BY5.1 Flashcards
When does semi-conservative replication occur?
interphase of mitosis
What are the 2 major functions of DNA?
1) REPLICATION in dividing cells
2) carrying information for PROTEIN SYNTHESIS in all cells
describe the process of semi-conservative replication:
1) DNA molecule unzips then unwinds - H bonds break
2) Separated DNA polynucleotide strands act as templates
3) Free DNA nucleotides complementary base pair with exposed bases on template strands - H bonds form
4) DNA Polymerase moves along complimentary nucleotides - catalyses condensation reactions - Phosphodiester Bonds form
5) result -> copied polynucleotide strands
6) the 2 new DNA molecules rewind into helices
Why is the replication of DNA considered SEMICONSERVATIVE?
1) BOTH of the ORIGINAL DNA ARE COPIED
2) The DNA MOLECULES produced EACH contain 1/2 ORIGINAL DNA, 1/2 NEW DNA
What evidence suggests that DNA self-replicated by semi-conservative replication?
The Meselson Stahl experiment:
1) E.COLI cultured on 15N - denser than 14N
2) Absorbs 15N to synthesise NITROGENOUS BASES
3) DNA EXTRACTED by BREAKING OPEN THE CELLS
4) DNA CENTRIFUGED -> ONE BAND - LOW IN TUBE
5) Culture remaining N15 bacteria on 14N ONCE
6) repeat steps 2-4
7) result - ONE INTERMEDIATE BAND in tube - DNA must be 1/2 14N and 1/2 15N
Why is E.coli used to demonstrate semi - conservative replication?
1) EASY TO CULTURE
2) NO NUCLEAR MEMBRANE around DNA - easy to extract
How does DNA determine the primary structure of a protein?
The SEQUENCE OF DNA BASES
determines the SPECIFIC SEQUENCE of AMINO ACIDS in the polypeptide chain
What is the genetic code?
a 3 LETTER (base), or TRIPLET CODE for ONE SPECIFIC AMINO ACID
(one amino acid can be coded for by more than one type of triplet, called the DEGENERATE CODE.)
Why is the genetic code a 3 base code?
1) 2 base code codes for only 16 (4^2) combinations - i.e. less than the 20 amino acids humans have
2) 4 base code codes for 256 (4^4) combinations - copying ERRORS would occur MORE FREQUENTLY
3) 3 Base code codes for 64 (4^3) possible combinations - enough for all 20 Amino Acids and START/STOP CODES
What do start/stops codes do?
ENSURE THE CODE IS READ IN THE RIGHT DIRECTION
What is the name of mRNA bases that the triplet code is transcribed on to?
CODONS
What is a gene (CISTRON)?
The portion of DNA which codes for a WHOLE POLYPEPTIDE - basis for the ‘ONE GENE, ONE POLYPEPTIDE’ hypothesis
what is meant when it is said that all the triplet codes/codons are UNIVERSAL?
The codes are exactly the same for every organism
What is meant when it is said that the code is non-overlapping?
Each triplet is read separately
What are the 2 Stages of protein synthesis?
1) Transcription - formation of mRNA
2) Translation - translation of code into primary structure protein
What is the definition of transcription?
The mechanism by which the base sequence of a gene on a DNA strand
is converted into the COMPLEMENTARY BASE SEQUENCE of mRNA
Describe the method of transcription in protein synthesis?
1) DNA is too big to leave the nucleus via the nuclear pores
2) RNA Polymerase binds to DNA at the gene to be copied
3) DNA unwinds and unzips
4) one of the DNA strands is used as a template
5) Free RNA nucleotides align to the complementary DNA base e.g - uracil joins to adenine
6) DNA polymerase joins the nucleotides forms single mRNA strand
7) mRNA now carries complementary codons which code for specific amino acids
8) at end of the sequence mRNA detaches and DNA rezips and rewinds
9) mRNA transfers nucleotides through the nuclear pores to the cytoplasm
10) mRNA attaches to ribosomes (made of rRNA and protein)
What is tRNA?
-transfer RNA
Single strand
clover leaf shape due to BASE PAIRING between sections
exposed bases at bottom - ANTICODON
Why is the tRNA anticodon important in protein synthesis?
1) Determines SPECIFIC AA that attaches to the tRNA molecule, and therefore the SEQUENCE OF THE AA IN THE POLYPEPTIDE
2) Only the anticodon can base pair with the complementary mRNA codon
What is the description of Translation in protein synthesis?
a mRNA code is converted into an Amino Acid sequence
Describe the Process of Translation in protein synthesis
1) mRNA moves out of the nuclear pores to the cytoplasm
2) Ribosome attatches to mRNA
3) (meanwhile) AA is activated by ATP
4) AA attaches to specific tRNA molecule at a specific linkage site - has AA at one end and the anticodon at the other
5) tRNA brings specific AA’s to mRNA
6) tRNA anticodon CBP’s to mRNA start codon - forms H bonds
7) tRNA no.2 CBP’s to mRNA’s second codon
8) A ribosomal enzme catalyses peptide bond formation between one tRNA’s AA and another’s growing polypeptide (ATP NEEDED)
9) tRNA No.1 is released and returned to cytoplasmic pool - To join to same specific AA
10) Ribosome passes along mRNA, tRNA with specific anticodon fills vacant slot
11) AA forms a peptide bonds with the last member of the chain using ATP energy
12) Until a stop codon is reached
What is the function of the Ribosome in protein synthesis?
- Acts as a framework moving along the mRNA, reading the code -
holds the CODON-ANTI CODON COMPLEX together until two AA’s join
