DNA Flashcards
(29 cards)
What are the components of DNA
- deoxyribose as the pentose sugar
- phosphate group
- nitrogenous base ( ag or tc)
What is a DNA molecule made of
- two complementary polynucleotide strnds wound around each other in a double helix held together b hydrogen bonds
What are the 4 organic nitrogenous bases in DNA and how do they pair
Pyrimidines - cytosine and thymine
Purines - adenine and guanine
Complementary base pairing
A-T joined by 2 hydrogen bonds
C-G joined by 3 hydrogen bonds
Why are hydrogen bonds important in base pairing
They maintain the shape of the double helix
Describe the sugar phosphate backbone
The deoxyribose sugar and phosphate groups are on the outside of the molecule forming the backbone
Feature of DNA
- very long 8mm
- thin
- tightly coiled
How are the nucleotide strands in DNA arranged
- the nucleotides in one strand are arranged in the opposite direction from those in the opposite strand
- so the strands of nucleotides are described to be antiparallel
(5’ 3’ and 3’,5’ the other)
How is DNA suited to its functions
- very stable molecule - and it information content passes essentially n changed from enervation to generation
- very large molecule - carries a large amount of genetic information
-the two strands are able o separate as they’re held together by hydrogen bonds - as the base pairs are on the inside of the helix within the sugar phosphate backbone the genetic information is protected
Functions =of DNA
- replication - if each polypetide strand of the double helix are separated two identical double helices can be formed
- each parent strand acts as a template for the synthesis of a new complementary strand
Protein synthesis -the sequence of bases represents the information carried in DNA and determines the sequence of amino acids in the proteins
Why must chromosomes make copies of themselves
So that when the cells divide each daughter cell receives an exact copy of the genetic information
Describe conservative replication of DNA
Where the parental double helix remains intact - conserved and whole new double helix is made
Describe semi conservative replication of DNA
The parental double helix separates into 2 strands each of which acts as a template for the synthesis of a new strand
Describe dispersive replication of DNA
2 new double helices contain fragments from both strands of the parental double helix
Describe the stages of semi conservative replication
Helicase breaks the hydrogen bonds holding the base pairs together
The dna unwinds catalysed by the enzyme helicase - the 2 strands of the molecule separate
The enzyme DNA polymerase catalyses the condensation reaction between the 5’ phosphate group of a free nucleotide to a the 3’-OH on the growing DNA chain
- each chain act as a template and free nucleotides are joined to their complementary bases
- there are now 2 newly synthesised daughter strands carrying bases complementary to the parent strands which acted as templates for their synthesis
Describe how genetic code is a triplet code
Biochem experiments showed
- a polynucleotide strand has 3x the number of bases as the amino acids chain it coded for
- if three bases are removed from a polynucleotide chain the polypeptide made would have one fewer amino acid
- if the polynucleotide has 3 extra bases the polynucleotide would have one more amino acid
- so 3 bases code for 1 amino acid
Characteristics of genetic code
3 bases code for 1 amino acids so it is a triplet code
- is is described as degenerate/redundant aas there are 64 possible codes but only 20 amino acids, so more than 1 triplet can code for the same amino acid
- the code is punctuated - there are three codes that do not code for an amino acid in mRNA they are called stop codons
- the code is universal - in all organisms the same triplet codes for the same amino acid
- the code is non overlapping. Each base occurs in only one triplet
What is a codon
Triplet of bases in mRNA that codes for a particular amino acids or a punctuational signal
What is an intron
Non coding nucleotide sequence in DNA and pre mRNA that is removed from pre mRNA to produce mature mRNA
What is an exon
Coding region in the nucleotide sequence of DNA and pre mRNA that remains in the final mature mRNA after the introns have been removed
How are introns and extrons involved in eukaryotes
- dna contains the info to make polypeptides
- an RNA version is made from the DNA
- as this initial RNA version of the code is much longer than the final mRNA it contains sequences of bases that have to be removed
- these sequences to be removed are introns
- they ae cut out of the pre mRNA using endonucleases
- the sequences left are exons which are joined together or spliced with ligases
What is transcription
One strand of DNA acts as a template for the production of mRNA ( a omplementary section of part of the DNA sequence
This occurs in the nucleus
What is translation
The mRNA acts as a template strand to which complementary tRNA molecules attach
The amino acids they carry are linked to form a poly peptide
- this occurs on the ribosomes in the cytoplasm
Describe the process of translation initiation
Initiation- a ribosomes attaches to a start codon at one end of the mRNA molecule at the smaller subunit
- the first tRNA with an anticodon complementary to the first codon on the mRNA attaches to the ribosome ( at 1/2 of the larger subunits )
The 3 bases on the codon of mRNA bind to the three complementary bases on the anticodon of tRNA with hydrogen bonds
- a second tRNA mowith an anticodon complementary to the second codon on the mRNA attaches to the other attachment site(2/2 larger subunits) and the codon and anticodon bond itch hydrogen bonds
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Describe the process of elongation in translation
The 2 amino acids are close enough soft a ribosomal enzyme to catalyse the formation of a peptide bond between them
The first tRNA leaves the ribosome leaving its attachment site vacant
The tRNA returns to the cytoplasm to bind to anther copy of its specific amino acid
The ribosome moves one codon along the mRNA strand
The next tRNA binds
