Gene expression Flashcards
How does an organism’s genotype determine its phenotype?
Genetic information flows from DNA to protein
DNA replication
DNA—Transcription–> RNA
RNA—Translation–> Protein
Function of DNA
DNA is used to carry the genetic code, which is used to synthesize specific polypeptides
Gene
Each gene is a sequence of nucleotides as part of a DNA molecule and it codes for one polypeptide
how the information on DNA is used to synthesize polypeptides.
The huge length of DNA molecule is a single chromosome which carries the codes for a very large number of polypeptides 🡪 within this extremely long molecule, the relatively short length of DNA (thousands of nucleotides) that codes for a single polypeptide is called a GENE
The nitrogenous bases of the DNA code dictates the order in which specific amino acids are assembled and combined together
The DNA code lies in the sequence of one of the 2 strands of the DNA double helix – coding strand read in the 5’ to 3’ direction
RNA vs DNA
Length:
DNA– Very long strands, several million nucleotides
RNA– Relatively short strands (100-1000 nucleotides long)
Sugar:
DNA– Deoxyribose
RNA– ribose
Bases:
DNA– Cytosine, guanine, adenine, thymine
RNA– Cytosine, guanine, adenine, uracil
Forms:
DNA– Consists of 2 polynucleotide strands in the form of a double helix with complementary base pairs:
- C with G, A with T
held by hydrogen bonds
RNA– Consists of single strands and exist in 2 functional forms: messenger RNA and Transfer RNA
Location:
DNA– occurs in the chromosomes of the nucleus
RNA– some RNA also occurs in the nucleus but most are found in the cytoplasm - particularly in the ribosomes.
Main features of genetic code
- Universal
- Continuous and non-overlapping – ribosomes are read continuously at successive group of 3 nucleotides, one DNA triplet/codon at a time without skipping any nucleotides
- Degenerate, but unambiguous (every DNA codon codes just for 1 amino acid and most amino acids are encoded by degenerate DNA codon that differ in the 3rd position of the DNA codon)
-Has punctuation DNA codons – start and stop codons
Protein synthesis
DNA instructs the cell to make specific polypeptides and proteins in 3 stages:
1. Transcription – transfer of genetic material into an RNA molecule (initiation, elongation, termination) (in the nucleus)
2. Amino Acid activation (tRNA + amino acid using aminoacyl bond) (in cytoplasm)
2. Translation – transfer of information from RNA into a protein (initiation, elongation, termination) (in cytoplasm)
Transcription: Initiation
- Initiation
- Double helix unwinds and hydrogen bonds are broken at the site of the gene being transcribed
- RNA Polymerase recognizes and binds to the promoter
Transcription: Elongation
-Template strand is used as a template for transcription
-RNA polymerase matches free nucleotides by complementary base pairing, working in the 5’->3’ direction
-Hydrogen bonds then form between complementary bases
- Each free nucleotide is then joined by condensation reaction between the sugar and the phosphate groups of the adjacent nucleotides of the RNA strand
Transcription: Termination
-When the RNA Polymerase reaches the terminator, transcription is terminated
-mRNA is released and leaves the nucleus through the pores of the cytoplasm
amino acid activation
occurs in the cytoplasm
amino acids are activated by combining with tRNA
- anticodon: 3 consecutive bases of tRNA, complementary to the codon of mRNA that codes for the specific amino acid
Translation
proces in which protein chain is assembled
occurs at the ribosomes of the cytoplasm, which consists of a large and small subunit
- each subunit is made of proteins and ribosomal RNA
- ribosomes move along the mRNA one triplet code at a time, in the gap between the large and small subunit
Translation: Initiation
On arrival at the ribosome, the mRNA binds to the small subunit at an attachment site.
In this position, there are 6 bases (2 codons) of the mRNA exposed to the large subunit at any time
The first 3 exposed bases (start codon) of the mRNA are always AUG
A molecule of tRNA with the complementary anticodon UAC forms hydrogen bonds with this codon, the amino acid methionine is attached to this tRNA molecule
Translation: Elongation
- A second tRNA bonds with the next 3 bases of mRNA bringing another amino acid alongside the methionine molecule.
- Whilst the 2 amino acids are held close together within the ribosome, a peptide bond is formed between them by condensation reaction catalysed by an enzyme found in the large subunit, forming a dipeptide
- The ribosome moves along the mRNA in the 5’–> 3’ direction and the next codon is read
- At the same time, the 1st tRNA without its amino acid leaves the ribosome
- Then, a third tRNA bonds with next codon of the mRNA, bringing another amino acid alongside the 2nd amino acid residue of the dipeptide –> a peptide bond is formed immediately, forming a tripeptide that starts to emerge from a hole within the large subunit
- Again, the ribosome moves along the mRNA and the next codon is read
- At the same time, the 2nd tRNA leaves the ribosome
- by these steps, constantly repeated, a polypeptide is formed and emerges from the large subunit.
Translation: Termination
Eventually, a “stop” codon. is reached
At this point, the completed polypeptide is released from the ribosome into the cytoplasm
Role of mRNA
responsible for the transfer of information from the nucleus to the cytoplasm
Role of tRNA
Construction of proteins at the ribosomes in the cytoplasm
Anticodon in tRNA is complementary to the codon in mRNA that codes for the specific amino acid
Role of ribosomes in translation
- Ribosomes move along the tRNA one triplet codon at a time
- tRNA anticodons align with mRNA codons by complementary base pairing in ribosomes- amino acid attached to tRNA forms peptide bonds by condensation reaction with the 2nd amino acid while the 2 amino acids are held close together within the ribosome.
Mutagen
alters the DNA sequence of the cell
types of mutagens and effects on the rate of mutation in DNA
-Chemical mutagens: work by blocking DNA replication, resulting in a shorter DNA synthesised
- Physical mutagens: radiation : They damage DNA by breaking it into smaller fragments
- Biological mutagens: Human Papillomavirus: A virus works by inserting its DNA into the chromosomes/genome of human cells, thereby altering the DNA sequence and disrupting gene function
Result from nucleotide substitution (brief summary would do)
- Missense Mutation: new nucleotide alters the codon and hence produces an altered amino acid in the protein
-Nonsense mutation: The new nucleotide changes a codon that codes for an amino acid to a STOP codon - translation would stop prematurely and protein formed is truncated
-Silent mutation: They do not cause a change in their resulting polypeptide sequence and proper folding of a protein can still occur - Silent mutation:
Result from nucleotide deletions or insertions
Frameshift mutation: alters all the subsequent codons downstream from the mutation site– change the properties of the protein, particularly its tertiary structure - or to cause no protein to be coded for at all
Sickle Cell Anaemia
Mutation in the allele of B-haemoglobin –> results from substitution of a single base in the sequence of bases that make up for all the codons for B-haemoglobin.
DNA level: Adenine is replaced by thymine
mRNA level: codon GAG is replaced with GUG
Protein level: polar glutamic acid is replaced with non-polar valine
Presence of non-polar valine in the beta chain of haemoglobin gives a hydrophobic spot in the otherwise hydrophilic outer section of the protein – tends to attract other haemoglobin molecules to bind to it.
