DNA, RNA, Protein Synthesis, Meiosis Flashcards
Describe the structure of eukaryotic DNA
Eukaryotic DNA is linear, double-stranded helix with hydrogen bonds between the bases. DNA molecules are long and coil up around histones (proteins). This undergoes supercoiling to form chromosomes, which are stored in the nucleus.
Describe the structure of prokaryotic DNA
Prokaryotic DNA is double stranded, but is shorter and circular. The DNA is not associated to histones and is stored in the cytoplasm of the cell as a part of the chromosomal loop or plasmids. This DNA is similar in structure to the DNA found within mitochondria and chloroplasts of eukaryotic cells.
What is a gene?
A gene is a base sequence of DNA which codes for a specific polypeptide or functional RNA.
What is a cell’s genome?
The complete set of genes in a cell (the entire of the cell’s genetic material)
What is a cell’s proteome?
The full range of proteins a cell is able to produce
Where are genes located within DNA?
A gene occupies a specific fixed position on a DNA molecule, called a locus
Do all of the base sequences in DNA code for an amino acid/nucleotide?
In prokaryotic DNA, yes all bases code for amino acids (there are no introns). However, in eukaryotes, the DNA does contain introns (non-coding regions of DNA), and multiple repeats (another kind of non-coding DNA generally found between genes). Only exons are coding regions.
What are alleles?
Alleles are alternative forms of a gene which have a slightly different order of bases, and as such, code for a slightly different polypeptide. They arise from mutations.
What are homologous pairs?
A homologous pair is two chromosomes which are the same size and shape and code for the same genes. They are identical (unless they contain different alleles).
What is protein synthesis?
The process by which new proteins are made by a cell, involving transcription and translation.
Describe the structure of tRNA (transfer RNA)
tRNA has a cloverleaf shape. It is a single polynucleotide strand held together by hydrogen bonds to specific base pairs (on the other side of the structure, it is still single stranded despite this bonding). It has a anticodon consisting of three bases which binds to a specific mRNA codon. It also has an amino acid binding site at the other end, consisting of three bases (it binds to the amino acid coded for by the mRNA codon).
Describe the process of transcription.
Transcription occurs the nucleus (eukaryotes) or in the cytoplasm (prokaryotes)
- DNA helicase binds to DNA and breaks the hydrogen bonds between bases, separating the two DNA strands
- RNA polymerase binds to the promoter region of DNA
- free RNA nucleotides bind to their complementary DNA bases (complementary base pairing)
- RNA polymerase forms phosphodiester bonds between adjacent RNA nucleotides. When RNA polymerase reaches a stop signal, mRNA detaches from DNA
- in prokaryotes, mRNA is formed and transcription is complete
- in eukaryotes, pre-mRNA is formed, which contains introns (non-coding DNA) and exons (coding DNA)
- splicing occurs, which removes the introns, forming mRNA
- the mRNA leaves the nucleus through a nuclear pore
Describe the process of translation
Translation occurs either in the cytoplasm (free-floating ribosome) or on a ribosome associated to the RER.
- the mRNA strand attaches to a ribosome and a tRNA molecule with a complementary anticodon to the mRNA’s codon binds to the mRNA codon delivers a specific amino acid
- another tRNA molecule with a complementary anticodon to the next codon binds to the other attachment site on the ribosome, delivering the next amino acid in the polypeptide chain. A peptide bond forms between the adjacent amino acids and the first tRNA molecule detaches from the ribosome.
- the ribosome moves a distance of three bases (one codon) down the mRNA molecule and the process repeats until a stop codon is reached
- at this point, the ribosome detaches from the mRNA and the polypeptide is formed
- the polypeptide will then fold in a specific way to complete protein synthesis
What is the genetic code?
The genetic code is a sequence of base triplets (codons) in mRNA which code for specific amino acids.
What does non-overlapping mean?
Each base is only part of one triplet (and each triplet contains three bases)
What does degenerate mean (genetic code)?
There is more than one base triplet for each amino acid (multiple different base combinations can code for the same amino acid).
What does universal mean (genetic code)?
The same base triplets code for the same amino acids in all organisms
What are gene mutations?
Gene mutations involve a change in the DNA base sequence of chromosomes. This may result in different amino acids being coded for.
How do gene mutations occur?
Gene mutations occur spontaneously during DNA replication.
What are the types of gene mutation?
- base substitution - one base is swapped for another (e.g. ATTCGA becomes ATTCAA). This is a point mutation as it only changes the one amino acid (potentially)
- base deletion - one base is deleted/removed from the sequence (e.g. GCTTCA becomes CTTCA). This is a frameshift mutation as it will affect all the amino acids downstream of the mutation (which may cause a very different protein to form).
- base addition - one base is added to the sequence (e.g. TTTCGA becomes TTTGCGA). This is also a frameshift mutation as it affects all amino acids downstream of the mutation, which could result in the formation of a very different protein.
What is the effect of genetic mutations?
The effect can vary dramatically depending on the type of mutation. A simple base substitution is unlikely to affect the protein much at all (it may not have any impact if the same amino acid is still coded for). However, base additions/deletions can have a huge impact on the final protein as many of the wrong amino acids could be coded for. If the mutation occurs in a non-coding region, it is much less likely to have an effect (although it may impact how well RNA polymerase can bind, so may result in more/less protein being made). Generally, gene mutations are detrimental to the protein, but they can be advantageous.
What are mutagenic agents?
Mutagenic agents are factors which increase the rate of gene mutation. Some examples are UV radiation, ionising radiation, certain chemicals, and some viruses.
What are chromosome mutations?
Chromosome mutations occur during meiosis and they are when the incorrect number of chromosomes are present in a cell. If the homologous chromosomes do not separate during meiosis 1, or the sister chromatids do not separate during meiosis 2, at least one cell may end up with a trisomy of a particular chromosome (3 copies of the chromosome rather than two), and other cells may get no copies or only one copy of the chromosome (these cells generally die). This is called chromosome non-disjunction and can lead to a variety of conditions, such as Down’s syndrome.
What is the different between haploid and diploid cells?
Diploid cells have the diploid number (2n) of chromosomes (so has two copies of every chromosome (the full amount)). Haploid cells have the haploid (n) number of chromosomes (one copy of every chromosome (half the full amount)). Gametes are haploid, all other somatic (body) cells are diploid in humans. Other organisms may spend a large portion of their life cycle comprising mainly haploid cells.
