Nucleic acids and their functions Flashcards
Nucleic acids are monomers/polymers
Polymers
What are the monomers of Nucleic acids?
Nucleotides
What is the name of a molecule containing many nucleotides?
Polynucleotides
they may be millions of nucleotides long
What are the 3 components of nucleotides?
- a phosphate group (same structure in all nucleotides)
- a pentose sugar
- an organic nitrogenous base
What is the pentose sugar in RNA?
What is the pentose sugar in DNA?
Ribose
Deoxyribose
What are the two groups of organic bases?
Pyrimidine & purine
What are the Pyrimidie bases?
Thymine and cytosine
What are the purine bases?
Adenine and guanine
In biological systems what energy makes the changes?
Chemical energy
What must happen for chemical reaction to occur?
Make or break bonds
Where does heterotrophic organisms (e.g. Animals) derive their chemical energy from?
Food
Where does autotrophic organisms (e.g. Plants) derive their energy from?
Light energy (which they convert to chemical energy in photosynthesis)
Where do organisms mainly tutors their chemical energy?
In lipids and carbohydrates
What is the molecule that makes energy available when it’s needed?
Adosine trophosphate (ATP)
How much ATP do we break down every day?
50g
The body only contains 5g of ATP, what does this suggest?
That ATP is not an energy store
When is ATP synthesised?
When energy is made available
When is ATP broken down?
When energy is needed
What are the components of ATP?
- Adenine
- ribose
- 3 phosphate groups
What is the enzyme that breaks down ATP?
How does it work?
ATPase
- It hydrolyses the bond between the second and third phosphate groups in ATP
- This removes the third phosphate group leaving only two
What is the ATP molecule hydrolysed into?
Adenosine diphosphte (ADP) and an inorganic phosphate ion
What is released when ATP is hydrolysed?
Chemical energy
How much energy is released when 1 mole of ATP is hydrolysed?
30.6KJ
What name is given to a reaction that releases energy?
An exergonic reaction
ADP + an inorganic phosphate ion (in a condensation reaction) =
(this requires energy inout)
What kind of reaction is it?
ATP + water
Endergonic reaction
The addition of phosphate to ADP is called?
Phosphorylation
How does ATP work?
- ATP transfers free energy from energy rich compounds like glucose to cellular reactions where it is needed
- but energy transfers are inefficient and some energy is always lost as heat
What would the incontrolled release of energy from glucose produce?
A temperature increase that would destroy cells
How to living organisms release energy?
Gradually in a series of small steps called respiration, producing energy in the form of ATP
Name 2 advantages in having ATP as an intermediate in providing energy compared with using glucose directly
- the hydrolysis of ATP to ADP involves a single reaction that releases energy immediately. The breakdown of glucose involves many intermediates and it takes longer for energy to be releaded
- only one enzyme is needed to release energy from ATP but many are needed to release energy from glucose
Name 2 other advantages in having ATP as an intermediate in providing energy compared with using glucose directly
- ATP releases energy in small amounts. When and where it is needed but glucose contains large amounts of energy which would be released all at once
- ATP provides a common source of energy for many different chemical reactions increasing efficiency and control by the cell
ATP provides the necessary energy for cellular activity: (MA)
- METABOLIC PROCESSES- to build large complex molecules from smaller, simpler molecules, such as DNA synthesis from nucleotides
- ACTIVE TRANSPORT- to change the shape of carrier proteins in membranes and allow molecules or ions to be moved against a concentration gradient
ATP provides the necessary energy for cellular activity: (MNS)
- MOVEMENNT- for muscle contraction
- NERVE TRANSMISSION- sodium potassium pumps actively transport sodium and potassium ions across the axon membrane
- SECRETION-the packing and transport of secretory products into vesicles in cells
What is the relative molecular mass of ATP?
507.2
Describe structure of DNA:
1) DNA is composed of two Polynucleotide stands wound around each other in a double helix
2) The pentose sugar in the nucleotides In deoxyribose
3) there are 4 organic bases in DNA: two purines (Adenine & guanine) and two Pyrimidines (Thymine and cytosine)
4) the deoxyribose sugar and phosphate groups are on the outside of the DNA molecules and form the ‘backbone’
5) the bases of the two strands face each other pointing inwards. Adenine always likes with Thymine and guanine always likes with cytosine. Hydrogen bonds join the bases and form complememtsry pairs. The hydrogen bonds between then bases maintain the shape of a double helix
Continue to describe the structure of DNA
6) a DNA molecule is very long and thin and is tightly coiled within the chromosome. The double helix is only 2nm Ian diameter but the DNA molecule in human chromosome number 1, our longest, is estimated to be 85nm long
7) the nucleotides in one strand are arranged in the opposite direction from those in the complementary strand. The strands are anti-parallel (parallel but facing in opposite directions)
How is DNA suited to its function?
- it is a very stable molecule and its information content passes essentially unchanged from generation to generation.
- it is a very large molecule and carries a large amount of genetic information
- the two strands are able to separate ad they are held together by hydrogen bonds
- as the base pairs are on the inside of the double helix within the deoxyribose phosphate backbones the genetic information is protected
Describe the structure of RNA
- RNA is a single stranded Polynucleotide
- RNA contains the pentose sugar ribose
- RNA contains the purine bases Adenine and guanine and the Pyrimidine bases cytosine and unracil (but not Thymine )
What are the 3 types of RNA
- messenger RNA
- Ribosomal RNA
- Transfer RNA
Describe Messenger RNA
- long single stranded molecule
- it is synthesised in the nucleus and carries the genetic code from the DNA to the ribosomes in the cytoplasm
- different mRNA molecules have different lengths related to the genes from which they are synthesised
Describe Ribosomal RNA
- it is found in the cytoplasm and comprises large, complex molecules
- ribosomes are made of Ribosomal RNA and protein.
- They are the site of translation of the genetic code into protein
Describe transfer RNA
- tRNA is a small single stranded molecule, which folds so that in places, there are base sequences forming complementary pairs
- its shape is described as a cloverleaf
- the 3’ end of the molecule has the base sequence cytosine-cytosine-adenine where the specific amino acid the molecule carries is attached
- it also carries a sequence of 3 bases called anticodon.
- molecules of tRNA transport specific amino acids to the ribosomes in protein synthesis
Who proposed the molecular structure of DNA?
Watson and Crick (1953)
How is the DNA stored in eukaryotic cells vs prokaryotic cells?
- it is enclosed in the nuclei of eukaryotic cells
- it is loose in the cytoplasm of prokaryotes
One of DNA’s two main roles is replication, explain this
- DNA comprises two complementary strands, the base sequence of one strand determining the base sequence if the other
- if two strands of a double helix are separated, two identical double helices can be formed, as each parent strand acts as a template for the synthesis if a new complementary strand
One of DNA’s two main roles is protein synthesis, explain this
The sequence if bases represents the information carried in DNA and determines the sequence of amino acid in proteins
Why do chromosomes make copies of themselves?
So that when cells divide each daughter cell receives an exact copy of the genetic information
(copying of DNA is called replication and takes place in the nucleus during interphase)
Initially there were 3 possibilities imagined for the mechanism of DNA replication- describe conservative replication
- Where the parental double helix remains intact I. E. is conserved and a whole new double helix is made
Initially there were 3 possibilities imagined for the mechanism of DNA replication- describe semi-conservative replication
In which the Parental double helix separates into two strands, each of which acts as a template for synthesis of a new molecule each containg an original parental stand and a newly synthesised complementary daughter strand
Initially there were 3 possibilities imagined for the mechanism of DNA replication- describe dispersive replication
The two new double helices contain fragments from both stands of the parental double helix
What did Watson and Crick realise when the built their model?
They realised that complementary base pairs implied that if the two strands were separated they would each make another complementary strand.
Two new identical molecules would form each with one old strand and one newly synthesised strand (semi-conservative replication)
Describe step 1 of Meselson-Stahl experiment
1) Meselson-Stahl carried out an experiment in which they cultured the bacterium (Escherichia Coli) for several generations in a medium containing amino acids made with the heavy isotope of nitrogen 15N instead of the normal light isotope 14N
1i) the bacteria incorporated the 15N into their nucleotides and then into their DNA so that eventually the DNA contained only 15N
1ii) they extracted the bacterial DNA and centrifuged it. The DNA settled at a low point in the tube because the 15N made it heavy
Describe step 2 of Meselson-Stahl experiment
1) the 15N bacteria were washed then transfered to a medium containg the lighter isotope of nitrogen, 14N and were allowed to divide once more
1i) the washing prevented contaminated the 14N medium with 15N was not incorporated into any new DNA strands
Describe step 3 of Meselson-Stahl experiment
1) DNA from this first generation culture was centrifuged and had a midpoint density
1i) this ruled out Conservative replication because that would produce a band showing the parental molecule that was
entirely heavy
1ii) the intermediate position could imply one strand of the new DNA molecule was an original strand of 15N DNA and the other half was newly made with 14N as in semi-conservative replication or it could imply that all strands contaminated a mixture of light and heavy as in dispersive replication
Describe step 4 of Meselson-Stahl experiment
1) DNA from the second generation grown in 14N settled at the midpoint and highpoint in the tube, in equal amounts
1i) the sample at the midpoint had intermediate density and the sample at the highpoint was light containing nitrogen that was 14N only
1ii) this rules out dispersive replication because if that were the case there would always be a mixture of light and heavy in every strand and one band only would form
1iii) one parental strand is conserved so this is conclusive evidence for the semi-conservative hypothesis
DNA is a store of…
Genetic information, coded in the sequence of bases in the DNA, in thousands of sections along its length, called genes
The base sequence…
Determines which proteins are made and because enzymes are proteins it determines which reactions can take place in an organism
What did biochemical experiments show?
That a Polynucleotide strand always had 3 times the number of bases than the amino acid chain it coded for
What would happen if 3 bases were removed from a Polynucleotide chain?
The polypeptide made would have one fewer amino acid
If the Polynucleotide had 3 extra bases…
The polypeptide would have one more amino acid
3 bases coded for
1 amino acid
If one base coded for 1 amino acid how many amino acids could be made?
4 amino acids could be made
If two bases coded for 1 amino acid how many amino acids can be made?
16 amino acids
If 3 bases Coded for 1 amino acid how many amino acids can be made?
64 amino acids
The code is a triplet code because
3 bases encode each amino acid
Why is the genetic code redundant?
All amino acids have more than one code
Why is the genetic code a degenerate code?
You cannot work out what the DNA code if you are given the amino acid
Why is the code described as universal?
In all living organisms the same triplet codes for the same amino acid
The code is non-overlapping
Each base occurs in only one triplet
DNA contains the information for doing what?
Making polypeptides
What are exons?
Regions of dna that code for proteins
What are introns?
Non-coding dna between exons
What is RNA splicing?
The introns must be cut out of the mRNA before the mRNA leaves the nucleus.
What is transcription?
One strand of the DNA acts as a template for the production of mRNA, a complementary section of part of the DNA sequence. This occurs in the nucleus
What is translation?
The mRNA acts as a template to which complementary tRNA molecules attach, and the amino acids they carry are linked to form a polypeptide. This occurs on ribosomes in the cytoplasm
Protein synthesis can be summarised as:
DNA transcription in nucleus->mRNA-> translation at ribosome - >polypeptide
State the relevant steps in transcription
1) the enzyme DNA helicase breaks the hydrogen bonds between the bases in a specific region of the DNA molecule. This causes the two strands to separate and unwind, exposing nucleotide bases
2) the enzyme RNA polymerase binds to the template strand of DNA at the beginning of the sequence to be copied
3) free RNA nucleotides align opposite the template strand, based on the complementary relationship between the bases in DNA and the free nucleotides. For example, a nucleotide containing cytosine-guanine Adenine-thymine.
4) RNA polymerase moves along the DNA forming bonds that add RNA nucleotides, one at a time, to the growing RNA strand. This results in the synthesis of a molecule of mRNA alongside the unwound portion of the DNA. Behind the RNA polymerase the DNA strands rewind to reform the double helix
5) the RNA polymerase separates from
the template strand when it reaches a ‘stop’ signal
6) the production of the transcript is complete and the newly formed RNA detatches From the DNA
State the steps in translation
1) INITIATION- a ribosome attaches to a ‘start’ codon at one end of the mRNA molecule
2) the first tRNA with an anticodon complementary to the first codon on the mRNA attaches to the ribosome. The three bases of the codon on the mRNA bond to the three complementary bases of the anticodon on the tRNA with hydrogen bonds
3) A second tRNA with an anticodon complementary yo the second codon on the mRNA attaches to the other attachment site and the codon and anticodon bond with hydrogen bonds
4) ELONGATION- the two amino acids are sufficiently close for a Ribosomal enzyme to catalyse the formation of a peptide bond between them
5) the first tRNA leaves the ribosome leaving its attachnent site vacant. It returns to the cytoplasm to bind to
another copy of its specific amino acid
6) the ribosome moves one codon along the mRNA strand. The next tRNA binds
7) TERMINATION- the sequence repeats until a ‘stop’ codon is reached. The ribosome mRNA polypeptide complex separates
What is a polysome?
Usually several ribosomes bind to a single mRNA strand, each reading the coded information at the same time. This is called a polysome. Each ribosome produces a polypeptide, so several are made at once
Energy from ATP is needed to attach the amino acid
What is this process called?
To the tRNA
Amino acid activation
What led to the one gene one polypeptide hypothesis?
It was realised that many proteins such as haemoglobin contain more than one polypeptide
(A gene is a sequence of DNA based that codes for a polypeptide)
Polypeptides can be chemically modified by combinations with non-proteins such as:
- carbohydrates, making glycoproteins
- lipids, making glycolipids
- phosphate, making phosphoproteins
What led to the one gene one enzyme hypothesis
- Once people understood that the genetic material was made of DNA, they wondered how the information for, not only the structure and workings of the cell, but also a whole organism can be encoded
-Experiments on the fungus ‘Neurospora Crassa’ in the 1940’s showed that radiation damage to DNA prevented a single enzyme from being made led to the one gene-one enzyme hypothesis
~ but enzymes are a particular type of protein , so the idea was extended to become the one gene -one protein hypothesis
The base sequence of a gene determines…
the primary structure of a polypeptide
What functions do proteins have in living organisms?
- enzymes
- Antibodies
- Hormones
- Transport proteins
- Forming structures
Briefly describe post-translational modification
- Polypeptides made on ribosome are transported through the cytoplasm to the Golgi body
- Occasionally the primary strucure of a polypeptide is functional but usually, in the Golgi body, the polypeptide is folded into secondary, tertiary and quaternary structures and it may be chemically modified as well
- The modification of a polypeptide is called ‘post translational modification’
Name a highly modified protein (molecule) and explain why it is
~ HAEMOGLOBIN
- Each polypeptide has alpha helix regions (secondary structure) and is folded (tertiary structure)
- Four polypeptides are combined (quaternary structure)
- The protein is modified by combinations with 4 non-protein haem groups to make the functional molecule
Th importance of tRNA
- The molecule bridges the gap between coding information and directing the synthesis of protein
