DNA and chromosomes Flashcards
State some differences of DNA in prokaryotic cells and eukaryotic cells:
prokaryotic - DNA is loose in the cytoplasm. It’s not associated with proteins (histones). Some contain plasmids. It’s smaller and circular.
Eukaryotic - DNA is linear. It’s associated with HISTONE proteins which help condense the DNA during prophase.
What is a gene?
A sequence of bases that code for a sequence of amino acids in a polypeoptide.
A section of DNA that contains the coded info for making polypeptides and functional RNA.
What are the four features of the genetic code?
non-overlapping
degenerate
universal
triplet code
What does non-overlapping mean?
The bases are read s separate consecutive groups of 3. No base is read twice during transcription.
What does triplet code mean?
3 bases code for 1 amino acid.
What does degenerate mean?
Most amino acids can be coded for by more than 1 CODON. Few amino acids are coded for by 1 single triplet.
What does universal mean?
The same codon is codes for all amino acids in every single organism, minus a few exceptions. This is indirect evidence for evolution.
Why do 3 bases code for each amino acid?
Because only 20 amino acids regularly occur in proteins. 3 bases produce 64 different codes which is more than enough to satisfy the requirements of amino acids.
What are exons?
Non-coding sequences made up of multiple repeats of base sequences. They don’t code for polypeptides.
What are introns?
Portions of DNA within a gene that don’t code for a polypeptide. The introns are removed from pre mRNA after transcription.
Differences between DNA and mRNA;
DNA mRNA
Double helix Single-stranded
Thymine Uracil
Lives longer Short-lived
Deoxyribose sugar Ribose sugar
Longer molecule Shorter molecule
(multiple genes) (1 gene copy)
Why does transcription occur? Where does it occur?
mRNA is a short lived copy of a gene. It means that DNA can remain in the nucleus. Only genes activated in that particular cell need to be transcribed.
Transcription occurs in the nucleus.
What is the first stage of transcription? and what happens during this stage?
Initiation- Enzyme attachment breaks the hydrogen bonds on 1 section of the DNA, causing the DNA to unwind.
RNA polymerase binds to 1 strand of DNA (the template strand) Transcription factors initiate the binding of RNA polymerase.
What is the second stage of transcription? and what happens during this stage?
Elongation - As RNA polymerase moves along the template strand, it binds new complementary bases of individual RNA nucleotides with bases on template strand. Adenine binds with uracil rather than thymine. Double helix re-joins as the pre mRNA strand is made - RNA polymerase joins together the sugar phosphate backbone.
What is the third stage of transcription? and what happens during this stage?
Termination - When DNA polymerase reaches a stop codon, it detaches. The strand of pre mRNA leaves the nucleus.
