Nucleic Acids And Their Functions

Question 1

What is a nucleic acid?

Answer 1

Polymers, made up of monomers called nucleotides. A molecule containing many nucleotides is a polynucleotide. Polynucleotides may be millions of nucleotides long.

Question 2

What are the three components of a nucleotide?

Answer 2

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

Question 3

How are the three components of a nucleotide combined?

Answer 3

By condensation reactions

Question 4

What are the two groups of organic bases?

Answer 4

The pyrimidine bases are thymine, cytosine and uracil

The purine bases are adenine and guanine

Question 5

Structure of DNA: (7)

Answer 5

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

Question 6

DNA is suited to its functions because : (4)

Answer 6

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.

Question 7

Structure of RNA (3)

Answer 7

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

Question 8

What is ATP?

Answer 8

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

Question 9

What are the three types of RNA involved in the process of protein synthesis?

Answer 9

Messenger RNA

Ribosomal RNA

Transfer RNA

Question 10

Describe messenger RNA

Answer 10

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

Question 11

Describe ribosomal RNA

Answer 11

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

Question 12

Describe transfer RNA

Answer 12

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

Question 13

Difference in DNA and RNA?

Answer 13

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

Question 14

Who proposed the molecular structure of DNA in 1953?

Answer 14

Watson and crick. They used the information obtained by many scientists, including Franklin and Wilkins to build three dimensional model of DNA

Question 15

Where is DNA in eukaryotic cells?

Answer 15

Enclosed in the nuclei

Question 16

Where is DNA in prokaryotes?

Answer 16

Loose in the cytoplasm

Question 17

What are the two main roles of DNA?

Answer 17

Replication

Protein synthesis

Question 18

What, briefly, happens in DNA replication?

Answer 18

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

Question 19

What briefly happens in protein synthesis?

Answer 19

The sequence of bases represents the information carried in DNA and determines the sequence of amino acids in proteins

Question 20

What were the three possibilities imagined for the mechanism of DNA replication?

Answer 20

Conservative replication

Semi conservative replication

Dispersive replication

Question 21

What was the conservative replication theory?

Answer 21

Where the parental double helix remains intact and a whole new double helix is made

Question 22

What was the semi conservative theory?

Answer 22

Where the parental helix separates into two strands, each of which acts as a template for synthesis of a new strand

Question 23

What was the dispersive replication theory?

Answer 23

Where the two new double helices contain fragments from both strands of the parental double helix

Question 24

Which is the correct replication theory?

Answer 24

Semi conservative

Question 25

Explain the meselson-stahl experiment

Answer 25

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

Question 26

Proof that the genetic code is a triplet code?

Answer 26

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

Question 27

What are the characteristics of genetic code?

Answer 27

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

Question 28

Stages of semi conservative replication?

Answer 28

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

Question 29

What is a intron?

Answer 29

Non coding nucleotide sequence in DNA and pre-mRNA, that is removed from pre-mrna to produce mature mRNA

Question 30

What is an exon

Answer 30

Nucleotide sequence in DNA and pre mRNA that remains present in the final mature mRNA, after introns have been removed

Question 31

What are the two stages of protein synthesis?

Answer 31

Transcription

Translation

Question 32

Briefly describe transcription?

Answer 32

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

Question 33

Briefly describe translation

Answer 33

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

Question 34

Stages of transcription?

Answer 34

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

Question 35

Stages of translation

Answer 35

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