Functions of Nucleic Acids Flashcards
(52 cards)
What are the components of a nucleotide?
•A phosphate group
•A pentose sugar, either deoxyribose sugar or ribose sugar
•An organic nitrogenous base
What is the process of DNA replication?
- DNA unwinds
- Hydrogen bonds holding complementary base pairs break, catalysed by DNA helicase and the 2 strands separate.
- Each DNA strand acts as a template.
- Free DNA nucleotides align opposite their complementary bases.
- DNA polymerase catalyses the condensation reaction between 2 DNA nucleotides. This occurs from the 5 prime to the 3 prime end of the chain.
- Each new DNA molecule is made from one original template strand and one replicated strand. This is known as semi-conservative replication.
What are the three theories of how DNA may replicate?
-Conservative
-Semi-Conservative
Dispersive
What is conservative?
the parental double helix remains intact, completely conserved, and a whole new DNA molecule is made
What is semi conservative?
Each strand in the parental double helix acts as a template to synthesise a new polynucleotide strand.
Each new DNA molecule contains one template strand and one newly replicated strand
What is dispersive?
Each new DNA molecule contains fragments of the parental double helix and newly synthesises DNA
What is evidence for semi-conservative replication is provided by?
an experiment devised by Matthew Meselson and Franklin Stahl
why is e.coli used?
because it is easy to culture and the DNA is not contained within a nucleus (i.e. bound by a nuclear membrane).
What is the process of the experiment?
- E.coli was grown in a medium containing amino acids made with the heavy isotope 15N. The bacteria produced nucleotides containing 15N. All the bacteria’s DNA contained this heavy isotope. The DNA was extracted and the suspension then centrifuged.
- The bacteria were washed and then transferred to a medium containing amino acids with the lighter isotope - 14N - and allowed to divide once (one generation). The DNA was extracted and the suspension then centrifuged.
- The bacteria were allowed divide again (two generations) on the 14N medium. Again the DNA was centrifuged.
Explain why this was conclusive evidence of semi conservative replication rather than conservative or dispersive-
After one generation conservative replication would not give a band in the middle, only a band at the top (light) and at the bottom (heavy). Dispersive would only produce one band which got progressively higher in the tube, rather than two
What are the two stages of protein synthesis?
Transcription
Translation
What is transcription?
occurs in the nucleus. The genetic code for a specific protein is copied. A complementary strand of mRNA is formed from one template strand of DNA. mRNA leaves the nucleus via the nuclear pore and travels to the ribosome.
What is translation?
occurs at the ribosome. The genetic code is translated into a polypeptide.
Amino acids that correspond to the codons on the mRNA are brought to the ribosome by tRNA.
The amino acids are joined together at the ribosome to form the polypeptide chain.
What is the process of transcription?
Only one strand of DNA is used as a template strand for the transcription of mRNA.
The enzyme DNA helicase breaks the hydrogen bonds holding together the base pairs in a specific region of the DNA molecule and the two strands unwind.
The enzyme RNA polymerase binds to the template strand at the beginning of the sequence to be copied.
Free RNA nucleotides align opposite complementary bases on the template strand
RNA polymerase moves along the template strand, catalysing the addition of RNA nucleotides to each other until it reaches a stop codon. Behind the RNA polymerase, the DNA strands rewind.
mRNA leaves via a nuclear pore and travels into the cytoplasm.
In eukaryotes, what is the initial mRNA molecule produced by transcription compared with the final one?
initial mRNA molecule produced by transcription is longer than the final mRNA that is translated at the ribosome.
What is this initial version called?
pre-mRNA, which needs sequences of bases to be removed.
What are introns?
non-coding nucleotide sequences in DNA that are removed after transcription by the enzyme endonuclease.
What are exons?
are the coding sequences which are left behind and are spliced together by ligase enzymes to form the final mature mRNA.
What are the stages involved in the creation of mature mRNA?
-Transcription
-Introns removed using endonuclease enzymes
-Exons spliced together using ligase enzymes
-Exits via nuclear pore to the ribosome in the cytoplasm
Suggest how more than one polypeptide could be produced from one gene?
The exons could be spliced back together in a different order thus coding for a different sequence of amino acids, which would result in a different polypeptide
What happens in translation?
sequence of codons on the mRNA is translated into a sequence of amino acids to form a polypeptide.
This takes place on the ribosome and involves tRNA.
What is the shape of tRNA?
clover leaf shape.
What is the function of tRNA?
to carry specific amino acids to the ribosome.
