Biological molecules: DNA, RNA and protein synthesis

Question 1

what is meant by point mutation?

Answer 1

a mutation affecting a single nucleotide

Question 2

why is DNA replication semi conservative?

Answer 2

each DNA molecule contains one strand of the original DNA molecule and one brand new.

Question 3

what is a nucleic acid?

Answer 3

large molecules discovered in the cell nuclei. The two types are RNA and DNA

Question 4

nucleic acids are made of many nucleotides. What are the three components of a nucleotide?

Answer 4

-pentose sugar
-phosphate group
-nitrogenous base

Question 5

how are nucleotides linked?

Answer 5

they are linked by condensation reactions to form a polynucleotide. The phosphate group on carbon 5 of the pentose sugar creates a covalent bond with the carbon 3 on the adjacent pentose sugar. This is called a phosphodiester bond which can be broken by hydrolysis

Question 6

what is the pentose sugar called in DNA and RNA?

Answer 6

in DNA, the pentose sugar is called deoxyribose(one fewer oxygen) and in RNA, the pentose sugar is called ribose.

Question 7

name the nitrogenous bases in DNA and RNA

Answer 7

in DNA, the nitrogenous bases are thymine, guanine, adenine and cytosine. It is the same as in RNA but thymine is replaced with uracil.

Question 8

what are pyrimidines and purines?

Answer 8

pyrimidines are the smaller bases which contain single ring carbon structures. These are thymine and cytosine. Purines contain double ring carbon structures, these are adenine and guanine.

Question 9

describe the structure of DNA

Answer 9

double helix and is anti-parallel as each strand has a phosphate group on one end and the other is a hydroxyl group, these strands run in opposite directions. Sugar phosphate backbone.

Question 10

how are the nitrogenous bases paired?

Answer 10

COMPLEMENTARY BASE PAIRING-adenine and thymine form two hydrogen bonds but cytosine and guanine form three hydrogen bonds

Question 11

why does a purine always pair with a pyrimidine?

Answer 11

so the distance between the sugar phosphate backbones is constant all down the DNA molecule.

Question 12

what are the differences between RNA and DNA (3 points)?

Answer 12

-DNA is found in chromosomes, RNA is found in cytoplasm
-DNA is long but RNA is short
-DNA is double stranded but RNA is single stranded

Question 13

Describe the first step of DNA replication

Answer 13

DNA helices cases hydrogen bonds the break between the complementary base pairs causing the two polynucleotide chains to separate.

Question 14

In DNA replication what occurs after the two polynucleotide chains seperate?

Answer 14

free nucleotides line up with their complementary bases. At this stage, the DNA are only held by hydrogen bonds, not phosphodiester bonds. The free nucleotides are called activated nucleotides and they contain 3 phosphate groups.

Question 15

In DNA replication, what occurs after the activated nucleotides line up with the original strand?

Answer 15

DNA polymerase moves down the molecule and catalyses the formation of phosphodiester bonds between the activated nucleotides which is an example of a condensation reaction. When phosphodiester bonds form, the two extra phosphate groups leave, providing more energy for the reaction.

Question 16

what is a DNA mutation?

Answer 16

when an incorrect base is inserted causing the DNA sequence to change.

Question 17

what is the role of messenger RNA?

Answer 17

to send instructions for protein synthesis from the genetic information of DNA.

Question 18

what is discontinuous and continuous DNA replication?

Answer 18

Discontinuous replication is the synthesis of the new strand of a DNA molecule as a series of small fragment (nucleotides) are joined. Continuous replication is seen by the other strand (leading strand) where the continuous addition of nucleotides take place.

Question 19

In the 1950s, what was the other way of DNA replication mentioned apart from semi-conservative replication?

Answer 19

conservative replication: DNA molecules is formed with two new strands, no original DNA strands.

Question 20

Describe the experiment that took place, to determine whether DNA replication was conservative or semi-conservative

Answer 20

-A sample of bacteria was used. The nitrogen atoms would all be nitrogen 14. DNA is extracted which is placed in a solution and spun in a centrifuge. DNA moves down to a band which scientists can detect (higher). The position of the DNA band depends on how heavy the DNA is.
-next scientists cultured the bacteria in a growth medium where they can be replicated many times. Most of the nitrogen atoms were nitrogen 15- the band formed for this is near the bottom (lower)of the centrifuge.
-then a sample of bacteria with nitrogen 15 was transferred to the bacteria of nitrogen 14. It is replicated once. Using a centrifuge, this band was in between the bands go nitrogen 14 and 15 showing that the replicated DNA contain half of original and half new.

Question 21

when the DNA was replicated twice, there were two bands, why?

Answer 21

4 strands are produced- two represent nitrogen 14 and two represent nitrogen 15

Question 22

what is a codon?

Answer 22

a sequence of three bases (triplet code) which codes for an amino acid

Question 23

what is a gene?

Answer 23

a section of DNA that contains the complete sequence of bases(codons) to code for an entire protein.

Question 24

what are the three key features of the genetic code?

Answer 24

-degenerate, as a single amino acids can be coded from multiple codons
-universal, as every living organism has it
-non overlapping as no base is read more than once

Question 25

what is transcription?

Answer 25

the first step in protein synthesis, the base sequence of a gene is copied into the complementary base sequence of a mRNA.

Question 26

describe the steps in transcription

Answer 26

1)RNA helicase breaks the hydrogen bonds between the strands
2)free RNA nucleotides move into place by complementary base pairing and form hydrogen bonds
3)RNA polymerase joins the RNA nucleotides by forming phosphodiester bonds.
4)once the mRNA strand is synthesised, it is detached and the DNA goes back to its double helix structure.
5)mRNA leaves through the nuclear pores, into the cytoplasm where translation occurs

Question 27

mRNA is complementary to what strand and similar to what strand?

Answer 27

it is complementary to the anti-sense/template strand but similar to the sense strand. However, thymine is replaced with uracil.

Question 28

Describe the structure of tRNA

Answer 28

at the stop is the amino acid binding site and at the bottom, there is a triplet of bases called the anti-codon. The tRNA anti-codon is complementary to the mRNA codon for that specific amino acid.

Question 29

Describe the steps in translation

Answer 29

1)the small subunit of a ribosome binds with the mRNA at the start codon
2)complementary tRNA molecule binds to the start codon by hydrogen bonds. The same occurs for the next codon.
3)peptide bonds form in between the amino acids which is catalysed by peptidyl transferase.
4)now the ribosome moves to the next codon and forms a peptide bond with the next amino acid. The first tRNA molecule is released.
5) this continues but when the ribosome arrives at a stop codon, it detaches and the polypeptide chain is released.

Question 30

describe the structure of ATP

Answer 30

nitrogenous base, adenine bonded to ribose. The ribose is bonded to three phosphate groups.

Question 31

how is energy released from ATP?

Answer 31

adding water breaks the covalent bond between the phosphate groups which releases energy- hydrolysis reaction. It is catalysed by ATP hydrolase. The equation:
ATP+water->ADP + Pi +energy

Question 32

what does the i present in Pi?

Answer 32

inorganic phosphate, not attached to a carbon contains molecule.

Question 33

what is phosphorylation?

Answer 33

when adding the phosphate group back into ADP which is catalysed by synthase- condensation reaction.

Question 34

what kind of energy store is ATP?

Answer 34

immediate energy store, short term energy store

Question 35

what is a degenerate codon?

Answer 35

there is more than one codon that specifies the single amino acid which prevents mutations.

Question 36

what is the maximum number of different base combinations in DNA that could code for each amino acid?

Answer 36

64 calculated by 4 to the power of 3

Question 37

describe how a nucleotide base sequence in a gene is used to synthesise a polypeptide (7 marks)

Answer 37

transcription occurs where DNA is copied to mRNA. Free nucleotides line up by COMPLEMENTARY base pairing to one TEMPLATE DNA strand, catalysed by RNA POLYMERASE.
Then translation occurs, mRNA moves to ribosomes, tRNA binds to mRNA. Anticodons bind with codons, specific amino acid attached to tRNA.

Question 38

how many base pairs are there in one full turn of the DNA double helix?

Answer 38

10

Question 39

which technique is used to determine the double-helical structure of DNA?

Answer 39

X-ray crystallography

Question 40

what is reverse transcriptase?

Answer 40

the enzyme which catalyses the formation of DNA from RNA

Question 41

Explain the similarities in DNA replication and transcription

Answer 41

-both have hydrogen bonding
-both require free nucleotides
-both require DNA polymerase to form phosphodiester bonds
-both require complementary bas pairing
-however in transcription only a small section of DNA unzips
-different helices enzymes are needed
-mRNA leaves nucleus but new DNA strand remains bound.

Question 42

suggests suitable substance to extract DNA?(2marks)

Answer 42

detergent as it acts as an emulsifier to breakdown the nuclear membrane