Nucleic acids

Question 1

what are nucleotides and describe their structure

Answer 1

nucleotides are biological molecules that participate in nearly all biochemical processes. they are phosphate esters of pentose sugars, where a nitrogenous base is linked to carbon 1, and a phosphate group is linked to either carbon 5 or carbon 3, by covalent bonds formed by condensation reactions.

Question 2

describe the phosphorylation of nucleotides

Answer 2

nucleotides become phosphorylated nucleotides when they contain more than one phosphate group; for example, Adenosine diphosphate and Adenosine triphosphate. ATP is energy-rich and it is used in most metabolic processes.

Question 3

what is the pentose sugar within DNA molecules

Answer 3

Deoxyribose sugar

Question 4

what is the pentose sugar within RNA molecules

Answer 4

Ribose sugar

Question 5

what is the structure of DNA

Answer 5

-DNA is a polymer of
many repeating monomers known as nucleotides
-the two polynucleotide strands run in opposing directions and are known as anti-parallel
-each nucleotide consists of a deoxyribose sugar, phosphate group, and one of four nitrogenous bases ( A G T C )
-there are phosphodiester (covalent) bonds that join nucleotides by one nucleotide pentose sugar and another nucleotide phosphate group.
-the two chains are joined by their complimentary nitrogenous base pairs with either 2 or 3 hydrogen bonds depending on the base type.

Question 6

what are the four nitrogenous bases found in DNA

Answer 6

adenosine
thymine
cytosine
guanine

Question 7

what are the two purine nitrogenous bases (TWO RINGS)

Answer 7

adenine and guanine

Question 8

what are the two pyrimidine nitrogenous bases (ONE RING)

Answer 8

cytosine and thymine

Question 9

how many hydrogen bonds are between adenine and thymine

Answer 9

2

Question 10

why do DNA chains have hydrogen bonds between them instead of a stronger bond

Answer 10

because it allows the chain to be unzipped when replication is taking place

Question 11

how many hydrogen bonds are between guanine and cytosine

Answer 11

3

Question 12

why is the sugar-phosphate backbone described as antiparallel

Answer 12

-the upright part of the large DNA molecule that resembles a ladder is formed by sugar and phosphate. the opposite directions of the two strands refer ti the direction that the 3rd and 5th carbon molecules on the 5-carbon sugar, deoxyribose, are facing.
-the 5’ end of the molecule is where the phosphate group is attached to the fifth carbon atom on the deoxyribose sugar.
-the 3’ end is where the phosphate group is attached to the third carbon atom of the deoxyribose sugar.

Question 13

how is DNA organised within cells

Answer 13

the majority of DNA content is in the nucleus. each molecule of DNA is tightly wound around specific histone proteins into chromosomes. each chromosome is therefore one molecule of DNA. there is also sometimes loops of DNA without histone proteins within chloroplasts and mitochondria

Question 14

describe the process of semi conservative replication

Answer 14

to make a new copy of DNA, each DNA molecule:
-unwinds- the double helix is untwisted, a bit at a time, catalyzed by a gyrase enzyme.
-unzips- the hydrogen bonds between the two nitrogenous bases are broken. this is catalyzed by DNA helicase, resulting in two single strands of DNA with exposed nucleotide bases.
-free phosphorylated nucleotides, present in the nucleoplasm within the nucleus, are bonded to the exposed bases.
-the enzyme DNA polymerase catalyzes the addition of the new nucleotide bases, in the 5’ to 3’ direction, it uses the unzipped DNA as a template
-the leading strand is synthesised continously, whereas the lagging strand is in fragments that are later joined, catalysed by ligase enzymes.
-hydrolysis of the activated nucleotides, to release extra phosphate groups, supplies energy to make phosphodiester bonds between the sugar residue and phosphate of the next nucleotide.
-the product of this is two DNA molecules, identical to each other and the parent module. each of these molecules contains one old and one new strand, and so it is termed semi conservative replication

Question 15

how is RNA structurally different to DNA

Answer 15

• the sugar molecule in each nucleotide is ribose
  -the nitrogenous base uracil, which is a pyrimidine replaces the pyrimidine base thymine
  -the polynucleotide chain is usually single stranded
• the chain is shorter
  -there are three forms of RNA. these are- mRNA, tRNA, and rRNA.

Question 16

what is a gene?

Answer 16

a gene is a specific length on a molecule of DNA that contains the code that determines the sequence of amino acids in a particular polypeptide or protein.

Question 17

where are proteins made within cells

Answer 17

within the cytoplasm and ribosomes

Question 18

what are the instructions inside genes

Answer 18

within each gene there is a sequence of DNA base triplets that determines the amino acid sequence, or primary structure of a polypeptide. As long as the primary structure of this is correct, it will fold correctly and can be held in shape enabling its function such as working as an enzyme.

Question 19

what happens instead? chromosomes cannot pass out of the nucleus to formulate a protein

Answer 19

as instructions inside the genes, on chromosomes cannot pass out of the nucleus, a copy of the gene has to be transcribed into a length of mRNA. in this form, the sequence of base triplets, now called codons, can pass out of the nucleus and to the ribosome, ensuring that the coded instructions are translated and the protein is assembled correctly from amino acids.

Question 20

summarise the process of transcription

Answer 20

• a gene unwinds and unzips
  -hydrogen bonds between complimentary nucleotide bases break
  -the enzyme RNA polymerase catalyses the formation of temporary hydrogen bonds between RNA nucleotides and their complementary unpaired DNA bases. A bond with T. C with G and U with A. The DNA strand is called the template strand.
• a length of RNA that is complimentary to the template strand of the gene is produced. it is therefore a copy of the other DNA strand- the coding strand.
• the mRNA now passes out of the nucleus, through the nuclear envelope and attaches to a ribosome.

Question 21

How are ribosomes made

Answer 21

ribosomes are made in the nucleolus and pass out of the nucleus into the cytoplasm as two smaller subunits, these subunits are combined in the cytoplasm with the help of magnesium ions that help the binding of the units. the subunits are Ribosomal RNA and a protein component.

Question 22

summarise the process of translation

Answer 22

Transfer RNA molecules are made in the nucleolus and pass out of the nucleus into the cytoplasm. they are single-stranded polypeptides but can twist into a hairpin shape. At one end is a trio of nucleotide bases that recognize and attach to a specific amino acid. at the loop of the hairpin is another triplet of bases, called an anticodon, that is complementary to a specific codon of bases on the mRNA

Question 23

describe polypeptide synthesis on a molecule tRNA

Answer 23

-transfer RNA molecules bring the amino acids and find their place when the anticodon binds by temporary hydrogen bonds to the complimentary codon on the mRNA molecule
-as the ribosome moves along the length of mRNA, it reads the codes, and when two amino acids are adjacent to each other a peptide bond forms between them.
-energy, in the form of ATP, is needed for polypeptide synthesis
-the amino acid sequence for the polypeptide is therefore ultimately determined by the sequence of triplets of nucleotide bases on the length of DNA- the gene.
-after the polypeptide has been assembled and it has left the ribosome. the mRNA breaks down and it is recycled
-finally, the polypeptide folds into its specific tertiary structure.

Question 24

what are the properties of the genetic code

Answer 24

-the genetic code is near universal because in almost all living organisms the same triplet of DNA bases codes for the same amino acids
-it is described as degenerate, because for all amino acids, there is more than one base triplet. this may reduce the effect of mutations, as a change in the triplet code could still code for the same amino acid
-the genetic code is also non-overlapping, and it is read starting from a fixed point in groups of 3 bases.