Genetic Code, Protein Synthesis, Protein Degradation and Protein Secretion

Question 1

state what it is meant by the key term ‘gene’

Answer 1

a gene is a sequence of DNA which codes for a specific protein

Question 2

state what it is meant by the key term ‘genome’

Answer 2

1. the genome is the total genetic information within a typical cell within an organism
2. human genome contains roughly 20,000 genes
3. approximately 3 billion nucleotides in the whole human genome

Question 3

state what it is meant by the key term ‘nucleosomes’

Answer 3

nucleosomes are long DNA molecules, longer than the Nuclei’s diameter, which coil around histones at frequent intervals to fit inside the nucleus

Question 4

state what it is meant by the key term ‘promotor region’

Answer 4

the promotor region is a specific sequence of RNA bases in which RNA Polymerase binds onto

Question 5

what are the 4 steps of Transcription

Answer 5

1. DNA Helices unzips DNA strands by breaking H-bonds
2. free RNA nucleotides bind to DNA bases, via H-bonds, via complementary base pairing
3. RNA polymerase binds to promotor region and forms phosphodiester bonds between RNA nucleotides via hydrolysis reactions
4. RNA reaches a stop signal/sequence and releases from the pre-mRNA strand

Question 6

describe the process of ‘splicing’

Answer 6

1. splicosomes identify specific sequences of nucleotides at the beginning and end of intron regions and removes the intron segment from the pre-mRNA strand
2. leaves an exon rich mRNA strand and occurs in the Nucleus

Question 7

explain how splicing leads to there being more proteins in the human body than there is genes

Answer 7

1. exon derived segments from a gene can be spliced together in different sequences or some exon-derived areas can be deleted all together
2. results in the formation of different mRNA strands from the same gene so forms different proteins
3. known to occur in >50% of genes
4. after splicing, mRNA moves through the nuclear pores into the cytoplasm

Question 8

what is rRNA

Answer 8

rRNA = ribosomal RNA

Question 9

describe the structure of ribosomes

Answer 9

ribosomes are formed of around 70-80 different proteins associated with a class of rRNA molecules

Question 10

describe a quick overview of ribosomal synthesis (4 things)

Answer 10

1. genes for rRNA transcribed similarly to mRNA except with a different RNA polymerase
2. synthesised in ribosomes specific to rRNA
3. translated proteins move back into nucleus, via nuclear pores, where they combine with newly formed rRNA to form a small and a large ribosome sub unit
4. subunits are then individually transported to cytoplasm where they combine to form a functional ribosome for translation

Question 11

state 3 facts about Transfer RNA (tRNA)

Answer 11

1. tRNA binds to mRNA codon with it’s specific anticodon via complementary base pairings
2. tRNA is covalently linked to a specific amino acid the enzyme aminoacyl-tRNA synthetase
3. 20 differed aminoacyl-tRNA synthetase enzymes, each of which catalyses the linkage of a specific amino acid to a specific tRNA

Question 12

what are the 3 stages, in order, of translation

Answer 12

1. the initiation phase
2. the elongation phase
3. the termination phase

Question 13

describe, in 3 steps, the initiation phase of protein translation

Answer 13

1. tRNA, containing Methionine, binds to the small ribosomal sub-unit
2. initiation factors (proteins), establish an initiation complex which positions the Methionine containing tRNA opposite there mRNA codon signalling the start site
3. large ribosomal sub-unit then binds, enclosing the mRNA between the two sub-units

Question 14

describe, in 1 step, the termination phase of protein translation

Answer 14

when the ribosome reaches a termination site, known as a stop codon, the link between the polypeptide chain and the last tRNA is broken, and the completed polypeptide chain is released

Question 15

describe, in 5 steps, the elongation phase of protein translation

Answer 15

1. a ribosome has 2 binding sites for tRNA
2. site 1 holds the tRNA portion linked to the protein chain that has been assembled unto this point
3. site 2 holds the tRNA containing the next amino acid to be attached to the polypeptide chain
4. ribosomal enzymes catalyse the linkage of the protein chain to the newly arrived amino acid
5. site 1 then releases the tRNA, and the tRNA in site 2 is transferred to site 1

Question 16

once a polypeptide chain has been formed, what may it undergo after translation

Answer 16

once a polypeptide has been formed via transcription, it may undergo post-translational modifications to it’s amino acid sequence

Question 17

state 3 examples of post-translational modifications

Answer 17

1. hydrolysing enzymes break polypeptide chain into smaller polypeptides, each of which perform it’s own function
2. covalently link to carbs/fats to protect the protein from rapid degradation by photolytic enzymes
3. covalently bind to fatty acids to anchor binding to a membrane

Question 18

state what it is meant by the key term ‘protein degredation’

Answer 18

protein degradation is the process of recognising and rapidly degrading proteins within a cell, eliminating consequences of mistakes in protein synthesis

Question 19

state 2 facts about protein degradation

Answer 19

1. different proteins degrade at different rates as some proteins have higher affinity for proteolytic enzymes than others
2. a denatured (unfolded) protein is more readily digested than a protein with an intact conformation

Question 20

describe how protein degradation occurs

Answer 20

1. proteins are targeted for degradation but the attachment of a small protein called ‘Ubiquitin’
2. Ubiquitin directs the protein to a protein complex, known as a proteasome, which unfolds the protein and breaks it down into smaller peptides

Question 21

why is protein degradation important

Answer 21

protein degradation is important for ensuring the activity of a protein is only there for a precise window of time

Question 22

during the first 30 amino acids of translation, what is discovered

Answer 22

proteins that are going to form integral membrane proteins or proteins that are to be exported from the cell are discovered in the first 30 amino acids during translation

Question 23

what do the first amino acids act as and what do they do (protein secretion)

Answer 23

1. these amino acids act as the signal sequence/signal peptide
2. signal peptide binds to a signal recognition particle which temporarily inhibits further translation of the polypeptide chain

Question 24

what is the function of the signal recognition particle

Answer 24

the signal recognition particle binds to specific membrane proteins on the RER and then restarts the assembly of the polypeptide

Question 25

what is the function of the RER during protein secretion (3 points)

Answer 25

1. polypeptide is fed though a complex in the RER
2. when polypeptide is completed, proteins are secreted into the lumen of the RER
3. the signal sequence are removed from the protein by enzymes in the lumen of the RER so aren’t present in the final protein

Question 26

what role do vesicles play at the RER during protein secretion (3 things)

Answer 26

1. vesicles form from proteins in the RER membrane
2. vesicles transport proteins to the Golgi apparatus and fuse with the golgi membrane
3. protein undergoes further modification in the golgi (eg - carbohydrates added)

Question 27

explain exocytosis of proteins (from the golgi)

Answer 27

1. proteins are sorted and packaged into vesicles depending on their final destination
2. the vesicles involved in exocytosis fuse with the plasma membrane and release the proteins into the extracellular fluid