Section 4 - Genetic information, variation and relationships between organisms: 8. DNA, genes and protein synthesis Flashcards
What is a gene
A section of DNA that codes for a polypeptide (protein) or functional RNA (eg. mRNA, tRNA)
What are the 4 bases in the DNA sequence
- Adenine
- Thymine
- Cytosine
- Guanine
Why do 3 bases code for each amino acid
Triplet code gives 64 (4^3) possible combinations, as there are 4 possible bases.
(enough to code for the 20 naturally occurring amino acids)
What are the features of the genetic code
- Degenerate
- Non-overlapping
- Universal
Why is the genetic code described as degenerate
Most amino acids are coded for by more than one triplet, to limit the impact of mutations
Why can the genetic code be described as non-overlapping
Each base sequence is read once, in one particular direction, where the triplets don’t overlap
Why can the genetic code be describes as universal
Each triplet code codes for the same amino acid in all organisms
What is always the first Amino acid coded for by a gene
Methionine, Start Codon AUG
Later removed if not part of the chain
How is DNA stored in Prokaryotes (+ in Mitochondria and chloroplast)
Short, circular structures (plasmid), with no associated proteins
No chromosomes
What is the structure of a chromosome
- DNA molecule wound up and wrapped around histones (proteins)
- DNA-Histone complex is coiled
- Coils form loops that pack together into chromatids
- Chromatids branch off of the centromere
(contains only 1 DNA strand)
What is a homologous pair
A pair of chromosomes, with one from the mother, and one from the father
What is the Diploid number
Number of chromosomes in homologous pairs (Humans = 46)
What is an allele
One of a number of forms of the same gene
- One from the mother and one from the father
- Each has different base sequence
What is RNA
Single stranded nucleic acid transcribed from sections of DNA
What is a Codon
Sequence of 3 bases on the mRNA that codes for one amino acid
What is the genome
The complete set of genes in a cell (including those in the mitochondria/chloroplast)
What is the proteome
The full set of proteins produced by the genome
(Complete proteome = Proteins produced by 1 type of cell under certain conditions)
What is the structure of RNA
Polymer of nucleotides:
- Ribose sugar
- Organic base (AUCG)
- Phosphate group
What is mRNA
Messenger RNA
What is the function of mRNA
Acts as a template for protein synthesis
What is the structure of mRNA
- Long, single stranded polymer of nucleotides
- No H bonds between nucleotides, so no folding
- Base sequence transcribed from DNA
What is tRNA
Transfer RNA
What is the function of tRNA
Binds to a specific amino acid, allowing for their arrangement in the polypeptide chain, for protein synthesis
What is the structure of tRNA
- Single strand of nucleotides folded into a clover shape by H bonds
- One end binds to a specific amino acid
- The other end has a specific anticodon, complementary to the codon that codes for the carried amino acid
What is the process of Protein synthesis
- DNA provides ‘instructions’ in the long base sequence
- A complementary strand of pre-mRNA is made for a section of this sequence (transcription)
- Pre-mRNA is spliced to form mRNA
- mRNA is used as a template, to which tRNA molecules attach, aligning their amino acids to form the polypeptide chain (Translation)
How is protein synthesis different in prokaryotes and eukaryotes
Prokaryotes directly transcribe mRNA form the DNA, whereas eukaryotes make pre-mRNA first, which needs to be spliced
What is transcription
The formation of pre-mRNA from the DNA base sequence
Where does transcription occur
In the Nucleus
What is the process of transcription
- The enzyme DNA helicase breaks the hydrogen bonds between the two DNA stands, exposing the nucleotides
- Free nucleotides align with the complementary bases of the template strands
- The Enzyme RNA polymerase moves along the strand, forming phosphodiester bonds between the loose nucleotides
- DNA strands re-join after the free nucleotides are joined together)
- When the RNA polymerase reaches a terminal sequence on the DNA, it detaches, releasing the pre-mRNA
What is the function of DNA helicase in transcription
Breaks the H bonds between the two strands of DNA, exposing the bases
What is the function of RNA polymerase in transcription
Moves along the strand of free nucleotides, forming phosphodiester bonds between them
How many DNA bases are exposed at the same time during transcription
~12, as the double helix re-attaches together after the RNA polymerase moves past
What is RNA splicing
The process that forms mRNA from pre-mRNA, to be used in translation
Where does RNA splicing occur
In the Nucleus
What are the non-coding sections of DNA
Introns
What are the sections of DNA that code for polypeptides
Exons
What is the process of splicing
The non-coding introns are spliced out by the enzyme Spliceosome, and the exons join together to form the mRNA
What is alternative splicing
When the exons re-join in a different order, so the same gene can code for multiple proteins, as this gives a different mRNA base sequence
What is translation
The formation of a polypeptide from mRNA
Where in the cell does translation occur
In the ribosomes
What is the process of Translation
- Ribosome attaches to the start codon at one end of the mRNA
- tRNA molecule with the complementary anticodon moves into the ribosome, carrying the specific amino acid
- The tRNA binds to the mRNA, positioning the amino acid in the correct location in the polypeptide
- A second tRNA moves into the ribosome, carrying the next amino acid, and attaches to the mRNA
- A peptide bond forms between the two amino acids the bond between the first tRNA and the mRNA is broken
- Ribosome moves along the the RNA to the next codon, repeating the process with two triplet codes within at any time
- Synthesis continues until a stop codon is reached, when the polypeptide is released to begin folding into a protein
What is the start codon
AUG - codes for Methionine
