Nucleic Acids And Their Functions Flashcards
What is a nucleic acid?
Polymers, made up of monomers called nucleotides. A molecule containing many nucleotides is a polynucleotide. Polynucleotides may be millions of nucleotides long.
What are the three components of a nucleotide?
A phosphate group, which has the same structure in all nucleotides.
A pentode sugar. The pentode is ribose in RNA and deoxyribose in DNA
an organic base/nitrogenous base
How are the three components of a nucleotide combined?
By condensation reactions
What are the two groups of organic bases?
The pyrimidine bases are thymine, cytosine and uracil
The purine bases are adenine and guanine
Structure of DNA: (7)
DNA is composed of two polynucleotide strands wound around each other in a double helix
The pentose sugar in the nucleotides is deoxyribose
There are four organic bases on DNA: 2 purines, adenine and guanine and 2 pyrimidines, cytosine and thymine
The deoxyribose sugar and phosphate groups are on the outside of the DNA molecule and form the ‘backbone’
The bases of the two strands face each other, pointing inwards. Adenine always lines up with thymine and guanine with cytosine. Hydrogen bonds join the bases and they form complimentary pairs. Adenine is complimentary to thymine and guanine is to cytosine. The hydrogen bonds between the bases maintain the shape of the double helix.
A DNA molecule is very long and thin and is tightly coiled within the chromosome. The double helix is only 2nm in diameter.
The nucleotides in one strand are arranged in the opposite direction from those in the complimentary strand. The strands are antiparallel
DNA is suited to its functions because : (4)
It is a very stable molecule and its information content passes essentially unchanged from generation to generation
It is a very large molecule and carries a large amount of genetic information
The two strands are able to separate, as they are held together by hydrogen bonds
As the base pairs are on the inside of the double helix, within the deoxyribose phosphate backbones, the genetic information is protected.
Structure of RNA (3)
RNA is a single stranded polynucleotide
RNA contains the pentose sugar ribose
RNA contains the purine bases adenine and guanine and the pyrimidines bases cytosine and uracil, but not thymine
What is ATP?
ATP is a nucleotide.
The initials stand for adenosine triphosphate, which indicates that it contains the base adenine, the sugar ribose and three phosphate groups
What are the three types of RNA involved in the process of protein synthesis?
Messenger RNA
Ribosomal RNA
Transfer RNA
Describe messenger RNA
It is a long, single stranded molecule.
It is synthesised in the nucleus and carries the genetic code from the DNA to the ribosomes in the cytoplasm. Different mRNA molecules have different lengths, related to the genes from which they are synthesised
Describe ribosomal RNA
Its founding the cytoplasm and comprises large, complex molecules. Ribosomes are made of ribosomal RNA and protein
They are the site of translation of the genetic code into protein
Describe transfer RNA
Is a single stranded molecule which folds do that in places, there are base sequences forming complementary pairs.
It’s shape is described as cloverleaf
Carries a sequence of three bases called the anticodon
Molecules of tRNA transport specific amino acids to the ribosomes in proteins synthesis
Difference in DNA and RNA?
Pentose : DNA has deoxyribose, RNA has ribose
Bases (purines): DNA has guanine and adenine, RNA has guanine and adenine
Bases (pyrimidines): DNA has cytosine and thymine, RNA has cytosine and uracil
Strands: DNA has two in a double helix, RNA is single stranded
Length: DNA is long, tRNA and rRNA are short; mRNA varies but shorter than DNA
Who proposed the molecular structure of DNA in 1953?
Watson and crick. They used the information obtained by many scientists, including Franklin and Wilkins to build three dimensional model of DNA
Where is DNA in eukaryotic cells?
Enclosed in the nuclei
Where is DNA in prokaryotes?
Loose in the cytoplasm
What are the two main roles of DNA?
Replication
Protein synthesis
What, briefly, happens in DNA replication?
DNA compromises two complimentary strands, the base sequence of one strand determining the base sequence of the other.
If two strands of a double helix are separated, two identical double helices can be formed, as each parent strand acts as a template for the synthesis of a new complementary strand
What briefly happens in protein synthesis?
The sequence of bases represents the information carried in DNA and determines the sequence of amino acids in proteins
What were the three possibilities imagined for the mechanism of DNA replication?
Conservative replication
Semi conservative replication
Dispersive replication
What was the conservative replication theory?
Where the parental double helix remains intact and a whole new double helix is made
What was the semi conservative theory?
Where the parental helix separates into two strands, each of which acts as a template for synthesis of a new strand
What was the dispersive replication theory?
Where the two new double helices contain fragments from both strands of the parental double helix
Which is the correct replication theory?
Semi conservative
Explain the meselson-stahl experiment
They cultured the bacterium escherichia coli for several generations in a medium containing amino acids made with the heavy isotope of nitrogen, 15N instead of the normal light isotope 14N. The bacteria incorporated the 15N into their nucleotides and then into their DNA so. That eventually, the DNA contained only 15N. They extracted the bacterial DNA and centrifuged it.the DNA settled at a low point in the tube because the 15N made it heavy.
The 15N bacteria were washed, then transferred to a medium containing the lighter isotope of nitrogen, 14N and were allowed to divide once more, the washing prevented contaminating the 14N medium with the 15N so that 15N was not incorporated into any new DNA strands
DNA from this first generation culture was centrifuged, and had a mid point density, this ruled out conservative replication because that would produce a band showing the parental molecule that was entirely heavy. The intermediate position could imply one strand of the new DNS molecule was an original strand of 15N DNA and the other half was newly made, with 14N as in semi conservative replication or it could imply that all strands contained a mixture of light and heavy as I’m dispersive replication
DNA from the second generation grown in 14N, settled at the mid points down high point in the tube, in equal amounts. The sample at the mid point had intermediate density and the sample at the high point was light, containing nitrogen that was 14N only, this rules out dispersive replication because if that were the case there would be a mixture of light and heavy in every strand and one band would only form, one parental strand is conversed, so this is conclusive evidence for the semi conservative hypothesis
Proof that the genetic code is a triplet code?
Biochemical experiments showed that a polynucleotide strand always has three times the number of bases than the amino acid chain is coded for
If three bases were removed from a polynucleotide chain, the polypeptide made would have one fewer amino acid
If the polynucleotide had three extra bases, the polypeptide would have one more amino acid
What are the characteristics of genetic code?
Three bases encode each amino acid so the code is a triplet code
There are 64 possible codes for only 20 amino acid’s are found in proteins are found in proteins more than one triplet can encode each amino acid so the code is described as degenerate or redundant
The code is punctuated: there are three triplet codes that do not code for amino acids. In mRNA, they are called stop codons and mark the end of a portion to be translated.
The code is universal, in all organisms known, the same triplet codes for the same amino acid
The code is non-overlapping: each base occurs in only one triplet
Stages of semi conservative replication?
DNA double helix
The hydrogen bonds holding the base pairs together break
DNA unwinds, catalysed by the enzyme helicase and the two strands of the molecules separate
The enzyme DNA polymerase catalysed the condensation reaction between the 5’-phosphate group of a free nucleotide to the 3’ OH on the growing DNA chain. Each chain acts as a template and free nucleotides are joined to their complimentary bases.
There are now two newly synthesised daughter strands. They carry bases complimentary to the bases on the original strands, which acted as the templates for their synthesis
What is a intron?
Non coding nucleotide sequence in DNA and pre-mRNA, that is removed from pre-mrna to produce mature mRNA
What is an exon
Nucleotide sequence in DNA and pre mRNA that remains present in the final mature mRNA, after introns have been removed
What are the two stages of protein synthesis?
Transcription
Translation
Briefly describe transcription?
One strand of the DNA acts as a template for the production of mRNA, a complementary section of part of the DNA sequence. This occurs in the nucleus
Briefly describe translation
The mRNA acts as a template to which complementary tRNA molecules attach, and the amino acids they carry are linked to form w polypeptide. This occurs on ribosomes in the cytoplasm
Stages of transcription?
The enzyme DNA helicase breaks the hydrogen bonds between the bases in a specific region of the DNS molecule. This causes the two strands to separate and unwind, exposing nucleotide bases
The enzyme RNA polymerase binds to the template strand of DNA at the beginning of the sequence to be copied
Free RNA nucleotides align opposite the template strand, based on the complementary relationship between the bases in DNA and the free nucleotides. For example, a nucleotide containing cytosine aligns opposite guanine
RNA polymerase moves along the DNA for,OMG bonds that add RNA nucleotides, one at a time, to the growing RNA strand. This results in the synthesis of a molecule of mRNA alongside the unwound portion of DNA. Behind the RNA polymerase, the DNA strands rewind to reform the double helix
The RNA polymerase separates from the template strand when it reaches a stop signal
The production of the transcript is complete and the newly formed RNA detaches from the DNA
Stages of translation
Initiation
A ribosome attaches to a ‘start’ codon at one end of the mRNA molecule
The first tRNA, with an anticodon complementary to the first codon on the mRNA, attaches to the ribosomes. The three bases of the codon on the mRNA bond to the three complementary bases of the anticodon on the tRNA, with hydrogen bonds
A second tRNA, with an anticodon complementary to the second codon on the mRNA, attaches to the other attachment site and the codon and anticodon bond with hydrogen bonds
Elongation
The two amino acids are sufficiently close for a ribosomal enzyme to catalyse the formation of a peptide bond between them
The first tRNA leaves the ribosome, leaving its attachment site vacant. It returns to the cytoplasm to bind to another copy of its specific amino acid.
The ribosome moves one codon along the mRNA strand
The next tRNA binds
Termination
The sequence repeats until a ‘stop’ codon is reached
The ribosome- mRNA- polypeptide complex separates
