Topic 7: DNA Replication, Transcription, Translation Flashcards
Compare the genetic material of prokaryotes and eukaryotes. [6]
Prokaryotic DNA: Plasmids present, circular, one chromosome, naked DNA, no introns, found in nucleotide region
Eukaryotic DNA: No plasmids, linear, many chromosomes, has histones, introns and extrons, in the nucleus
Both: Use DNA as genetic material
Explain the process of DNA replication. [8]
- Semi-conservative replication
- Helicase unwinds the double helix
- And separates strands by breaking hydrogen bonds
- Each strand of parent DNA used as templates
- DNA polymerase adds nucleotides
- Synthesis from 5’ to 3’
- Complementary base pairing
- Adenine-Thymine, Cytosine-Guanine
- Synthesis is continuous on the leading strand, non-continuous on the lagging strand
- Okazaki fragments form on the lagging strand
- DNA polymerase I replace RNA primer with DNA
- DNA ligase joins Okazaki fragments
State the components of a nucleosome.
DNA and histone
State a chemical modification of a nucleosome to impact gene expression.
Methylation, acetylation, phosphorylation, epigenetic tags
Outline the structure and functions of nucleosomes. [4]
- Found in eukaryotes
- DNA wrapped around histones
- Histones in a group of eight
- Regulates transcription
- Supercoils chromosomes
Identify enzymes used in DNA replication.
DNA polymerase, helicase, DNA ligase, RNA primase
Explain the role of Okazaki fragments in DNA replication. [3]
- Formed by non-continuous synthesis on the lagging strand
- Because replication must occur in the 5’ to 3’ direction
- Replication starts repeatedly, moving away from the replication fork
Describe the relationship between genes, polypeptides, and enzymes. [4]
- Genes are a sequence of DNA bases
- Genes code for a specific sequence of amino acids
- Enzymes are proteins composed of polypeptides
- Sequence of amino acids determines tertiary structure
- Enzymes involved in replication/transcription/synthesis of polypeptides
Distinguish between RNA and DNA. [3]
DNA
- double stranded
- deoxyribose
- adenine, thymine, guanine, cytosine
- all helical
RNA
- single stranded
- ribose
- adenine, uracil, guanine, cytosine
- three forms (mRNA, tRNA, rRNA)
Explain how translation is carried out, from the initiation stage onwards. [9]
- Steps: Initiation, elongation, termination
- mRNA binds to the small ribosomal unit of tRNA
- Ribosome slides along mRNA to the start codon (AUG)
- Triples of anticodons on tRNA bind to triples of codons on mRNA
- Complementary base pairing
- Adenine-Uracil, Cytosine-Guanine
- Second tRNA enters A site and binds to codons
- Peptide bond forms between the amino acids
- Ribosome moves along mRNA in a 5’ to 3’ direction
- tRNA that lost its amino acid detaches in the E site
- Translation terminated when stop codon reached
Explain the methods and aims of DNA profiling. [8]
- Obtain DNA sample
- From hair or blood
- DNA amplified through PCR
- DNA cut into fragments
- Using restriction enzymes
- DNA fragments separated through gel electrophoresis
- Using electrical fields
- Separated by size
- Forms pattern of bands unique to the individual
- Used in crime scene investigation
- Used in paternity testing
Explain the significant of complementary base pairing for replication, transcription, and translation. [8]
- A-T, C-G in DNA
- A-U, C-G in RNA
- Replication: CBP ensure identical nucleotide sequence of new complementary strands
- Semi-conservative replication
- Transcription: CBP produces mRNA sequence complementary to DNA sequence
- Translation: CBP converts mRNA sequence into specific amino acid sequence
- tRNA carries triplets of bases - anticodons
- mRNA carries triples of Nucleosomes - codons
- Anticodons bind to complementary codons
Describe ribosome structure. [6]
- Made of protein
- Made of mRNA
- Large subunit and small subunit
- Three tRNA binding sites on large subunit
- A site, P site, E exit
- One mRNA binding site
- 70s in prokaryotes, 80s in eukaryotes
- Can be free or bound (rough ER)
Explain the process of transcription leading to mRNA formation. [8]
- Unwinds double helix and separates strands
- RNA polymerase binds to promoter on DNA
- Binds to antisense strand of DNA
- Synthesis in 5’ to 3’ direction
- Using CBP A-U, C-G
- Until terminator signal is reached
- RNA detaches, DNA rewinds
- RNA polymerase detaches from DNA
- Introns removed from eukaryotes to form mature mRNA
State the bonds that i) Connect base pairs in a DNA molecule; ii) Link DNA nucleotides into a single strand.
I) Hydrogen bonds
Ii) Covalent bonds
Explain tRNA in translation. [3]
- tRNA attaches to the amino acids
- tRNA moves to the ribosome
- Triplets of anticodons bind to triplets of codons on mRNA
Explain the roles of specific enzymes in prokaryotic replication. [7]
- Helicase: Unwinds DNA double helix
- And separates strands by breaking hydrogen bonds
- DNA Primase: Adds primer to DNA
- DNA Gyrase: Relieves strain on replication fork
- DNA Polymerase III: Begins replication at primer
- Synthesis in 5’ to 3’ direction
- DNA Polymerase I: Replaces RNA with DNA
- DNA Ligase: Links Okazaki fragments
- DNA Polymerase I and III: Proofread for mistakes
Outline how translation relies on complementary base pairing. [3]
- Specific amino acids attached to specific tRNA
- Anticodons on tRNA bind to codons on mRNA
- A-U, C-G
- Translation converts sequence of mRNA nucleotides to polypeptide chain
Describe PCR including Taq DNA polymerase. [4]
- Amplifies DNA
- Cycles through high and low temperatures
- Taq withstands high temperatures without denaturing
- High temperatures break hydrogen bonds between DNA strands
- Taq adds complementary bases
Explain benefits and risks of GMO crops. [8]
Environmental benefits: Pest-resistant crops made, less insecticides, longer shelf life, shorter growing times
Environmental risks: Affects non-target organisms, monoculture lessens biodiversity, overuse of herbicides
Human health benefits: Increased nutritional value, produce toxin-free crops
Human health risks: Proteins from transferred genes could be toxic, GMO effects on human unclear
Outline role of ribosomes in translation. [4]
- Produces polypeptide from mRNA sequence
- mRNA and tRNA bind to ribosome
- tRNA base anticodons are complementary to mRNA codons
- Amino acids bind by peptide bonds
- Ribosome moves along the mRNA
Describe the genetic code and its relationship to polypeptides and proteins. [5]
- Genetic code: Triplets of nucleotides called codons
- Bases: DNA - ATCG, RNA - AUCG
- Each codon codes for a specific amino acid
- DNA transcribed to mRNA via CBP
- mRNA translated to polypeptide chain
- Some codons are start or stop codons
- Each gene codes for a polypeptide
Outline a gene transfer technique. [5]
- Plasmid removed from bacteria
- Plasmid is a small circle of DNA
- Restriction enzyme forms sticky ends
- Restriction enzyme cuts DNA with desired gene
- DNA added to open plasmid
- DNA Ligase joins nicks
- Recombinant DNA inserted into host cell
Explain DNA replication. [3]
- Semi-conservative replication
- Helicase unwinds double helix
- Separated strands are templates for new strands
- Nucleotides join template strands through CBP
- DNA polymerase joins nucleotides in new strands
Compare prokaryotic and eukaryotic DNA.
Prokaryotic: In cytoplasm, has plasmid, circular
Eukaryotic: In nucleus, no plasmid, linear
Both: Double helix of bases ATCG
Describe steps to process a small sample of DNA. [6]
- Use PCR to amplify the DNA
- Restriction enzymes cut DNA
- Use gel electrophoresis to separate fragments in size
- Creates DNA profiles
- Make comparisons between patterns
- DNA can be processed long after
Explain how DNA base sequence is conserved during replication. [5]
- Semi-conservative replication
- Uses DNA polymerase
- Helicase unwinds double helix and separates strands by breaking hydrogen bonds
- Forms leading and lagging strand
- DNA polymerase replicates template strands via CBP (A-T, C-G)
- Replicates from 5’ to 3’ direction
- Newly formed strand is identical to the other template strand
The Hershey and Chase experiment supported DNA as the hereditary material. Describe the experiment. [3]
- Radioactive isotopes used to label virus
- Proteins labelled with radioactive sulfur, DNA labelled with radioactive phosphorus
- Phage infects bacterium
- Only viral DNA enters bacterium
- Parts of phage remaining outside bacterial cell are removed
- Bacteria contain the labelled DNA
State one other function of DNA sequences that do not code for protein.
- Regulate gene expression
- Act as promoter
- Introns
Describe the Meselson-Stahl experiment.
- Proved semi-conservative replication
- Bacteria grown in broth containing N15 (heavy nitrogen isotope)
- Sample was spun in a centrifuge, all heavy nitrogen settled at the bottom
- N15 sample added to N14 (light nitrogen) broth
- After one round of replication, DNA spun in a centrifuge again
- DNA settled in the middle of the tube - mixture of both N15 and N14
Outline the use of named enzymes in gene transfer using plasmids. [6]
- plasmids removed from bacteria
- restriction enzymes cut plasmids at target sequence
- DNA fragments of other organism cut using same restriction enzyme
- complementary sticky ends produced
- DNA segment added to open plasmid
- sealed using ligase
- reverse transcriptase makes DNA copies of mRNA
- recombinant plasmid inserted into host cell
- cloned to produce new genes
Explain the control of gene expression in eukaryotes. [8]
- mRNA conveys genetic information from DNA to ribosomes
- gene expression requires production of specific mRNA through transcription
- most genes are turned off
- some genes only expressed in certain cells
- transcription factors increase/decrease transcription
- hormones affect gene expression
- nucleosomes limit access of transcription factors to DNA
- DNA methylation/acetylation to control gene expression
- introns contain positive/negative gene regulators
Explain the consequences of altering a DNA base in the genome of an organism. [8]
- altering a base in DNA is a point mutation
- only has an effect if base is in a gene
- when mRNA is produced by transcription one mRNA base is different
- one codon in mRNA is different
- one amino acid is different in the polypeptide
- polypeptide produced by translation of mRNA
- some base changes do not change the amino acid coded for
- structure of polypeptide altered
- usually polypeptide does not function as well
- e.g. sickle cell anemia mutation from GAG to GTG
Distinguish between transcription and translation. [4]
- DNA is transcribed, mRNA is translated
- transcription produces RNA, and translation produces protein
- RNA polymerase is used only in transcription, ribosomes used only in translation
- transcription in the nucleus of eukaryotes, translation in cytoplasm
- tRNA needed for translation but not for transcription
- sugar-phosphate bonds in transcription and peptide bonds in translation
Describe the primary, secondary and tertiary structure of a polypeptide. [3]
- Primary: sequence and number of amino acids
- Secondary: formation of alpha helices and beta pleated sheets stabilized by hydrogen bonding
- Tertiary: further folding stabilized by interactions between R groups
- Quaternary: exists in protein with >1 polypeptide chain
Outline the process of DNA profiling. [4]
- sample of DNA obtained (hair/blood)
- PCR used to amplify
- using Taq DNA polymerase
- tandem repeats amplified
- gel electrophoresis to separate bands
- according to length/size
- pattern of bands unique to individual
- e.g. forensics/paternity testing
Outline the role of DNA polymerase III in DNA replication. [4]
- binds to template strand at primer
- adds nucleotide to template strand
- using complimentary base pairing
- links nucleotides with sugar-phosphate bonds
- builds new strand in 5’ to 3’ direction
- synthesis on lagging strand is discontinuous (Okazaki fragments)
Explain how living organisms ensure that the amino acid linked to tRNA is always correct. [2]
- enzymes ensure that a specific amino acid binds to tRNA
- tRNA activating enzyme
- enzyme only binds to this tRNA
- different activating enzymes for different tRNAs
- attached amino acid corresponds to anticodon
During translation, three binding sites for tRNA molecules are used. Outline how each of the binding sites is used. [3]
- binding sites on the ribosome
- A: binding of an tRNA carrying an amino acid
- P: where amino acid links to polypeptide via peptide bond
- E: exit/detachment of tRNA from ribosome
Outline how proteins can be separated by gel electrophoresis. [3]
- separates molecules based on size and charge
- proteins differ in size/charge
- placed in a block of gel
- gel placed in an electrical field, an electric current is ran through
- proteins move through gel
- separated according to size, small proteins move further
- size markers/ladders used
