1.3 Control of Gene expression Flashcards
What forms the genetic code?
the base sequence of DNA
Where is the genetic code found?
in all forms of life suggesting all life originated from a common ancestor
What are genes?
Genes are the units of genetic code that make up the genotype of an organism
Genes are expressed to produce proteins
These proteins form the structure and control the functions of the organism
What is the phenotype?
the phenotype of an individual is determined by the proteins produced by the expression of its genes
Only a fraction of the genes in a cell are expressed depending on the proteins required by the cell
TOPTIP
order of DNA bases
| (genotype)
order of mRNA bases
| (phenotype)
order of amino acids in proteinsWhat is the basic structure of a protein?
protein contains the element carbon, hydrogen, oxygen and nitrogen
Protein molecules are polypeptide chains
Polypeptide is a chain of amino acids held together by peptide bonds
What is the 3D protein structure?
polypeptide chains fold into the final protein shape
the shape is held by hydrogen and ionic bonds and links between sulphur atoms
Describe the different levels of protein structure?
primary - a single polypeptide chain (sequence of amino acids joined by peptide bonds
secondary - folding of polypeptide chain held by hydrogen bonds (alpha helices and beta sheets)
tertiary - Interaction between amino acids forming a complex protein molecule. hydrogen, ionic and sulphur bonds hold the proteins shape
Quaternary - interaction between polypeptide chains
The final protein structure can be fibrous or globular. Explain these terms?
Fibrous form from parallel polypeptides into cable shapes for structural roles.
example: keratin in hair, nails and hooves and collagen
Globular proteins are folded into a specific spherical shape that allows it to carry out a functional role
Example: enzymes, hormones, antibodies
TOPTIP
the shape of a protein allows it to perform its specific function
Name the four protein groups?
structural
enzymes
hormones
antibodies
Function of structural proteins and an example?
building blocks of cell structure e.g. actin and myosin from contractile fibres in muscle cells
Function of enzymes and example?
speed up the rate of chemical reactions e.g. pepsin speed up the breakdown of proteins in the stomach during digestion
Function of hormones and example?
chemical messengers involved in regulations e.g. Insulin is involved in the regulation of sugar levels in blood
Function of antibodies and example?
Defensive substances that give immunity against specific diseases e.g. measles immunoglobulin specifically recognises measles antigens and renders them in active
What is chromatography?
A technique used to separate a mixture which differ in the components solubility in a solvent
As the solvent passes up through absorbent paper it carries the components of the mixture different distances
The more soluble the component the further it travels along the paper
How to calculate RF values?
distance travelled by component
divided by
distance travelled by solvent front
What is gene expression?
Genes are transcribed and translated during gene expression
Gene expression is controlled by the regulation of transcription and translation
Gene expression can be influenced by intracellular and extracellular environmental factors
What are the stages of gene expression?
DNA (sequence of bases) –> mRNA (sequence of bases) –> Protein (sequence of amino acids)
What is RNA?
a second type of nucleic acid involved in gene expression
Types of RNA?
Messenger (mRNA)
Transfer (tRNA)
Ribosomal (rRNA)
What is mRNA?
formed in the nucleus during transcription
transports a copy of the base sequence of gene to the cytoplasm where it attaches to a ribosome
Compare DNA and mRNA?
DNA sugar - deoxyribose bases - Adenine, thymine, cytosine, guanine number of strands - two location - nucleus
mRNA sugar - ribose bases - adenine, uracil, cytosine, guanine number of strands - one location - nucleus and cytoplasm
What is transfer (tRNA)?
tRNA are molecules in the cytoplasm which carry specific amino acids to mRNA at the ribosome, placing them in the order determined by the base sequence on the mRNA to form a polypeptide chain.
this is called translation
Shape of tRNA?
tRNA folds due to complementary base pairing and forms a triplet anti-codon site and an attachment site for a specific amino acid
DONT FORGET
Base pairing with tRNA causes it to fold into a clover leaf shape with two distinct regions: an exposed triplet anticodon site and an attachment site for a specific smino acid
The anticodon matches its complementary triplet codon on the mRNA strand, putting its specific amino acid in the correct sequence.
What is rRNA?
ribosomal RNA forms ribosomes together with proteins
Ribosomes are where protein synthesis takes place
Where are introns and exons found?
in DNA and RNA
What are introns?
regions of DNA that do not code for proteins
they are non coding regions
What are exons?
regions of a nucleic acid molecule which are active in gene expression (code for proteins) and are known as coding regions
TOP TIP
exons are expressed (actively code for a protein)
What is the primary structure of a protein determined by?
by the sequence of nucleotide bases in a DNA strand which is trapped inside the nucleus
where are proteins synthesised?
outside the nucleus on ribosomes
The code is carried as a message from the DNA to the ribosome by mRNA
DONT FORGET
when a dna sequence is ‘transcribed’ it is changed to an RNA base sequence
This information contained within the base sequence is not changed at any time
What are the stages of transcription?
- DNA unwinds as RNA polymerase moves along a section that codes for a protein
- The molecule unzips when hydrogen bonds are broken. bases on the DNA strand are exposed
- mRNA nucleotides move in and form complementary base pairs. (A(DNA) U(mRNA), T(DNA) A(mRNA) )
- Strong chemical bonds from between the phosphate of one nucleotide and the ribosome of the next nucleotide, building on mRNA strand - RNA polymerase
- The weak hydrogen bonds that were holding the DNA and mRNA primary transcript to leave the nucleus and enter the cytoplasm
- Hydrogen bonds reform between the two DNA strands, and the DNA molecules rewinds to form a double helix
What is the primary transcript?
the mRNA formed in transcription contains both introns and exons and is called a primary transcript
What happens to the primary transcript before leaving the nucleus?
cut by enzymes to remove introns
the exons are then spliced together to form mature mRNA transcript
what does the mature mRNA transcript now do?
mature mRNA transcript then leaves the nucleus containing only exons which can then be expressed to make protein molecules
What is this process called?
mRNA splicing
DONT FORGET
a triplet of bases on mRNA is called codon and codes for one specific amino acid
What is translation?
the process of assembly of amino acids in the correct order to form a polypeptide chain, according to the base sequence on mRNA is called translation
where does translation occur?
on the ribosomes
Where are most ribosomes found?
on the rough endoplasmic reticulum (RER) in the cell
What are the sites on ribosomes?
there are two attachment sites on the ribosomes called site P and site A
A small mobile subunit of a ribosome holds site E
Function of site P
Site P is the attachment site for the first tRNA molecule to join on to the mature mRNA transcript
Function of site A
Site A allows all following tRNA molecules to attach to the mRNA
Function of site E
Site E is located on a small subunit molecule and releases the completed polypeptide chain
What are the stages of translation?
- Mature mRNA transcript leaves the nucleus and attaches to a ribosome
- Transfer RNA molecules (tRNA) carry specific amino acids from the cytoplasm to the ribosome
- A start codon ‘AUG’ positioned on the attachment site P allows the first tRNA molecule carrying a specific amino acid to attach to the ribosome
- The anticodons of the tRNA molecules attach to the codons of the mRNA according to the base pairing rule, bringing amino acids into the correct position
- Peptide bonds form between neighbouring amino acids and ‘empty’ tRNA molecules leave the ribosome and return to the cytoplasm to collect another specific amino acid molecule
- There are three stop codons on mRNA: ‘UGA’, ‘UAG’ and ‘UAA’. When a stop codon moves onto the ribosome at attachment site A, no tRNA molecules can now attach and the finished polypeptide chain is released.
DONT FORGET
the tRNA molecule fold to form a three dimensional shape, held in position by base pairing
there are three bases at one end of the molecule called the anticodon and an amino acid attachment site at the other end. The specific amino acid carried by the tRNA molecule depends upon the anticodon
What is a codon?
group of three bases on mRNA
TOPTIP!
Be able to identify a ‘stop codon’ and a ‘start codon’.
Remember mutations can cause the formation of stop codons in the middle of a gene causing the translation of a protein to stop.
Explain protein synthesis?
- DNA base sequence is transcribed to form a complementary primary mRNA transcript
- Introns are removed from the primary transcript and exons are spliced together to form the mature mRNA transcript
- Transfer RNA carries a specific amino acid determined by its anticodon to the ribosome
- Amino acids are arranged in the correct sequence through complementary base pairing between mRNA codons and tRNA anticodons . Peptide bonds form between the amino acids to make a polypeptide
How is a peptide bond formed?
a strong peptide bond forms between the carbon end of one amino acid and the nitrogen end of another through the removal of water (condensation reaction)
How can a gene code for more than one protein?
through
- alternative splicing of the mRNA primary transcript
- post translational modification
Explain Alternative RNA splicing?
Different types of mRNA molecules can be produced from the same primary transcript depending on which parts of the mRNA molecules are treated as exons and which are treated as introns.
this is controlled by regulatory proteins specific to the cell type
this results in one gene coding for more than one protein
What is post translational modification?
post translational protein structure may be modified by molecular addition and cleavage
Explain molecular addition?
some proteins must be combined with a non-protein element before they can work properly
this is called molecular addition
Explain 1 - Haemoglobin?
Haemoglobin which carries oxygen in the blood, is a globular protein molecule combined with iron atoms
Haemoglobin is an example of conjugated protein
Explain 2 - Mucus?
Mucus is a globular protein molecule combined with a polysaccharide (sugar) atoms
Mucus is an example of a glycoprotein
Explain 3 - Egg yolk?
Egg yolk contains phosphoric acid combined with protein molecules
What is cleavage?
some polypeptide chains must be ‘cut’ by an enzyme for the protein and combined to be ‘activated’
Example 1- Insulin?
the hormone insulin only becomes active when the polypeptide chain is cut into 2 by enzymes following translation.
the two separate parts of the polypeptide chain each fold over and their shapes are stabilised by sulphur bridges
