Unit 1 Topic 2B Proteins and DNA Flashcards
Describe the function of carrier proteins in a cell surface membrane.
- Involved in facilitated diffusion
- Movement of {large / polar molecules}
- (Facilitated diffusion from a high concentration to a low concentration
- Involved in active transport
- Needs ATP to move molecules against concentration gradient
Describe the roles of RNA in the synthesis of translation of haemoglobin
- a mRNA molecule(produced by transcription) codes for each of the polypeptide chains of haemoglobin
- mRNA leaves nucleus and moves to cytoplasm
- Each tRNA carries its own specific amino acid to ribosome
- anticodon on tRNA binds to codons on mRNA by complementary base pairing
- tRNA holds amino acid in place while peptide bonds are formed between amino acids
- Reference to start and stop codons on mRNA (start and stop transcription)
Process of DNA replication
- The 2 strands of DNA molecule unzips along the line of hydrogen bonds by action of DNA helicase. (catalyzes the breakage of hydrogen bonding)
- 2 strands of unzipped DNA act as templates for synthesis of new DNA strands
- Exposed nitrogenous bases attract free DNA nucleotides; new hydrogen bonds are formed between matching complementary base pairs
- New DNA strands are formed along the template strands by DNA polymerase between adjacent free nucleotides (5’ to 3’ end)
- New and short sections of DNA are formed by DNA ligase between nucleotides between adjacent segments of new DNA
- Identical DNA molecules are formed
Properties of triplet code
(DONUT)
- Degenerate - codes for limited amino acids
- Overlapping between codons
- Non (goes with above)
- Universal - codes for same amino acid in all organisms
- triplet code - 3 consecutive bases codes for 1 amino acid
Describe how the induced-fit mechanism differs from the ‘lock-and-key’ model.
- The shape of the active site of enzyme is modified when the substrate binds to it according to induced-fit mechanism
- The shape of the active site is fixed and perfectly complementary to the shape of the substrate according to the lock-and-key model
Describe the roles of transcription and translation in the synthesis of globular protein by a muscle cell.
- The {gene / sequence of DNA} for the (globular protein is transcribed
- Complementary base pairing between RNA nucleotides and DNA (to produce mRNA)
- mRNA leaves the nucleus and attaches to a ribosome
- Pairing between codons on mRNA and anticodons on tRNA
- tRNA provides specific amino acids
- The sequence of {bases / codons} determines the {sequence of amino acids / primary structure of the protein}
Function of enzymes
- Can lower activation energy
- Increase rate of reactions, without itself being changed or used up
Process of enzymes
- Substrates bind to the active site of an enzyme molecule to form an enzyme-substrate complex
- Enzymes lower the activation energy of the reaction to catalyse the reaction
- After the reaction, enzyme detaches from the product
Conservative replication
- Original DNA remains intact,
- New identical double helix formed from entirely new material.
Semi-conservative replication
- DNA unzips, new nucleotides align along each strand
- Each new double helix has one strand made of completely new material and the other strand of the original DNA
State the steps of Meselson and Stahl’s experiment
- Bacteria was grown in a 15N (heavy) medium for many generations
- Preparation of DNA by adding CsCl to make the solution dense enough for centrifugation.
- The sample (gen 0) was centrifuged and only 15N (heavy) bands were observed.
- The bacteria was transferred to a 14N (light) medium and allowed to replicate.
- A sample was taken every 20 minutes and each sample was centrifuged
- Gen 1: All DNA were of intermediate density and only 15N-14N bands were observed
- Gen 2: ½ DNA were of intermediate density and ½ were of light density.
- Both 15N-14N bands and 14N bands were observed.
State the steps of DNA replication
In semi-conservative DNA replication,
- DNA helicase unzips the two strands of DNA
- The nitrogenous bases are exposed, nucleotides line up along both strands.
- DNA polymerase facilitates the complementary base pairing between bases by catalysing the formation of covalent phosphodiester bonds between sugars and phosphate groups by condensation reactions.
- DNA ligase binds the DNA fragments together by catalysing the formation of covalent phosphodiester bonds between newly-bound nucleotides by condensation reactions.
- The completed DNA strands recoil as a double helix.
State the process of transcription
- DNA helix unwinds, expose base to act as template, hydrogen bonds between complementary strands are broken. (catalysed by DNA helicase)
- Free RNA mononucleotides in nucleus line up against and attach to one coding strand of DNA. There is complementary base pairing between ribonucleotides by RNA polymerase.
- RNA nucleotides are joined together by the formation of phosphodiester bonds in a condensation reaction by RNA polymerase.
- The completed pre-mRNA strand detaches from the DNA strand
- Pre-mRNA modified to become mRNA (splicing, remove introns) and moves out of the nucleus to the cytoplasm for translation.
Process of translation
- mRNA molecules bind to the small subunit of ribosomes
- tRNA molecules bind to a specific amino acid corresponding to the anticodon
- anticodon of tRNA line up against and attach to the codon of mRNA and hydrogen bonds are formed between the bases of tRNA and mRNA
- another tRNA molecule binds to the next codon
- peptide bond formed between two adjacent amino acids by condensation reaction
- polypeptide chain is formed as the process repeats
- when stop codon reached, polypeptide chain released, ribosome complex disassembles and is ready for reuse
Stop codon properties
a) Occurs at the end of the gene
b) Recognised by the ribosomes and Indicates when polypeptide synthesis ends and
c) Indicates when tRNA molecules stop attaching on the mRNA strand.
d) The polypeptide chain would then be released and the ribosome complex disassembles and is ready for reuse.
tRNA vs mRNA (differences and similarities)
Differences:
- tRNA is folded/ is clover shaped but mRNA is straight and unfolded
- tRNA has a fixed length but mRNA does not and the length depends on the size of the gene
- tRNA has anticodon but mRNA has codon
- tRNA has an amino acid binding site but mRNA does not.
- tRNA has hydrogen bonds holding the structure together but mRNA does not
Similarities:
- Both contain RNA mononucleotides and have uracil, ( adenine, cytosine and guanine )
- Both are single stranded
Explain how the three-dimensional structure of lactase affects the mechanism of action of this enzyme. (3)
- lactase is soluble because of its {globular shape / external polar R groups} (1)
- (and therefore) lactase collides with lactose (1)
- active site of lactase is complementary to the lactose (1)
- Formation of enzyme-substrate complex lowers the activation energy (1)
What is RNA
- polymer of a nucleotide formed of a ribose, nitrogenous base, and phosphate group
What are the nitrogenous base of RNA
- adenine
- guanine
- cytosine
- uracil
What is the function of the RNA
- copy and transfer genetic code from DNA in the nucleus to ribosomes
What is mRNA
- copy of gene from DNA
- created in the nucleus
- leaves the nucleus to carry the copy of genetic code of one gene to a ribosome in the cytoplasm
Why don’t DNA just leave nucleus and we need mRNA
- mRNA is much shorter (length of one gene)
- DNA is too large to leave the nucleus and would be at risk of being damaged by enzymes
What are the properties of mRNA
- short lived: only for temporarily helping to create a protein, would have carried out its function before enzyme break it down
- single-stranded
What is a codon
every 3 bases in the sequence code for a specific amino acid
Where is tRNA found at
cytoplasm only
What are the shape and properties of tRNA
- single stranded
- cloverleaf shape (folded)
- held in place by hydrogen bonds
What is the function of tRNA
- attach to one of the 20 amino acids
- transfer the amino acid to ribosome to create peptide chain
What is the mechanism behind amino acid attaching to tRNA
- specific amino acids attach to specific tRNA molecules and brought to the mRNA (at ribosome)
- determined by the 3 bases (exposed on top) found on the tRNA
anticodon
tRNA
- complementary to the codon on mRNA
What is rRNA
- combiens with protein to make up the bulk of ribosomes
Differences between DNA and RNA monomers
DNA
- contains base thymine
- pentose sugar: deoxyribose
RNA
- contains uracil
- pentose sugar: ribose
Differences between DNA and RNA polymers
DNA
* larger
* 23000 genes
* double-stranded
RNA
* shorter
* only the length of one gene
* single-stranded
(past paper: spec) Suggest one way in which the cells use uridine monophosphate
- synthesis of RNA
(past paper: spec) Explain why each codon for the DNA genetic code must contain at least three bases? (3)
- enough codons needed for 20 different amino acids
- four bases used in genetic code
- triple codes provide enough possible (43 / 64) codons for amino acids
(past paper: spec) Collagen is an insoluble, fibrous protein. Describe the roles of RNA in the synthesis of collagen. (5)
- mRNA molecule codes for each of the polypeptide chains in collagen
- mRNA carries copy of genetic code for collagen out of the nucleus to the ribosomes
- each tRNA carries its own specific amino acid to the ribosome
- anticodon on tRNA binds to codon on mRNA
- tRNA holds the amino acid in place while peptide bonds form
- start and stop codons (signal start and end of translation)
(past paper: spec) Explain how the primary structure of collagen determines its properties. (4)
insoluble
- amino acids
- many hydrophobic R groups
strong
- triple helix
- many repeating amino acid sequences
- many small R groups so the triple helix can form
