribosome (made up of rRNA and proteins). tRNA carries amino acids to the ribosomes to be joined into polypeptide chains.
Summarise the process of transcription of a gene into a length of mRNA
Gene unwinds and unzips.
Weak hydrogen bonds break.
RNA polymerase catalysed the formation of temporary hydrogen bonds between RNA nucleotides and their complementary unpairs DNA bases.
The mRNA now passes out of the nucleus, through the nuclear envelope, and attaches to a ribosome.
What are the 2 strands in transcription?
The template strand - the DNA strand.
The coding strand - the mRNA that is complementary to the DNA (template) strand.
Where is tRNA made?
Explain its structure.
The nucleolus, it then passes out of the nucleus into the cytoplasm.
They are single stranded polynucleotides, but can twist into a hairpin shape. At one end is a trio of nucleotide bases that recognises and attaches to a specific amino acid. At the loop of the hairpin is another triplet of bases, called an anticodon, that is complementary to a specific codon (triplet) of bases on the mRNA.
What do ribosomes catalysed the synthesis of?
Polypeptides
Explain how translation occurs at a ribosome
tRNA molecules bring the amino acids and find their place when the anticodon binds by temporary hydrogen bonds to the complementary codon on the mRNA molecule.
As the ribosome moves along the length of mRNA, it reads the code, and when 2 amino acids are adjacent to each other a peptide bond forms between them.
Energy, in the form of ATP, is needed for polypeptide synthesis.
The A.A. sequence for the polypeptide is therefore ultimately determined by the sequence of triplets of nucleotide bases on the length of DNA - the gene.
After the polypeptide has been assembled, the mRNA breaks down. Its component molecules can be recycled into new lengths of mRNA, with different codon sequences.
The newly synthesised polypeptide is helped, by chaperone proteins in the cell, to fold correctly into its 3D shape/tertiary structure, in order to carry out its function.
State and explain the 3 nature factors of genetic code
Universal - because in almost all living organisms the same triplet of DNA bases codes for the same amino acid.
Degenerate - for most A.A’s, there is more than 1 base triplet. This may reduce the effect of point mutations.
Non-overlapping - it is read starting from a fixed point in groups of 3 bases. If a bass is added or deleted then it causes a frameshift.
