bio unit 3 Flashcards
What is a nucleic acid and what it it’s function?
A polymer of nucleotides, such as DNA and RNA. Their function is to store and express genetic information in order to synthesise proteins.
What are the three types of RNA and what are their functions?
messenger RNA (mRNA) - the product of transcription, which carries genetic information from the nucleus to the ribosome in order to be translated into a protein. ribosomal RNA (rRNA) - an essential building block of the ribosome, which is where translation of mRNA takes place. transfer RNA - carries an amino acid to the corresponding codon (triplet base pair) which is complementary to the tRNA's anticodon to create a polypeptide chain.
What are the differences between DNA and RNA?
DNA contains deoxyribose sugar, whereas RNA contains ribose sugar. DNA is double stranded while RNA is single stranded. RNA also has the nitrogenous base of uracil instead of DNA’s thymine.
What is meant by DNA being ‘degenerate’ and ‘universal’?
Degenerate - different codons can specify the same amino acid e.g. GCU and GCC both code for Ala.
Universal - the same codons code for the same amino acid in every living organism.
Describe the steps of transcription and post-transcriptional RNA processing and mention the relevant enzymes (if any) at each step.
- Helicase unwinds the DNA double helix.
- RNA polymerase binds to the promoter region of the gene.
- RNA polymerase joins nucleotides A, C, G and U together in correspondence to their matching base pair from 5’ to 3’.
- RNA polymerase reaches a termination sequence and detaches from the DNA.
- A methyl cap is added to the 5’ end of the transcript and a poly A tail to the 3’ end. Introns (do not code for functional proteins) are spliced out of the transcript by spliceosomes while exons (code for functional proteins) are kept in.
Describe the steps of translation and post translational modifications and mention the relevant enzymes (if any) at each step.
- Ribosomal subunits form around start codon.
- tRNA brings a corresponding amino acid to the ribosome, which creates a polypeptide chain through a condensation reaction (releases water).
- The ribosome reaches a termination sequence and release factors separate the polypeptide chain from the ribosome.
- The polypeptide chain folds into it’s functional shape/joins with other polypeptide chains.
What are the four structure levels of proteins and how are they classified?
Primary - sequence of amino acids in a polypeptide chain.
Secondary - local folded structures due to atomic interactions between the backbones creating an α helix or a β pleated sheet.
Tertiary - R groups of amino acids interact forming hydrogen, ionic, and disulfide bonds.
Quaternary - 2 or more polypeptide chains joined together.
What is the function of the promoter and operator regions on a gene operon?
Promoter - RNA polymerase attaches here to begin transcription.
Operator - regulates the expression of structural genes. The repressor which stops the transcription of the trp operon bind here.
An operon is a cluster of genes that function under one promoter.
How are tryptophan levels regulated by transcription repression when levels are high?
When high levels of free tryptophan are present in a cell, it binds to the repressor, changing it’s shape so that it can bind to the operator. Therefore, transcription of the trp operon cannot occur as the RNA polymerase cannot get past the repressor.
How are tryptophan levels regulated by attenuation when there are low levels of free floating tryptophan but high levels being carried by tRNA?
In prokaryotic cells, transcription and translation take place at the same time. Within the leader sequence there are 4 domains of which certain parts are complementary to each other, 1 and 2, 2 and 3, 3 and 4. There is also an attenuator region of weakly bonded adenine and uracil. Between domain 1 and 2, there is an AUG stop codon. In domain 1, there are two codons for tryptophan. When there is tryptophan present, being carried by tRNA, the codons will be translated and the ribosome will stop at the stop codon. Since the complementary regions of 1 and 2 can’t join because of the ribosome, the complementary regions of 3 and 4 join, breaking the attenuator apart. As a result, the RNA polymerase breaks off and never gets to transcribe the genes of the operon.
How are tryptophan levels regulated by attenuation when there is no tryptophan present in the cell?
The ribosome stops at the codons for tryptophan in domain 1 and waits for the tryptophan amino acid. Therefore, the domains 1 and 2 cannot join because the ribosome is covering domain 1. Domain 2 and 3 then join, not breaking the attenuator, so the RNA polymerase can continue with transcribing the genes of the operon.
What are structural and regulatory genes? Provide an example for each one.
Structural genes code for the synthesis of a protein e.g. the genes of an operon. Regulatory genes control the expression of structural genes e.g. the operator region of a gene.
Describe the processes involved in the secretion of a protein from a cell.
- Newly processed proteins enter the rough endoplasmic reticulum (RER) where they are further processed and packaged into transport vesicles and sent to the golgi apparatus.
- Proteins are sorted by where they are needed either inside or outside of the cell. If they are leaving the cell, they are packaged into secretory vesicles.
- During exocytosis the vesicles merge with the plasma membrane to release the protein outside of the cell.
What enzymes are involved in DNA replication and how are they used?
Reverse transcriptase - builds single stranded DNA using mRNA as a template. DNA polymerase then builds the complementary strand creating a second strand.
Ligase - joins pieces of DNA together by creating covalent bonds in the sugar phosphate backbone.
Endonuclease - Identifies and cleaves a recognition site creating either ‘sticky’ or ‘blunt’ ends.
DNA polymerase I - replaces RNA nucleotides with DNA nucleotides.
DNA polymerase III - brings DNA nucleotides that are complementary to the template.
Taq polymerase - has the same function as DNA polymerase but can withstand high temperatures and so is used in PCR.
‘Rednecks Teach Lousy Energetic Pigeons Over Peach Tea Thoughtfully.’
What is the function of CRISPR-Cas9 in bacteria and how can it be used to edit an organisms genome?
The CRISPR genes in bacteria remember the genetic information of attacking bacteriophages so they can break it apart before it becomes viral, using the cas9 enzyme (an endonuclease). Because the CRISPR-Cas9 complex relies on guide RNA (gRNA) which is complementary to the bacteriophage spacers in order to cleave the virus, synthetic gRNA can be made to cut target DNA to edit an organisms genome.
