DNA and RNA Flashcards
(16 cards)
define DNA
give structure
DNA structure is a double-stranded helix, found in the nucleus of cells.
The 2 strands run in opposite directions = 3’ to 5’ and 5’ to 3’ (antiparallel). DNA is tightly wound into CHROMOSOMES (humans have 23 pairs/46).
Contains the code for all human proteins in the form of genes.
GENE = section of DNA, in the chromosomes, that codes for a polypeptide chain or protein.
NUCLEOTIDE = repeating unit of DNA, PHOSPHATE - DEOXYRIBOSE SUGAR - NITROGENOUS BASE
BASES = A (adenine), G (guanine) which are double ring PURINES
T (thymine), C (cytosine) which are single ring PYRIMIDINES
What is RNA
what are the different forms of RNA
RNA = RIBO-NUCLEIC ACID
mRNA = messenger RNA which is a complimentary copy of the template strand of the DNA. It carries the genetic instructions for protein synthesis from nucleus to ribosome.
CODON = 3 consecutive bases on the mRNA.
tRNA = transfer RNA, clover-leaf shape, picks up a specific amino acid to make the protein. There are 20 different amino acids, each tRNA only combines with 1 aa.
ANTI-CODON = 3 consecutive, exposed bases on the tRNA, mRNA binding site.
rRNA = ribosomal RNA, makes up the ribosome where protein synthesis occurs.
What are the differences between DNA and RNA
DNA is double stranded and RNA is single stranded.
DNA has deoxyribose sugar that has one less oxygen atom than ribose sugar.
DNA has the base thymine and RNA has uracil (pyrimidine).
RNA can leave the nucleus and DNA does not.
DNA is a large long lasting/stable molecule because it carries the entire genetic code and must act as a template, so it stays in the nucleus and is tightly
wound, and so is protected from damage when making proteins.
DNA = ACTG
RNA = ACUG
What is protein synthesis
What processes are involved
Protein synthesis is the creation of proteins by cells that uses DNA, RNA, and various enzymes.
Transcription
Translation
Transcription process
= the production of mRNA copy of the DNA template in the nucleus
1. The DNA helix unwinds and the hydrogen bonds between the complementary bases break.
2. The triplet code of the gene carried on the template strand of DNA is copied to make mRNA.
3. Spare RNA nucleotides pair with the exposed bases on template strand of DNA to build mRNA following
complementary base pairing rules: Uracil binds to Adenine, Adenine to Thyme & Cytosine binds to
Guanine.
4. The (single strand) mRNA leaves the nucleus/nuclear pore.
Translation process
- TRANSLATION = where the code sequence carried on the mRNA is used to sequence amino acids to create a functional protein in the ribosome.
- mRNA carrying the genes code attaches to a ribosome.
- Protein synthesis is initiated at the start codon AUG.
- Ribosome reads mRNA bases in 3’s called a codon.
- tRNA brings in specific amino acid it carries according to its anticodon. 3 bases on tRNA are called
anticodon. Anticodon on tRNA is complementary to codon on mRNA. - Codon and anticodon complementary base pair (A-U, C-G), bringing the correct amino acid in the
sequence required. - Peptide bonds form between adjacent amino acids building up the polypeptide chain.
- tRNA releases amino acid and then reloads new amino acid from cytoplasm.
- Protein synthesis is terminated by a stop codon.
What are:
-polypeptide bonds
-start codons
-stop codons
Peptide bonds form between the amino acids and a polypeptide chain is formed. The polypetide chain can then be folded into a protein.
A genetic codon table is used to look up the amino acids coded for and it is usually the CODONS that have to be looked up.
START CODON = A start codon initiates the translation. The start codon always codes for
the amino acid meth and is AUG.
STOP CODONS = A stop codon ends translation which causes the ribosome to stop translating and
release the mRNA and the polypeptide chain. UAA, UAG, UGA.
What is the degeneracy of the genetic code
Redundancy due to degeneracy:
because 4^3 or 64 codons code for only 20 amino acids the code is said to be DEGENERATE because several different codons can all code for the same amino acid eg CCU, CCC, CCA, CCG all code for proline.
This means that a mutation can happen in the last base of a codon and generally not change the amino acid
produced.
What is the structure and significance of proteins
Large molecules made of repeating units called AMINO ACIDS (aa). There are 20 possible amino acids that are
joined by PEPTIDE BONDS. Different order aa = different polypeptide chain. Polypeptide chains are then folded into proteins.
Proteins are structural (make up body structures like hair, nails, muscle fibres) or catalytic (enzymes). Recall
enzymes control all life processes including protein synthesis.
Shapes of proteins – GLOBULAR which are long chains coiled up into spherical shapes eg enzymes, hormones & FIBROUS which are long straight chains that are linked by H or disulfide bonds eg hair, muscle fibres
How is gene expression controlled
Gene expression – information from one gene is used to synthesise a functional protein using the DNA template by the process of transcription and translation – including protein folding.
DNA –> mRNA –> polypeptide or protein –>
One gene –> one polypeptide relationship
define a mutation and a mutagen agent
MUTATION = permanent change in the nucleotide sequence of DNA (usually harmful). Inheritable if the mutation occurs in the gametic cells that are fertilised.
MUTAGENIC AGENT = something that causes a mutation (e.g. chemicals, X-rays, UV…)
What is a gene point mutation?
what effects can they have?
GENE (POINT) MUTATIONS
When a single base/nucleotide or base pair is changed and affects only a single gene.
- NEUTRAL – no effect on the functioning of polypeptide chain produced because of degeneracy of the code (same aa included) or changed aa position does affect functioning because not at an active site that controls shape of protein
- BENEFICIAL – change of aa alters shape of the protein for better functioning
- HARMFUL – change of aa changes shape of protein and makes the protein non-functional (doesn’t work)
What types of mutations can occur?
substitution = base replaced
- samesense – redundancy due to degeneracy means the triplet code still codes for the same aa. Protein remains functional.
- missense – one aa is changed. Dependent on the location of the changed aa the protein may still be
functional.
- nonsense – the triplet change now codes for an early stop codon. The protein is much shorter and
will almost certainly be non functional.
deletion = base removed,
insertion = extra base added,
Deletion and insertion result in “frameshift” i.e. all the codons following the mutation are changed, resulting in a changed aa sequence after the mutation and a less or non-functional protein. May be fatal. Substitution only changes 1 aa, may or may not be significant.
What is a metabolic pathway?
A metabolic pathway is a series of enzyme controlled biochemical reactions that are connected by their
intermediates (so where the product of one reaction becomes the substrate of another).
Gene 1 ——– Gene 2 ——– Gene 3
——-Enzyme 1 —— Enzyme 2 ——– Enzyme 3
A __________>B __________>C __________>D
Biochemical reactions are catalysed by specific enzymes and every enzyme is coded for by a specific gene(s). Biochemical reactions do not occur in isolation but form part of a chain reaction so that the product of one becomes the substrate of another step in metabolism. Phenotype is determined by the presence, absence, or amount of specific metabolic products.
Mutations in the metabolic pathway
mutation at gene 2.
If gene 2 is mutated, then enzyme 2 will not catalyse, so reaction stops at this point. This means that substance B accumulates (may be toxic) and no C or D is produced. C and D may have important (necessary for life) biological functions.
There can be different causes of metabolic disorders as a mutation in Gene 1, 2 or 3 can cause a non
functional enzyme leading to a lack of D.
Normal children can be born from affected parents. Normal children can be born, because if the recessive mutated genes in the parents are different, then each one of those can be dominated by an allele inherited from the other, resulting in normal production of the enzyme.
Environmental effect on gene interactions?
The effect of environment on expression of phenotype involves ways that environmental factors may change
phenotype without changing genotype.
The genotype will provide the genetic potential but the environment can alter the expression of the genotype.
> Temperature e.g. Himalayan rabbit has black extremities and a white main body, only in parts that are cool enough will the black pigment develop, Drosophila flies can develop curly wings at above 25oC.
Light Intensity and Duration e.g. need light to produce chlorophyll, day length triggers flowering, size of leaves bigger in lower light conditions.
Wind e.g. prevailing wind can change the shape, height of plants
Epigenetic Tags:
Not every protein is needed in every cell (cells specialise) and proteins are not needed in cells all of the time. Therefore genes can be switched on and off.
Epigenetic tags are chemical tags that can switch certain genes on or off without changing the triplet base sequence. Epigenetic tags can be added during an organism’s life span and can be inherited.
