[HKALE 80 II]
5.
(a)
Briefly explain the meaning of the following terms:
(i)
(ii)
(iii)
(iv)
(v)
gene
allele
chromosome
linkage group
mutation
(10 marks)
(b)
(a)
(b)
Describe how the information carried on DNA is used in protein synthesis. (If you wish,
you may use labelled diagrams to answer the question.)
(10 marks)
(i)
(ii)
A gene is the basic unit of heredity , a number of which form a chromosome.
An allele is one of a pair of genes occupying the same locus on homologous
chromosomes which separate during meiosis.
(iii) A chromosome is a threadlike structure found in the nucleus of eukaryotic cells. A
number constitute the genetic material. They consist of DNA, RNA and protein.
(iv) A linkage group refers to all the gene present on the same chromosome. They do not
show independent assortment.
(v) Mutation involves a change in the amount or structure of DNA in the chromosomes
AND Mutation involves a change in the phenotype of an organism.
2
2
2
2
2
DNA is a double strand/ helix of 4 nucleotide bases with the bases in fixed association i.e. 2
adenine to thymine and vice versa, guanine to cystosine and vice versa
--A—T—G—C----T---A---C---G---The strand begins to unwind and an mRNA chain is produced alonside by complementary
base formation, Transcription, N.B. Uracil replaces thymine.
1
2
mRNA then leaves the nucleus and becomes associated with ribosomes to form polysomes.
The mRNA is coded in triplets and each such group is called a codon.
2
1
1
tRNA associates with these triplets in the anticodon area. This process being known as
translation , thus
1
(20)
And so the amino acids borne at the other end of the tRNA are arranged into a sequence
determined by the original configuration of the bases on the DNA. The amino acids are
then
linked
by
peptide
linkage
to
form
specific
proteins.
[HKALE 89 II]
8.
(a)
The following are codons for some of the amino acids and the start signal for translation
found in messenger RNA:
* The codon AUG serves as a start signal and initiates synthesis of a polypeptide.
Answer the following questions with reference to the synthesis of the peptide shown below:
(i)
Using ONE labelled diagram ONLY, explain the roles of the various components
involved in protein synthesis. In your diagram, you should indicate:
(1) the base sequence of the messenger RNA,
(2) the site of protein synthesis,
(b)
(c)
(i)
(ii)
(iii)
(3) the direction of translation,
(4) the base sequences of the anticodons of the transfer RNAs, and
(5)
the stage at which the amino acids, methionine and histidine, are linked
together while leucine is forming a bond to the growing peptide and cysteine is
still in the pool of free amino acids.
(10 marks)
(ii)
What is the base sequence of the corresponding segment on the DNA molecule ?
(2 marks)
Explain the significance of polysomes in protein synthesis.
(2 marks)
Explain the meaning of the following terms with reference to codons:
universal
degenerate
nonsense triplets
(6 marks)
Total 20 marks
8.
(a)
(i)
correct base sequence of m-RNA
indicate site of protein synthesis - ribosome
indicate correct direction of translation
correct base sequence of anticodons
showing Met and His
showing Leu joining to the growing peptide
showing Cys in the pool of free amino acids
clarity and presentation of diagram
(Diagram should show that:
t-RNAs are amino acid specific such that one end of the t-RNA links up with
a specific amino acid and draws it to the m-RNA on the ribosome. The three
unpaired bases (the anticodon) at the other end link up with the appropriate
codon in the m-RNA molecule. In this way the amino acids are lined up in
an order corresponding to the sequence of codons in the m-RNA.)
(ii)
TAC GTA GAA ACG
2
(b)
Polysomes are the association of a group (5 to 50) of ribosome to a single m-RNA 2
molecule. This enables a large number of polypeptides to be assembled on a single
m-RNA strand in a comparatively short period.
(c)
(i)
'Universal’ means the same code codes for the same amino acid in all kinds 2
of organisms.
(ii)
'Degenerate’ means that a single amino acid may be coded by more than one 2
triplet of bases.
(iii) Nonsense triplets refers to those triplets which do not code for any known 2
amino acid. They sometimes serve as the 'stopping’ code.
[HKALE 98 II]
3. (a) Explain the features of the genetic code.
(6 marks)
(b) Describe in detail the cellular processes that are necessary in the transfer and decoding of genetic
information for polypeptide synthesis.
(12 marks)
(c) In general, what additional processes are necessary for the formation of the three-dimensional
structure of proteins after polypeptide synthesis ?
(2 marks)
3.
(a) Features of the genetic code
(2 marks on features, 4 marks max. on explanation)
-
-
-
-
triplet code (½)
½
a group of three nucleotides is necessary to encode for a specific amino
acid (1)
1
universal (½)
½
same triplet of nucleotides is responsible for coding the same amino acid
(½) in all organisms (½)
1
degenerate (½)
4 different nucleotides to form the triplet (½)64 available codes(½)
½
(i.e. 43 = 64, but there are only 20+ amino acids (½) some amino acids
have, more than 1 code (½)
2
non-overlapping (½)
½
a nucleotide will only be used once for each translation process / will not
be used twice in adjacent codes in one translation (1)
(accept diagrammatic explanation)
1
(max.6)
(b) *Transcription(½)in the nucleus (½)
1
Genetic information on DNA (½). Region of DNA unwinds (½), one strand
of the unwound portion acts as template (½), attracts free ribonucleotides
(½)to it through complementary base-pairing (½):
2½
4½
adjacent ribonucleotides join up to form m-RNA (½)
m-RNA leaves the nucleus and attaches to ribosome at rough endoplsmic
reticulum (1) (RER) in the cytoplasm (½)
*Translation (½) at RER
free amino acids in the cytoplasm is activated by each joining to a specific
t-RNA (1) using ATP.
Each t-RNA has a specific anticodon consists of three ribonucleotide bases
(1), these pair up in a complementary manner with the codon bases on the
m-RNA (1) bringing the amino acid with it (½). Adjacent amino acids join
½
1½
max.8
(½)
1
4
by peptide bond (½)to form polypetide. Ribosome moves along m-RNA
(½), amino acids add on one at a time (½).
(max.12)
(c) Coiling (½)and folding (½)of the polypeptide due to intramolecular bonds,
e.g. H-bond / disulphide bond (½)formed between different parts of the
polypeptide (½).
(2)
[HKALE 96 II]
6.
Compare and contrast the processes of mitosis and meiosis. Discuss the roles and
significance of mitosis and meiosis in the life of flowering plants and mammals, illustrating
your answer with examples where appropriate.
(20 marks)
Ans
6.
Similarity between the processes of mitosis and meiosis:
3
½
Both involve nuclear division (½)
Both involve duplication of DNA/chromosomes (1) and
movement of chromosomes along the spindle fibres. (1)
Both involve spindle formation (1)
Differences between mitosis and meiosis:
Mitosis
Meiosis
2 identical daughter nuclei
are formed each is
genetically similar to the
parent nucleus (1)
4 daughter nuclei are formed
which are genetically
different from the parent
nucleus and each other (1)
2
only 1 division of the (½)
nucleus, same
2 divisions of the nucleus
(½), chromosome number
2
chromosome number as
the parent nucleus (½)
reduced to half (½).
no pairing up of
homologues, linear
alignment at the equator
(½).
homologous chromosomes
pair up at the equator in the
first division (1).
1
½
chromosome splits into
chromatids which move to
no splitting into chromatids
in first division entire
2
opposite poles (1)
chromosome migrates to
opposite poles homologues
separate (1).