Key area 3:control of gene expression Flashcards
Explain why only a fraction of genes in a cell are expressed
Only a fraction of the genes in a cell are expressed depending on the proteins required by that cell
Name the molecules in a RNA nucleotide
Phosphate, ribose sugar and base
Give the names of the 4 RNA bases
Adenine
Uracil
Guanine
Cytosine
Describe the base pairing rules for RNA bases
Cytosine pairs with guanine
Adenine pairs with uracil
Describe 3 differences between RNA and DNA molecules
DNA is double stranded while RNA is single stranded. DNA has deoxyribose sugar while RNA has ribose sugar.
DNA bases are A,T,C,G while RNA bases are A,U,C,G
State what mRNA is and describe its role
messenger RNA is formed in the nucleus from free nucleotides and carries a copy of the DNA code from the nucleus to the ribosomes where protein synthesis occurs
State what tRNA is and describe its role
Transfer RNA molecules collect amino acids and bring them to the ribosome to build proteins
State the location of transcription
Transcription takes place in the nucleus
State 4 things that must be present for transcription to occur
Template DNA
Supply of free RNA nucleotides
Enzymes
ATP
Describe the process of transcription
The enzyme RNA polymerase unwinds and unzips the double helix of the gene to be expressed and aligns free RNA nucleotides against the exposed DNA nucleotides of the template strand. Complementary base pairing ensures correct positioning of RNA nucleotides which are joined to form a primary transcript. Each transcript has introns and exons. The exons are spliced together to form a mature mRNA transcript
Describe the role of RNA polymerase in transcription
RNA polymerase binds at the promoter and unwinds the DNA . RNA polymerase adds nucleotides onto the 3’ end of the growing mRNA molecule
Explain what introns are
Introns are non-coding region and are removed from the primary transcript
Explain what exons are
Coding regions and are spliced together to form a mature mRNA transcript
Describe RNA splicing
After the mRNA has been transcribed the introns are removed. The remaining exons are spliced together to form a continuous chain. This is called the mature transcript. The mature transcript then leaves to travel to the cytoplasm
Describe the roll or rRNA
Ribosomal RNA molecules combine with proteins to create the ribosome-the organelle responsible for assembling proteins following the DNA code
Where does transcription start from
Starts from a region of a DNA known as the promoter
State the location in translation
The cytoplasm
Define amino acid, polypeptide and protein
Proteins are made of long chains of smaller molecules called amino acids. Amino acids are joined together by peptide bonds. The proteins fold into different shapes which dictates the role it will have in the cell
Describe the process of translation
Stage 1: the ribosome binds to the 5’ end of the mRNA at the start codon
Stage2: a tRNA carrying the amino acid methionine becomes attached to the ribosome
Stage3: the ribosome the moves to the next codon and looks for a tRNA with an anticodon that matches this second codon
Stage 4: the peptide Bon then forms between these two amino acids
Stage5: the ribosome then moves along to the next codon
Stage 6: steps 3-5 the rebate until it reaches a stop codon
Describe the structure of tRNA
Transfer RNA is made of a single chain of nucleotides and it folds into a 3D structure held together by hydrogen bonds.
Describe the function of tRNA
tRNA has an attachment site for a specific amino acids. The tRNA picks up its appropriate amino acid and takes it to the ribosomes to be matched with the mRNA
Define codon
The mRNA is made up of sequences of three nucleotides (a triplet of bases) called codons. Each codon is code fo one amino acid
Define anticodon
Each tRNA has an attachment site for a specific amino acid and a triplet of bases known as an anticodon
Describe the structure of a ribosome
Small, roughly spherical structures found in all cells. They contain the enzymes essential for protein synthesis. The ribosomes function is to the the tRNA molecules beating amino acids in contact with the mRNA
Describe the complementary pairing of bases between mRNA and tRNA
tRNA anticodons are recognised and align with mRNA codons and are attached together by peptide bonds
Explain how codons on mRNA recognise incoming tRNA
tRNA has an attachment site which is specific to an mRNA amino acid
Explain the function of start and stop codons
Start codons start the translation while stop codons causes translation to finish when the polypeptide is complete
Name the bond formed between amino acids of a polypeptide
Peptide bond
Explain the mechanism by which different proteins can be expressed from one gene
The same gene can be used to make different proteins by alternative RNA splicing or post translational modification
Define alternative RNA splicing
Under certain conditions alternative segments of RNA may be treated as exons and introns do during splicing
Define post translational modification
Once translation is completed proteins can be modified by cleavage or addition of other molecules
Cleavage
A single polypeptide chain can be cleaved (cut) by enzymes to make it active.
Addition of other molecules
Carbohydrates and phosphate groups can be added to proteins.
Describe the overall shape of protein molecules
Proteins fall into two distinct groups which are fibrous or globular
Describe what can happen to polypeptide chains as they are transformed into proteins
The polypeptide is folded to give a protein with a three dimensional shape held in place by a hydrogen bonds and other interactions between individual amino acids
Explain how interactions of amino acids determine the final shape of a protein
The shoe of the protein is linked to its function
