1
Q
What are DNA and RNA made up of
A
monomers called nucleotides
2
Q
What does each nucleotide contain
A
- phosphate group
- nitrogen containing organic base
- pentose sugar - ribose - RNA Deoxyribose - DNA
3
Q
What are the two groups of organic bases
A
- pyrimidines - single ring
- purines - double ring
4
Q
What are the four nitrogenous bases in DNA and state what group they are in
A
- guanine - purine
- cytosine - pyrimidine
- adenine- purine
- thymine - pyrimidine
5
Q
What is different about the nitrogenous bases of RNA and DNA
A
the pyrimidine uracil replaces thymine
6
Q
What does ATP stand for and what type of monomer is it
A
- adenosine triphosphate
- nucelotide
7
Q
What is ATP made up of
A
- ribose sugar joined to an adenine base with three phosphate groups attatched
8
Q
Describe what happends when an energy bond between the phosphate groups are broken in ATP what type of reaction is it and give an equation
A
- when the high energy bond between the second and third phosphate group is broken via hydrolysis by the enzyme ATPase 30.6KJ of energy is released for use in the cell and adenosine diphosphate is formed
- reversible requiring energy from respiration of glucose to reform the bond
- ATP -> ADP+Pi
9
Q
What are the advantages of ATP
A
- energy is released quickly from a one step reaction involving just one enzyme - hydrolysis of glucose takes many steps
- energy is released in small amounts 30.6KJ where it is needed by contrast one molecule of glucose containes 1880KJ which couldn’t safely be realeased all at once
- universal energy currency - common source of energy for all reactions in living things
10
Q
What are the role of ATP in cells
A
- used in many anabolic reactions eg DNA and protein synthesis
- active transport
- muscle contraction
- nerve impulse transmission
11
Q
Describe the structure of DNA
A
- consist of two polynucleotide strands that are arranged into a double helix
- the first dinucleotide is formed when a condensation reaction occurs between the first two nucleotides
- 5th carbon atom of a deoxyribose sugar is joined to the 3rd carbon atom of the deoxyribose sugar of the nucleotide above it via the phosphate molecule
- continues building a single strand of DNA in 5’-3’ direction
- DNA then forms a double stranded molecule from two strands one strand runs int he opposite direction to the other (anti parallel) both strands are held together by hydrogen bonds that form betwen complementary nitrogenous bases
- double strand then twists to form a double helix
- bases between both strands pair up - complementary base pairing guanine form 3 hydrogen bonds with cytosine molecule and adenine form 2 hydrogen bonds with adjacent thymine
12
Q
Why is a DNA molecule hard to break
A
- hydrogen bonds are weak however there a large number of them present in a molecule of DNA over a million nucleotides long means collectilvey they are very strong
13
Q
How can DNA be extracted
A
- grinding up a sample in a solution of ice cold salt and washing up liquid
- the detergent dissolves the lipids in the phospholipid membranes allowing DNA to be released
- cold temperature protects the DNA from cellular DNAases
- addition of protease will digest any remaining cellular enzymes and histones that the DNA is wound around
- add ethanol to the salt already present will cause the DNA to precipitrate out from the solution
14
Q
Descrive how RNA differs from DNA
A
- shorter
- single stranded
- sugar is ribosse
- one base thymine replaced with uracil
15
Q
What is mRNA
A
- messenger RNA is a single stranded molecule typically 300-2000 nucleotides long
- produced in the nucleus using one of the DNA strands as a template during transcription
16
Q
what is tRNA
A
- transfer RNA is a small molecule that winds itself into a cloverleaf shape
- has an anticodon at one end and an amino acid ad the other end
- transfer the correct amino acid to the growing polypeptide during translation
17
Q
What is rRNA
A
- ribosomal RNA forms ribosomes with the addition of protein
18
Q
What are the two functions of DNA
A
- protein synthesis
- replication
19
Q
What is protein synthesis
A
- the sequence of bases in one strand called the template strand determines the order of amino acids in the polypeptide
20
Q
What is replication
A
- when cells divide a complete copy of DNA in the cell needs to be made both DNA strands sepearate and each strand acts as a template to synthesise a complementary strand
21
Q
What are the three theories for how DNA replicates and explain each one
A
- conservative replication - original parent double stranded molecule is conserved and a new double tranded DNA molecule synthesised from it
- semi conservative replication - parental strands seperate and each strand acts as a template to synthesise a new strand the new molecule consists of one original parents stand and one newly synthesised strand
- dispersive - newly synthesised molecule contains fragments from the original parent strand and newly synthesised DNA
22
Q
Describe the process of semi conservative DNA replication
A
- process requires ATP, free nucleotides and enzymes
- DNA helicase breaks the hydrogen bonds between the bases causing the double helix to unwind and seperate into two strands
- the exposed bases bind to free floating nucleotides in the nucleoplasm
- DNA polumerase binds the complementary nucleotides forming the phosphodiester bond
- one strand act as the template for the new molecule so newly synthesised DNA contains one parent strand and a complementary newly syntheised strand
23
Q
What is the meselson stahl experiment
A
important experiment carried out to determine the exact mechanism for DNA replication
24
Q
Describe the meeselsohn and stahl experiment
A
- growing bacteria on a N15 medium. N15 is a heavy isotope of nitorgen so all DNA produced would be heavier wieght than normal when DNA was extracted by centrifuging in caesium chloride the DNA band appeared low down the tube
- bacteria were then grown on a N14 medium (normal wieght nitrogen)and after one generation the DNA extracted formed an intermediate band halfway up the tube this is because the DNA molecule contained on strand from the heavy parent DNA and one newly synthesised light DNA strand - one band only produced rules out conservative replication
- the bacteria were grown for a further generation using N14 medium the DNA extracted formed an intermediate band halway up the tube and a lighter band towards the top of the tube because half of the DNA was intermediae weight and half light this rules out dispersive replication
- DNA therefore replicates semi conservatiley
- if grown for further generations using N14 medium whilst intermediae weight DNA would remain the proportion of light DNA produced would increase
25
What is the genetic code
* sequence of nucletodie bases form a code
* each code has three letters called a triplet code or codon
* which codes for a specific amino acids
26
What are the characteristics of the genetic code
* code is universal - same in all living things
* degenerate - 20 amino acids coded by 4^3 vases - 64 diffferent combinations of bases therfore there are spare base codes
* Start codon - marking the point on DNA where transcription begins
* each gene found on DNA will code for a different polupeptide - one gene polypeptide hypothesis
27
Outline the basic of protein synthesis
* transcription occurs in the nucleus
* translation occurs in the ribosomes
* post translational modification occurs in the golgi apparatus prior to packaging of proteins into vesicles
28
codon
the triplet of bases in mRNA that codes for a particular amino acid or a punctuation signal
29
Describe the process of transcription
* DNA act as a template for the production of mRNA
* DNA helicase acts on a specific region of the DNA molecule called the cistron breaking hydrogen bonds between both DNA strands causing the strands to seperate and unwind exposing nucleotide bases
* free RNA nucleotides pair to exposed bases on the DNA template strand and RNA polymerase joins them by forming the phosphodiester bonds between the phosphate group on one nucleotide and the ribose sugar on the next
* this continues until the RNA polymerasse reaches a stop codon where RNA polymerase detaches and production of mRNA is complete
* the mRNA strand then leaves the nucleus via the nuclear pores and moves to the ribosomes
30
What happens to pre-mRNA after transcription in eukaryotes
* introns are present within many genes so are also transcribed producing pre mRNA
* coding regions are referred to as exons
* pre mRNA is spliced to remove the non coding redions before passing to the ribosomes
31
introns
non coding nucleotide sequence in DNA and pre mRNA that is removed from pre mRNA to produce mature mRNA
32
exons
nucleotide sequence on one strand of the DNA molecule and the corresponding mRNA that codes for the production of a specific polypeptide
33
What is tRNA
* involves another specific RNA molecule called transfer RNA
* at one end of the tRNA molecule there are three exposed bases called the anticodon these are complementary to the mRNA codon
* at the opposide end of the tRNA molecule is an amino acid attatment site where the relevant amino acid is found
* the attatchement of the relative amino acid to the attatchment site is called amino acid activation and requires ATP
34
Where to the mRNA and tRNA strands go in ribosomes and what is translation
* involves converting the codons on the mRNA into a sequence of amino acids known as a polypeptide
* each ribosomes found free in the cytoplasm or found attatched to the rough endoplasmic reticulum is made up of two subunits ribosomal RNa and protein
* the mRNA binds to the smaller subunite whilst tRNA binds to one of two attatchement sites on the larger subunit
35
Describe the process of translation
* initiation - ribosome attatches to the start codon
* tRNA molecule with a complementary anticodon to the first codon binds to the first attatment site on the ribosome
* a second tRNA molecule joins to the second attatchment site and a ribosomal enzyme catalyses the formation of a peptide bond between the two amino acids - elongation
* first tRNA molecule is released and the ribosome now moves one codon along the mRNA whcih exposes a free attatchement site and another tRNA molecule joins and process is repeated
* this repeats until a STOP codon is rreached when the polypeptide is released - termination
* usually several ribosomes bind to a single mRNA strand at the same time - polysome
36
Describe the process of post translational modification
* trannslation produces a polypeptide
* further modification needed to produce a protein with secondary, tertiary or quantenary structure
* occurs within the golgi body
* modification pccurs to produce glycoproteins , lipoproteins and complex quanternary structures such as haemoglobin
37
How id haemoglobin formed
* two alpha chains and two beta chains coded by two different genes assembled together with iron as a prosthetic group
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