U3 AOS 1 SAC

Question 1

Universal nature of the genetic code

Answer 1

a specific triplet codes for the same amino acid in all organisms

Question 2

Degenerate nature of the genetic code

Answer 2

provides some redundancy as changes to the original sequence of DNA due to mutations may not necessarily mean a different amino acid coded (except for AUG which codes for met and UGG which codes for trp)

Question 3

Unambiguous

Answer 3

each codon can only code for one specific amino acid (however multiple codons can code for the same amino acid)

Question 4

Gene expression

Answer 4

the process by which the information in a gene is turned into a polypeptide

Question 5

Transcription

Answer 5

the genetic code is copied into a complimentary pre-mRNA molecule
- DNA unwinds and unzips and the hydrogen bonds between the complimentary nitrogenous bases break
- RNA polymerase binds to the promotor region of the template DNA strand, reads the gene in a 3’ to 5’ direction and synthesises a complimentary strand of pre-mRNA from free-floating RNA nucleotides
- Continues until a stop triplet is reached at the terminator end of a gene and the RNA polymerase detaches from the template strand

Question 6

RNA processing (in eukaryotic cells)

Answer 6

Before leaving the nucleus, the pre-mRNA must undergo post-transcriptional modifications to become mature mRNA and exit the nucleus
- Introns are transcribed but not translated (removed and recycled back in the nucleus aka digestion of the pre-mRNA)
- Exons are expressed and joined together as they code for specific amino acids
- Methyl cap is added to the 5’ end and poly-A tail is added to the 3’ end (protects
the mRNA from degradation and enables the ribosome to read it in the correct direction)

Question 7

Transcription

Answer 7

The decoding of the mature mRNA to form the specific sequence of amino acids in a protein’s primary sequence
- After mature mRNA exits the nucleus via the nuclear membrane pores, it attaches to a ribosome (on rough ER or free-floating in cytosol) which reads the mRNA codons from the start codon
- A complimentary tRNA anticodon transfers the specific amino acid to the ribosome and when the tRNA anticodon binds to an mRNA codon, the amino acid is released
- Each released amino acid is connected to another one via a condensation polymerisation reaction producing peptide bonds, forming a polypeptide chain
- When the ribosome reaches a stop codon, the mRNA and polypeptide chain are released

Question 8

Alternative splicing

Answer 8

The process where different exons are rejoined in different orders, removed all together and/or introns may be retained (renamed exons) which results in a single gene producing multiple different mRNA strands that code for different proteins
- Reason why there are fewer genes in the genome than proteins in the proteome

Question 9

Structural gene

Answer 9

Segment of DNA that codes for a polypeptide; comprises the promotor, exons and introns

Question 10

Regulatory gene

Answer 10

Codes for gene products (regulatory proteins/repressor proteins) which activate or inactivate the expression of structural genes
- If active, also influence rate of protein synthesis

Question 11

Effects of mutations in structural & regulatory genes

Answer 11

Structural - render it non-functional
Regulatory - cause excess or lack of gene product

Question 12

3 key sections of a gene

Answer 12

Promotor (‘Upstream’ region) - 5’ end where RNA polymerase binds and allows for the transcription of the structural genes (regulatory gene)

Coding region - section of DNA that codes for the specific protein and is copied into complimentary pre-mRNA during transcription (structural gene)

Terminator (‘Downstream’ region) - represents a sequence of DNA which signals for the end of transcription

Question 13

Exons & Introns

Answer 13

Within the coding region of a gene
- EXONS are expressed as they are regions which contain the codons that code for amino acids
- INTRONS are interrupting genes that do not code for amino acids (non-coding regions) and are instead cut out and recycled in the nucleus

Question 14

Comparison of exons & introns

Answer 14

BOTH are transcribed but only exons are further translated and expressed as gene products (polypeptide chains/proteins) whereas introns are cut out during RNA processing

Question 15

Operator region

Answer 15

Section of DNA in which a regulatory/repressor protein binds to (e.g. trp repressor) binds to in order to activate or deactivate gene expression by blocking or enabling RNA polymerase to transcribe the gene

Question 16

Benefit of regulatory genes

Answer 16

Prevent the over or under expression of a particular protein, therefore allowing the cell to adapt to its needs

Question 17

Gene regulation

Answer 17

The process by which gene expression is activated or inactivated

Question 18

Benefits of gene regulation

Answer 18

• Save resources
• Saves energy (ATP)
• Prevents products accumulating

Question 19

Role of regulatory proteins

Answer 19

• Bind to the promotor region of a gene to inactivate the gene as RNA polymerase is prevented from transcribing structural genes -> repressor proteins
• Bind to target cell receptors and trigger signal which results in the activation or inactivation of gene expression -> signalling proteins

Question 20

Monomers of proteins

Answer 20

Amino acids

Question 21

Molecular structure of amino acids

Answer 21

• Central carbon atom bonded to a hydrogen atom
• Carboxyl group (COOH)
• Amino group (NH2)
• R-group (variable that differs in each of the 20 different amino acids)

Question 22

What reaction forms peptide bonds between amino acids?

Answer 22

Condensation polymerisation reaction as carboxyl group of one amino acid and the amino group of the other react, releasing water and forming a peptide bond

Question 23

Wild type

Answer 23

The genotype or phenotype in an organism (species) which is considered the ‘standard’ and found naturally occurring in the wild

Question 24

GMO

Answer 24

Genetically modified Organism - an organism altered or produced by genetic engineering techniques

Question 25

Transgenic organisms

Answer 25

An organism which has foreign DNA from another species inserted into its genome

Question 26

Knock-in organisms

Answer 26

An organism in which DNA from the same individual/species has been inserted into a specific locus

Question 27

Knock-out organisms

Answer 27

An organism where a specific section of DNA has been removed to disable the expression of a gene product or alter its function

Question 28

Endonucleases (restriction enzymes)

Answer 28

An enzyme which cuts DNA’s sugar-phosphate backbone/phosphodiester bonds at a specific recognition/restriction site (a specific nucleotide sequence)
- Used in restriction digest reactions where the substrate could be a plasmid, genomic DNA or PCR product
- Leaves blunt or sticky ends
- Site is usually a palindrome that is 4-8 bp long

Question 29

DNA ligase

Answer 29

Joins DNA fragments together by catalysing the formation of phosphodiester bonds