DNA and protein synthesis Flashcards
transcription (first)
Steps in Transcription
1. At the start of the gene, the DNA is made single-stranded by breaking H-bonds. (This is done by RNA Polymerase binding to the Promoter and unwinding the DNA.) 2. Only ONE of the two DNA strands is used as the template for copying into RNA. This is because only one strand carries the code for the aa sequence of the polypeptide (the other strand has the complementary base pairs). 3. RNA nucleotides bind to the template strand by forming H-bonds between complementary base pairs (Guanine and Cytosine opposite to each other, Adenine opposite to Thymine, and Uracil opposite to Adenine). 4. RNA Polymerase joins the nucleotides together by forming phosphodiester bonds between them to form a new RNA molecule. 5. At the end of the gene, the RNA molecule is released. It is known as pre-mRNA, because it contains both intron and exon sequences. 6. The pre-mRNA undergoes splicing / editing to remove the introns and join the exons together to form the mRNA. 7. The mRNA leaves the nucleus via the nucleopores and binds to ribosomes in the cytoplasm / RER.
Splicing does not occur in Prokaryotes, as they do not contain introns.
DNA replication vs transcription
replication
- entire genome is copied
- occurs once per cell cycle
- both strands are used as templates
- two DNA molecules are formed as product
- DNA helicase unwinds the DNA to make it ss
- DNA nt are used
- DNA polymerase makes the phosphodiester bonds
transcription
- only genes copied
- many times per cell cycle
- only one strand is used as template
- many mRNA molecules are formed as prodct
- RNA polymerase unwinds the DNA to make it ss
- RNA nucleotides are used
- RNA polymerase makes the phosphodiester bonds
genetic code
triplet code - 3 bases code for 1 amino acid - set of 3 bases on mRNA that code for 1 aa is codon
degenerate code - some amino acid are coded by more than one codon / multiple codons code for the same amino acid
universal code - each codon codes for the same amino acid in all species
non-overlapping code - bases are not shared between adjacent codons
tRNA
single stranded RNA
folded over using H-bonds to form a clover-leaf shape
many different types of tRNA - each with a specific anti-codon
an enzyme attaches to a specific aa to each tRNA (based on its anticodon)
this reaction requires ATP
each tRNA carries a specific amino acid
translation
occurs in ribosome - ribosome catalyses raction
ribosome made up of two components - proteins + ribosomal RNA
Steps in Translation 1. The mRNA leaves the nucleus through the nucleopores and attaches to the ribosome (in the cytoplasm or on the Rough ER). 2. The ribosome has two binding sites, which are occupied by the first two codons on the mRNA. 3. Two tRNA molecules which have the complementary anti-codons bind to the two codons by H-bonds. 4. Each tRNA carries a specific amino acid. 5. A peptide bond is formed between the two amino acids (ie, the first amino acid is transferred to the second amino acid by a peptide bond). This requires ATP. 6. The first tRNA (without its amino acid) leaves the ribosome, and the ribosome moves forward by one codon. 7. The process is repeated until the stop codon is reached. 8. At the stop codon, the newly formed polypeptide is released from the ribosome. 9. The polypeptide is folded into its secondary / tertiary / quaternary shape and used by the cell (role of ER, Golgi apparatus etc).
mutations
mutation - a change in the base sequence of DNA
can occur spontaneously (DNA replication causes errors) or induced artificially (eg radiation)
can occur at large scale - changes visible at a chromosomal level
small-scale level - need to fingerprint of sequence the DNA to see the mutation
types of mutatinos
Types of mutations:
1. Substitution = one base is replaced by another base. 2. Addition = one or more bases are inserted. 3. Deletion = one or more bases are removed.
shanti’s examples for mutations
What happens when mutations occur: (for convenience, I am showing the mutation on the mRNA rather than on the DNA)
1. In an intron = no change to the protein, no change to the phenotype of the individual. 2. Substitution a. Replace ACA with ACG: both code for thr. No change to the amino acid sequence because of degenerate code. b. Eg. GCA is replaced by GGA: ala is replaced by gly. In this case, one aa is replaced by a similar aa, the protein might still be functional. (This information about aa will be given in the question.) c. Eg. GAG is replaced by GUG: Glu (forms ionic bonds) becomes Val (is non-polar). Tertiary structure will change, protein will not be functional. This occurs in sickle-cell anaemia. (This information about aa will be given in the question.) d. Eg. CAG is replaced by UAG: Gln is replaced by a stop codon. Therefore, the polypeptide is shorter than normal, so not functional. 3. Deletions or additions of 3 bases or multiples of 3 bases: One or more amino acids would be deleted or added. IF this occurs in the active site or in regions involved in forming tertiary or quaternary structures, the protein will be non-functional. This is the mutation seen in cystic fibrosis. But if it occurs in a not very important part of the protein (like at the end), the protein might still be functional. 4. Deletions or additions that are not a multiple of 3 bases: All the codons after the mutation are changed, i.e., all the amino acids are changed, therefore protein becomes non-functional.
what is a genome
the complete set of genetic information contained in the cells of an organism
what is a proteome
the complete set of proteins that can be produced by a cell
describe the structure of mRNA
a long, single strand
its base sequence is complementary to the DNA it was transcribed from
suggest advantages of using mRNA rather than DNA for translation
shorter and contains uracil - breaks down quickly so no excess polypeptide forms
single-stranded and linear - ribosome moves along strand and tRNA binds to exposed bases
contains no introns
describe the structure of tRNA
a single strand of around 80 nucleotides that is folded over into a clover leaf shape
on one end is an anti-codon, on the opposite end is an amino acid binding site
what is produced by transcription
mRNA
where does transcription take place
in the nucleus
