1.5 Nucleic Acids Flashcards
Define a nucleotide
Monomer of nucleic acid comprising a pentose sugar (deoxyribose or ribose), a nitrogenous base and a phosphate group.
-all 3 sub components are joined in a condensation reaction
Pyrimidine bases
Thymine, cytosine and uracil
Purine bases
Adenine and guanine
In the structure of DNA, what bonds join the phosphate of one monomer to the pentose of another?
Phosphodiester bond
Define an autotrophic organism as well as the 2 sub types
-autotrophic: convert other forms of energy into chemical energy
-chemoautotrophic: use energy derived from oxidation of electron donors such as H2, Fe2+ (some bacteria and archaea)
-phototrophic: use light energy in photosynthesis (green plants)
Define a heterotrophic organism
-organisms that derive their energy from food, such as mammals and dentritvores
Where do organisms store chemical energy
Lipids and carbohydrates
How is energy released from ATP
-enzyme ATPase hydrolyses the bond between the 2nd and 3rd phosphate groups leaving only 2.
-ATP molecule is hydrolysed to ADP and an inorganic phosphate ion with the release of chemical energy
-exergonic reaction
How much energy does each hydrolysed mole of ATP produce?
30.6kJ
What is phosphorylation?
The addition of a phosphate to ADP
Advantages in having ATP as an intermediate in providing energy as oppose to glucose
-hydrolysis of ATP involves a small reaction releasing energy immiedately whereas breakdown of glucose involved many intermediates & takes longer
-only 1 enzyme needed for ATP but many for glucose
-ATP releases energy in small amounts when needed as oppose to large amounts at once
-AP provides a common source of energy for many different chemical reactions which increases the efficiency and control of the cell
Explain the roles of ATP
-metabolic processes - build large, complex molecules ie DNA synthesis
-active transport - allow molecules to be moved against concentration gradient
-movement - muscle contraction
-nerve transmission
-secretion
Describe the structure of DNA
-composed of 2 polynucleotide strands wound around each other in a double helix held together by hydrogen bonds.
- sugar-phosphate backbone joined by condensation reactions creating phosphodiester bonds
-pentose sugar is deoxyribose
Why is DNA antiparallel?
-polynucleotide strands run in opposite directions to allow for hydrogen bonding between complimentary base pairs
Why is DNA suited to its functions? (4)
- very stable molecule and its genetic information passes essentially unchanged through generations
- very large molecule so can carry lots of genetic information
- 2 strands are able to separate as held by hydrogen bonds
- base pairs are inside the double helix so the genetic information is well protected
Describe the structure of RNA
- single stranded polynucleotide
- contains pentose sugar ribose
- contains adenine, uracil, cytosine and guanine
What is mRNA?
- messenger RNA
- long, single stranded molecule
- synthesised in nucleus from a template strand of DNA and carries genetic code to ribosomes in cytoplasm via the nuclear pore
- 3 bases = codon
- varied length dependant on the genes they transcribe
What is rRNA?
- ribosomal RNA
- found in cytoplasm and comprises large, complex molecules
- ribosomes are made of rRNA and protein
- they are the site of translation of the genetic code into protein
What is tRNA?
- transfer RNA
- small, single stranded molecule that folds in so many places that there are base sequences forming complimentary pairs
- cloverleaf shape
- the 3’ end has CCA where the specific amino acid the molecule carries is attached
- carries sequence of 3 bases called the anticodon
Who proposed the molecular structure of DNA and when?
James Watson and Francis Crick in 1953.
Where is DNA found?
- eukaryotic cells: enclosed in the nuclei
- prokaryotic cells: loose in the cytoplasm
—> separate DNA also in mitochondria and chloroplasts
What is DNA replication?
Where 2 complimentary strands of DNA separate and two identical double helices can be formed with each original strand acting as a parent template
What is protein synthesis?
Where a sequence of bases represents the information carried in DNA and determines the sequence of amino acids in proteins
When does DNA replication take place in eukaryotic cells?
Interphase
What are the 3 theories of replication?
- conservative
- semi-conservative
- dispersive
What is the conservative theory?
-parental double helix remains in tact and a whole new double helix is made
—> INCORRECT
What is the semi-conservative theory?
- parental double helix separates into 2 strands, each of which acts as a template for the synthesis of a new strand
—> CORRECT
What is the dispersive theory?
- two new double helices contain fragments from both DNA strands
—> INCORRECT
What is the Meselson Stahl experiment?
- investigation into how DNA separates
Step 1 of MS experiment
- cultured bacterium e.coli for several generations in a broth containing amino acids and the heavy isotope 15N as oppose to the usual 14N found in nitrogenous bases
- bacterium incorporate 15N into nucleotides and then DNA until it only has 15N
- extract DNA and put in centrifugal tube
- DNA settled low due to heavy 15N
Step 2 of MS experiment
- 15N bacteria were washed and transferred to a medium containing 14N and divided once more
Step 3 of MS experiment
- DNA from first gen culture is centrifuged and had a midpoint density
- ruled out conservative as that would produce a heavy molecule
- intermediate position could imply one strand was 14N and one was 15N showing semi- conservative but it could also be mixed strands showing dispersive
Step 4 of MS experiment
- DNA from 2nd gen grown in 14N settled at high and middle in equal amounts which rules out dispersive as only 1 band would form
- one parental strand conserved which proves semi-conservative
Explain the role of the enzyme DNA helicase in DNA replication
- unwinds the double helix by breaking the hydrogen bonds between base pairs on the 2 antiparallel polynucleotide DNA strands to form 2 separate strands
Explain the role of the enzyme DNA polymerase in DNA replication
- catalyses condensation reactions between the activated nucleotides, forming phosphodiester bonds
- synthesises the new strands from template strands
—> releases 2 extra phosphates which gives ATP
What is the role of free nucleotides?
- also known as activated nucleotides or nucleoside triphosphates
- align with complimentary bases on template DNA and are held by hydrogen bonds
- when DNA polymerase forms the phosphodiester bonds, activated nucleotides lose 2 phosphate groups which releases energy needed for reaction
What is DNA?
- store of genetic information coded in the sequence of bases in the DNA
- determined the characteristics of an organism
What are the characteristics of DNA?
- triplet code
- 64 possible codes but only 20 amino acids - more than one code for the same amino acid (known as degenerate)
- code is punctuated - 3 codes that do not code for AA. Known as stop codons in mRNA
- universal - all organisms have the same triplet codes for the same amino acids
- do not overlap
What is an intron?
A non-coding nucleotide sequence in DNA and pre-mRNA that is removed from pre-mRNA to produce mRNA
What is an exon?
A coding region in the nucleotide sequence of DNA and pre-mRNA that remains present in the final mRNA after introns are removed
DNA makes new polypeptides. What has to happen in eukaryotic cells first?
- RNA has to be processed first
- initial code much longer than final
- introns are cut out by endonucleases and remaining exons are joined by ligases
Outline the process of protein synthesis
DNA is transcribed in the nucleus to RNA which is then translated to a polypeptide in the ribosome
Define transcription
One strand of DNA acts as a template for the production of mRNA, a complimentary part of the DNA sequence.
Explain transcription
- DNA helicase enzyme breaks the H bonds between bases so the double helix unwinds and separates, exposing the nucleotide bases.
- RNA polymerase binds to the template strand of DNA at the beginning of the sequence to be copied, forming hydrogen and phosphodiester bonds
- free RNA nucleotides align opposite the template strand in order of complimentary bases
- RNA polymerase moves along forming bonds that add nucleotides (synthesis of mRNA molecule)
- strands rewind to form a double helix and the RNA polymerase separates from the template when it reaches a ‘stop’ codon
- mRNA leaves via nuclear pore
Define translation
- sequence of codons on the mRNA used to generate a specific sequence of amino acids, forming polypeptides
Translation takes place on a ribosome and involves tRNA. What are the 2 subunits of a ribosome?
- large: 2 sites of attachment for the tRNA molecules
- small: binds to mRNA
Describe the initiation stage of translation
- a ribosome attaches to a ‘start’ codon at one end of the mRNA molecule
- first tRNA attaches to ribosome and the 3 bases on the codon of the mRNA bond to the 3 bases on the anticodon of tRNA by hydrogen bonds
- second tRNA attaches to other attachment site in same manner
Describe the elongation state of translation
- 2 AA are close enough for the ribosomal enzyme to catalyse a peptide between them
- the tRNA now leaves the attachment site so it’s vacant so the ribosome moves one codon along the mRNA so the next tRNA can bind
- process repeated until polypeptide chain is formed
Describe the termination stage of translation
- the sequence repeats until a ‘stop’ codon is met
- ribosome-mRNA-polypeptide complex separates
- protein synthesis is complete
2 uses of ATP in plant cell
Cell division, active transport
Creatine phosphate increases ATP availability. Under what conditions is ADP -> ATP, and what conditions for ATP-> ADP
ADP -> ATP = exercise so ATP generated quickly due to creatine
ATP -> ADP = relax, creatine phosphate creates to be store of ATP
