Nucleotides And Nucleic Acids 2.3 Flashcards
What is the role of helicase in DNA replication
-an enzyme that catalyses the breaking of hydrogen bonds between the nitrogenous bases in a DNA molecule
What is the role of DNA polymerase in DNA replication?
-enzyme that catalyses the formation of phosphodiester bonds between the nucleotides ,to form DNA
Why is DNA replication described as semi-conservative ?
One old strand is conserved in each new molecule.
Describe the process of semi-conservative DNA replication
1- the double helix is UNTWISTED a bit at a time this is catalysed by a gyrase enzyme
2-DNA unzips- hydrogen bonds between the nucleotide bases are broken. This is catalysed by HELICASE and results in two single strands of DNA with exposed nucleotide bases
3-free PHOSPHORYLATED NUCLEOTIDES present in the nucleoplasm in the nucleus , are bonded to the exposed bases , following complementary base pairing rules
4-DNA POLYMERASE catalyses the addition of new nucleotide bases in the 5’ to 3’ direction to the single strands of DNA , it uses each single strand of unzipped DNA as a template
5- the LEADING strand is synthesised continuously whereas the LAGGING strand is in fragments (discontinuous) that are later joined (this is catalysed by ligase)
6-HYDROLYSIS of the activated nucleotides to release the extra phosphate groups supplies the energy to make PHOSPHODIESTER bonds between the sugar residue of one nucleotide and the phosphate group of the next nucleotide
Two identical strands of DNA are formed.Both containing half of the original material.
What is the role of DNA ligase?
-an enzyme that joins the nucleotides together in sections
How are the number of mutations during DNA replication limited?
During DNA replication there are enzymes that can proofread and edit out incorrect nucleotides
What are some examples of spontaneous gene mutations?
•Base substitution – a nucleotide base is replaced with another
•Base deletion – a nucleotide base is deleted from the sequence
•Base insertion – a nucleotide is inserted into the sequence
State what structural detail of a polypeptide is altered by gene mutations
The sequence of amino acids (Primary structure)
Why is the genetic code universal?
-almost every organism uses the same code
Why is the genetic code described as degenerate?
. multiple codons can code for the same amino acid
.this can limit the effect of mutations
Describe the process of transcription
• DNA HELICASE breaks the hydrogen bonds between the bases in the two strands of DNA.
• This causes the DNA helix to unwind and one strand acts as a template.
•Free mRNA nucleotides align opposite exposed complementary DNA bases
• The enzyme RNA POLYMERASE joins together the adjacent RNA nucleotides, forming phosphodiester bonds, to create a new mRNA polymer chain.
• Once one gene is copied, the mRNA is modified and then leaves the nucleus through the nuclear envelope and attaches to a ribosome
What is transcription?
The process in which a complementary mRNA copy of one gene on the DNA is created in the nucleus
Describe the process of translation
-tRNA molecules bring the amino acids and find their place when their 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 (ATP provides energy for this)
-the ribosome will move along the mRNA molecule to the next codon and another complementary tRNA will attach to the next codon on the mRNA
-this continues until the ribosome reaches the stop codon at the end of the mRNA molecule causing the ribosome to detach
The polypeptide chain is now created and will enter the Golgi body for folding and modification
What replaces Thymine in RNA?
Uracil
What is the start codon?
AUG
-codes for amino acid methionine
What are the stop codons?
UAA, UAG and UGA
What is the role of RNA polymerase?
-an enzyme that catalyses the formation of phosphodiester bonds of adjacent nucleotides
What is the role of mRNA?
-transfers genetic information from DNA
What is the role of tRNA?
Responsible for bringing in amino acids to to the organelle where proteins are being formed.
What is the role of rRNA?
Makes up the large and small sub-unit of a ribosome
Why is the genetic code described as non-overlapping?
Each base is only read once in which codon it is part of.
What is the bond between two nucleotides called?
-a phosphodiester bond(carbon 5’ to 3’ bond)
-this is a covalent bond between the sugar residue and the phosphate group in a nucleotide
-these bonds are broken when polynucleotides break down (hydrolysis) and formed when polynucleotides are synthesised (in a condensation reaction)
What is a nucleotide?
The monomer from which nucleic acids are made
What is the difference between a purine and pyrimidine?
-a purine (guanine and adenine) have 2 carbon rings they are larger
-a pyrimidine (cytosine and thymine(uracil ) ) have 1 carbon ring they are smaller
What always pairs with a purine?
-a purine always pairs with a pyrimidine , giving equal sized ‘rungs’ on the DNA ladder
-these can then twist into the double helix ,giving the DNA molecule stability
List some differences between DNA and RNA
-DNA is found in chromosomes in the nucleus .Whilst RNA is found in the cytoplasm
-DNA is an extremely long molecule. RNA is a relatively short molecule
-DNA is 2 polynucleotide strands in a double helix.RNA is one polynucleotide strand
-DNA contains the nitrogenous base thymine whereas in RNA thymine is replaced with uracil
-DNA contains the pentose sugar deoxyribose. RNA contains the pentose sugar ribose
How are the two strands in DNA described?
They run in opposite directions, they are anti parallel
In DNA what does adenine always pair with?
-adenine always pairs with thymine
-by means of 2 hydrogen bonds
In DNA what does guanine always pair with?
-guanine always pairs with cytosine
-by means of 3 hydrogen bonds
What is a phosphorylated nucleotide?
A nucleotide that contains more than one phosphate group e.g. ADP and ATP
What is the structure of DNA ?
-polymer made up of repeating units called nucleotides
-a molecule of DNA consists of 2 polynucleotide strands
-the 2 polynucleotide strands run in opposite directions - they are described as ANTIPARRALEL
-two nucleotides are bonded by a PHOSPHODIESTER BOND between the pentose sugar and the phosphate of different nucleotides
-DNA molecules are long
What are the 2 antiparallel strands of DNA joined together by?
-they are joined to each other by hydrogen bonds between the nitrogenous bases
What is the importance of hydrogen bonds between the two antiparallel DNA strands?
-adenine always pairs with thymine, through 2 hydrogen bonds
-guanine always pairs with cytosine, by means of 3 hydrogen bonds
-a purine always pairs with a pyrimidine, giving equal sized ‘rungs’ on the DNA ladder
-meaning this can then twist into the DOUBLE HELIX , giving the molecule STABILITY
-hydrogen bonds also allow the molecule to unzip for transcription and replication
How is DNA organised in eukaryotes ?
-majority of the genome (DNA content) is in the nucleus
-DNA is tightly wound around histone proteins into chromosomes. Each chromosome is therefore 1 molecule of DNA
- a loop of DNA without histone proteins is found in the mitochondria and chloroplasts
How is DNA organised in prokaryotic cells?
-DNA is not enclosed in the nucleus, it is in a loop within the cytoplasm
-it is not wound around histone proteins and is described as naked
How do viruses that contain DNA organise it?
-viruses that contain DNA also have it in the form of a loop of naked DNA
A DNA molecule contains 2 polynucleotide chains
Describe how these 2 chains are held together
(2 marks)
-there are phosphodiester bonds in the backbone
-there are hydrogen bonds between bases
-purines are bonded to pyrimidine (A to T and C to G)
Twenty different amino acids are commonly used for protein synthesis. In theory ,this would only need 20 different base combinations.
Explain the uses of the remaining 44 combinations.
(2marks)
-several codons code for one amino acid
-some are used as stop or start codons
So that mutation may not result in a change in amino acids
How do you carry out DNA precipitation ?
-DNA can be extracted from plant material e.g. onions, strawberries , bananas, kiwis by:
1- add detergent and heat (60degrees Celsius for 15 mins).This will disrupt the phospholipid bilayer of the cell membranes and nuclear membranes, releasing the DNA. The heat also denatures enzymes released from the cell that would otherwise begin to digest the DNA. Cool the mixture to prevent the DNA itself from breaking down. Pour mixture into a blender and blend for 5 seconds this further breaks down the cell walls and cell membranes, releasing more DNA. Only blend for a short time to not break DNA strands apart
2-filter to remove large debris and membrane fragments
3- add salt to break hydrogen bonds between the DNA and water molecules
4-add protease to denature the proteins associated with the DNA
5- add ice-cold ethanol to precipitate out the DNA from the solution. The DNA appears as white strands- as nuclei acids are insoluble in ice-cold ethanol do forms a precipitate
Draw out the structure of DNA
