Translation in protein synthesis.

It is a process of formation of polypeptide chain of amino acids during protein synthesis as a result of information and direction present on m RNA.

It involves following steps-

A) Activation of amino acids:-

  The Amino acids are present in Cytoplasm in inactivated form which are activated with the help of ATP (Source of energy) and enzyme amino acyl synthetase.The enzyme amino acyl synthetase links with A.A.AMP to give A.A.AMP. Enzyme complex called as activated amino acid.

               

B)  Attachment of activated amino acid to t – RNA:-

The activated amino acid attaches itself with specific t – RNA molecule at 3’ end (CCA end). It forms amino acid t – RNA complex.

There is separate t – RNA for each amino acid.

C)  Formation of polypeptide Chain:-

It involves following steps.

I) Chain Initiation: a) the synthesis of polypeptide chain starts with the formation of initiation complex.

b) Its formation requires presence of initiation factors (IF) and GTP molecules.

c) The released mRNA binds itself to the site of protein synthesis i.e. r – RNA.

d) m-RNA attaches with the smaller subunit through 5’ end.

e) Smaller subunit attaches itself with larger subunit. Thus, initiation
        complex is formed.

II)     Chain elongation:

a) The addition of amino acids in a sequence to form a polypeptide chain is called as chain elongation.

b) The specific t – RNA molecule brings specific amino acids to the site of protein synthesis which link together with the help of peptide linkage (CONH bond)

c) The ribosomal unit moves along m– RNA strand. This shifting is called as translocation and it does not consume energy.

d) The process is continued till the last amino acid is added.

III) Termination & Release of Polypeptide Chain:-

a) The elongation of chain is terminated at right stage and the chain is released with the help of various releasing factors. (RF)

b) The process of termination takes place near 3 end of m – RNA strand.

c) It is brought about by any one of the three termination codons namely, UAA, UAG and UGA.

d) After termination of polypeptide chain no amino acid is added, process completely stops and the two ribosomal subunits detach.

Thus, polypeptide chain is synthesized.

 

PROCESS OF TRANSCRIPTION:-

The process of transcription involves following steps:-

A) Activation of Ribonucleotides.

The ribonucleotides occur freely in the nucleoplasm. Before transcription starts, the nucleotides are activated via phosphorylation for which the enzyme phosphorylase and energy are required. The activated ribonucleotides are adenosin

e triphosphate (ATP), guanosine triphosphate (GTP), uridine triphosphate (UTP) and cytidine triphosphate (CTP).
B) DNA Template.

On specific signals, segments of DNA corresponding to one or more cistrons become derepressed and ready to transcribe.

Enzymes required for chain separation are unwindases and helix destabilizing proteins or gyrases. Terminator region has either poly a base sequence or palindromic sequence.
C) Base Pairing.

Ribonucleoside triphosphates present in the surrounding medium come to lie opposite the nitrogen bases of the DNA template strand. They form complementary pairs and then the two extra phosphates present in the ribonucleoside triphosphates (ribonucleotide diphosphates) separate with the help of pyrophosphatase. Energy is released in the process.
D) Chain Formation. With the help of RNA polymerase the adjacent ribonucleotides held over DNA template join to form RNA chain. Once the RNA chain is initiated the sigma (s) factor separates. RNA polymerase core enzyme now moves along the DNA template elongating the chain at the rate of some 30 nucleotides per second. There is no proof reading during transcription. RNA synthesis stops as soon as polymerase reaches the terminator region. Rho factor (p) is required for this. Terminator region has a stop signal. It also possesses 4 - 8 A-nucleotides.
E) Separation of RNA.

Termination or rho factor has ATP-ase activity. It helps in the release of the completed RNA chain which is called a primary transcript. It undergoes processing to form functional RNAs. In many prokaryotes, the structural genes of related functions are grouped together in operons. An operon is transcribed as a single unit resulting into a polycistronic mRNA. In eukaryotes, transcription unit is monocistronic.
F) Duplex Formation.

After the release of primary transcript, the two strands of DNA again establish linkages between the complementary base pairs. The enzymes Gyrases, unwindases and HD proteins are released. Finally the double helical form of DNA is resumed.
G) Post-Transcription Processing.

Primary transcript is larger than the functional RNAs and is called heterogeneous or hnRNA (especially in case of mRNA). Primary transcript is converted into functional RNAs by these post transcription processes. They are of four types:
(i) Cleavage

Larger RNA precursors are cleaved to form smaller RNAs. Primary transcript of rRNA in eukaryotes is 45S. Primary transcript forms 5-7 tRNA precursors on being cleaved by ribonuclease-P (an RNA enzyme).

(ii) Splicing.

Eukaryotic transcripts possess extra segments (introns or intervening sequences). snRNAs along with some protein molecules function as 'enzymes for splicing. They are called SnRNPs or small nuclear ribonucleoproteins . SnRNPs get attached to 5' and 3' ends on introns. With the help of some more proteins, a complex called spliceosome develops. It requires energy from ATP Spliceosome removes the intron and joins the exons to produce mature mRNA. A ligase may be required for this. Ribozyme (an-RNA enzyme) is a self-splicing intron involved in some of these reactions as well as catalyzing polymerization. Self-splicing is usually seen in some primary r-RNA transcripts. The split-gene (genes having introns) arrangements represent probably an ancient feature of the genome. The presence of introns is reminiscent of antiquity, and the process of splicing represents the dominance of RNA-world.
(iii) Terminal Additions.

Some extra nucleotides are added to the ends of RNAs and they serve specific functions, E.g., CCA segment in tRNA, cap nucleotides at 5' end of mRNA or poly-A segments at 3' end of mRNA. Addition of cap nucleotides at 5' end is called capping. mRNA cap is formed from GTP & is called 7 methylguanosine (7MG). It is required for ribosomal recognition.
(iv) Nucleotide Modifications.

Certain nucleotides are methylated, ethylated, deaminated etc. to produce different chemicals. These are most common in tRNA- methylation (E.g., methyl cytosine, methyl guanosine), deamination (E.g., inosine from adenine), dihydrouracil, pseudouracil, etc.

In bacteria, since the mRNA does not require any processing to become active, and also since transcription and translation takes place in the same compartment (there is no separation of cytosol and nucleus in bacteria), many times the translation can begin much before the mRNA is fully transcribed. Consequently, the transcription and translation can be coupled in bacteria.

In vitro synthesis of RNA was first performed by Ochoa (1967). He discovered polynucleotide phosphorylase which could polymerize ribonucleotides to produce RNA without any template.

 

Transcription in protein synthesis: -

TRANSCRIPTION UNIT

1) The synthesis of MRNA chain over DNA is called as transcription.
2) Coded information for protein synthesis is obtained from
DNA called as cryptogram.
3) DNA dependent RNA polymerase enzyme plays the key sole in transcription. It is able to carry out all three steps involved which are initiation, elongation and termination.
4)  RNA polymerase associates transiently with initiation factor and termination factor for initiation and termination of the transcription process.
5) Eukaryotes have three different RMA polymerases while in Prokaryotes single RNA polymerases.
6) The segment of DNA which participates in transcription is the transcription unit. It is primarily consisting of three regions in the DNA.

a) Promoter
b) structural gene
c) terminator
Promoter - Sequence of DMA needed for RNA polymerase to bind to the template and accomplish the initiation reaction defines promoter.
The sequence of DNA required so that no further bases are added to the RNA chains and complex of RNAT and DNA comes apart is called as terminator. Only one strand is copied in transcription.

DNA Template. 

On specific signals, segments of DNA corresponding to one or more cistrons become derepressed and ready to transcribe. Each segment has a promoter region and a terminator region. A promoter region has RNA polymerase recognition site and RNA polymerase binding site. Chain opening occurs in a specific region occupied by 6bp i.e. ., TATAAG (Pribnow box) in most prokaryotes.

There is convention in defining two strands of the DNA in the structural gene of transcription unit. Since the two strands have opposite polarity and the DNA dependent RNA polymerase also catalyze the polymerization in only one direction that is 5’ to 3’ the strand that has the polarity 3’ to 5’ acts as a template and referred as template strand. The other strand which has polarity 5’ to 3’ which does not code for anything is called as coding strand.

The promoter and terminator flank the structural gene in a transcription unit. The promoter is said to be located towards 5' end (upstream) of the structural gene (the reference is made with respect to the polarity of coding strand). It is a DNA sequence that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that also defines the template and coding strands. The terminator is located towards 3'-end (downstream) of the coding strand. 

Dear students, 

Very soon  I am starting paper sessions on blog. The paper will be MCQ type. You can attempt paper for practice if you wish.

The paper will be based on 12 class syllabus. 

Monday, wednesday and Friday  chapter wise tests and on Sunday test will include questions based on chapters covered in the same week.

For revision you can refer to videos posted earlier with notes on this blog.

Take this advantage. You will be benefited a lot.

Amey Prakash Edlabadkar

 

TRY THESE QUESTIONS

CHAPTER 1. REPRODUCTION IN LOWER AND HIGHER PLANTS

QN. 1) ATTEMPT ANY THREE OF THE FOLLOWING :-( 2 MARKS EACH)

A) Describe the structure of embryo sac in angiosperms with the help of labelled diagram.

B) Differentiate between wind pollination and insect pollination.

C) Describe the internal structure of anther with the help of labelled diagram.

D) Give the significance of seed and fruit formation.

E) Explain any two outbreeding devices for cross pollination.

F) Draw a well labelled diagram of male gametophyte of flowering plants.

QN. 2) ATTEMPT ANY TWO OF THE FOLLOWING :-( 3 MARKS EACH)

A) Describe the development of female gametophyte in angiosperms with the help of labelled diagram.

B) Define double fertilization. Explain the process and state its importance.

C) Explain the development of dicot embryo in Angiosperms.

D) What is apomixis? Explain different types of apomixis with suitable examples.

 

 

CHAPTER 2. REPRODUCTION IN LOWER AND HIGHER ANIMALS

QN. 1) ATTEMPT ANY THREE OF THE FOLLOWING :-( 2 MARKS EACH)

A) Describe the structure of Graafian follicle with the help of diagram.

B) Write a note on uterus and also mention its functions...

C) Explain the process of spermatogenesis in humans with the help of labelled diagrams.

D) Give the significance of fertilization.

E) Explain various stages of parturition.

F) Write a note on ZIFT and GIFT.

QN. 2) ATTEMPT ANY TWO OF THE FOLLOWING :-( 3 MARKS EACH)

A) Describe internal structure of human testis with the help of diagram (T.S. is expected).

B) Write an account of blastulation and implantation. (Diagram not expected)

C) Explain the mechanical means of birth control.

D) Explain various phases of menstrual cycle.

E) Give an account of different accessary sex glands associated with human male reproductive system.

 

CHAPTER 3. INHERITANCE AND VARIATION

QN. 1) ATTEMPT ANY THREE OF THE FOLLOWING :-( 2 MARKS EACH)

A) Give different reasons for Mendel’s success.

B) Explain monohybrid cross and ratio with suitable example.

C) Define the terms: - 1) Trait    2) Genotype

D) Describe the structure of chromosome with the help of diagram.

E) Write an account of Down’s syndrome.

F) Write a note Autosomal inheritance.

QN. 2) ATTEMPT ANY TWO OF THE FOLLOWING :-( 3 MARKS EACH)

A) Write an account of various types of linkage.

B) Explain codominance with suitable example.

C) Explain inheritance of colorblindness.

D) Explain dihybrid cross and its ratio with suitable example.

E) Describe the mechanism of sex determination in bird and honey bees.

 

CHAPTER 4. MOLECULAR BASIS OF INHERITANCE

QN. 1) ATTEMPT ANY THREE OF THE FOLLOWING :-( 2 MARKS EACH)

A) Explain the packaging of DNA in eukaryotes.

B) Write a note on experiment that gave unequivocal proof that DNA is the genetic material.

C) Explain any four properties of genetic code.

D) What are the different objectives of Human Genome Project?

E) Name the three enzymes and their roles involved in digestion of lactose.

F) Explain the structure of transcription unit with the help of diagram.

QN. 2) ATTEMPT ANY TWO OF THE FOLLOWING :-( 3 MARKS EACH)

A) Write a note on structure of DNA.

B) Explain the process DNA replication.

C) Explain the process of translation in protein synthesis.

D) Explain the process of DNA fingerprinting in a stepwise manner.

E) Write a note on lac operon concept.

 

CHAPTER 5. ORIGIN AND EVOLUTION OF LIFE

QN. 1) ATTEMPT ANY THREE OF THE FOLLOWING :-( 2 MARKS EACH)

A) Give main features of mutation theory.

B) Explain various types of chromosomal aberrations.

C) Give an account of reproductive isolation.

D) What are the different types of fossils? Give their importance in evolution.

E) Write an account of skeletal features of Neanderthal man.

F) Write a note analogous and homologous organs.

QN. 2) ATTEMPT ANY TWO OF THE FOLLOWING :-( 3 MARKS EACH)

A) Write an account of Urey and Miller’s experiment.

B) Explain in brief different points of chemical evolution of life.

C) Give an account of RNA world hypothesis.

D) Explain Darwinism in detail.

E) Explain different types of Natural selection with proper diagrams.

 

1.    Transcription (DNA to mRNA) :-
Transcription is the first step segment in protein of synthesis one of the two strands takes of place DNA in into nucleus.

2.   It is the process of copying genetic information of a catalyzed by RNA polymerase.
During the process, a part of DNA (a specific gene) uncoils and unzipped by breaking the hydrogen bonds between the two strands. However, only one strand which is in 3' to 5' direction, acts as a template strand or sense strand to synthesize mRNA.

3.   The other strand is called antisense strand.

4.   The nitrogen bases of template strand attract complementary nitrogen bases. A attracts U, T attracts A, G attracts C and C attracts G.

5.   After the formation of ribose sugar phosphate backbone synthesis of mRNA is completed in 5' to 3' direction.

6.   In eukaryotes, the process is catalyzed by the enzyme RNA polymerase II and the mRNA synthesized is called precursor mRNA (pre-mRNA).

7.   It requires processing to become mature functional mRNA molecule. During processing, the capping enzyme adds a guanine nucleotide, most commonly 7-methyl guanosine to the 5' end.

8.   The cap is essential for the ribosome to bind to 5' end of mRNA.

9.   At the 3' end a sequence of about 50 to 250 adenine nucleotides is added. It is called poly (A) tail. It is essential to determine stability of mRNA.

10.                Initially coding sequence (exons) is not continuous.

11. It is interrupted by non-coding sequence (introns).

12.                The introns are removed and the exons are joined so that continuous coding sequence is available. This process is called mRNA splicing.

13.                In prokaryotes, only one type of RNA polymerase synthesizes mRNA and it is capable of functioning directly as mature mRNA, hence, its processing is not required.
Thus, the genetic information present on DNA (on specific gene) to synthesize a specific protein is transcribed on to mRNA which then leaves the DNA, passes through nuclear pore and enters the cytoplasm. The two strands of DNA recoil. If mRNA molecules are produced in excess, they are stored in nucleolus.
The mRNA acts as a messenger as it carries transcribed genetic information or "message" from DNA to This genetic information is nothing but copy of a specific sequence of nitrogen bases of a gene. Is always carried in triplet nitrogen bases called codons each of which codes for a specific amino acid. However, for any mRNA first or initiation codon is always AUG which always codes for methionine in and formyl methionine in prokaryotes. The last codon does not code for any amino acid, hence it is called stop codon. It is one of the three codons UAA, UAG or UGA.

3) TRANSCRIPTION IN PROKARYOTES

4) TRANSCRIPTION IN EUKARYOTES

 

 

RNA (RIBONUCLEIC ACID)

A)  RNA is type of nucleic acid containing ribose sugar.

B) It is present in all organisms except some viruses. (DNA viruses).

C) It is found in cytoplasm as well as nucleus.

D)  Chemical composition of RNA:-

It is composed of: -

1) Sugar: The sugar of RNA is ribose (C5H10O5).

2) Phosphoric acid: It is an inorganic molecule (H3PO4).

3)    N – Base: These are purines and pyrimidines. Purines are Adenine and Guanines whereas pyrimidines are cytosine and uracil. 

E) Molecular Structure of RNA:-

1) Single stranded Nature.

1) RNA is single stranded molecule consisting of single polynucleotide chain.

2) Polynucleotide chain: It is formed by linking of many smaller subunits called as ribonucleotides (functional and structural unit)

3) Nucleotide: Nucleotide of RNA is called as ribonucleotide.

These are of four different types namely:-

Adenine, guanine, Cytosine and uracil.

4) Formation of Poly nucleotide chain:-

Ribonucleotides are linked together with the help of Phosphodiester linkages to form a polynucleotide Chain.

5) Heteropolymer: RNA shows repetitive sequence of nucleotide such as A, G, C, U hence, it is a heteropolymer. It has high molecular weight.

6) Base pairing: RNA is a single stranded molecule which may form loops. Usually base pairing is absent, if present, found in coiled regions only.

The base pairing in loop follows Watson & Crick pairing.

 7) Purine – Pyrimidine Ratio: Purine pyrimidine ratio never equals to one.

8) Polarity: RNA is polarized molecule with definite direction 5’- 3’.

F) Functions of RNA.

1) Different types of RNAs play major role in protein synthesis.

2) RNA through formation of RNA primer participates in DNA replication.

3) RNA acts as genetic material in certain viruses.

Types of RNA (Additional)

Basically there are 2 types of RNA

A) Genetic RNA.

1) RNA which forms genetic material is called as genetic RNA.

2) It carriers and stores genetic information.

3) It is found in some viruses only. Ex: - TMV – Tobacco mosaic   virus 

B) Non Genetic RNA:

1) It does not carry any genetic information.

2) It is present in all organisms except some viruses.

3) It actively participates in protein synthesis and also carries out certain vital functions.

4) Non-genetic RNA is off three types namely, mRNA, r RNA, and t RNA.

m RNA

1) It is present in nucleus as well as cytoplasm.

2) It is longest of all RNA types and short-lived. (Ephemeral).

3) It forms (3-5) % of total RNA contents of cell.

4) It is known as messenger RNA.

5) The number, nature and sequence of nucleotides present in mRNA strand is decided b4y the nature of protein molecule to be synthesized and is complimentary to DNA strand from which it is synthesized. (Cistrons or sense strand).

6) It shows presence of initiation codon i.e. AUG at 5´ end and at 3´ end it shows termination codon which is either UAA, UAG UGA.

7) It is linear, unfolded, single stranded structure without base pairing.

8) It is synthesized from DNA from the process known as transcription.

(The information present on mRNA is termed as cryptogram.)

7) Functions:

1) It carries messages for protein molecule to be synthesized from DNA to site of protein syntheses i.e. r RNA.

rRNA.

1) It is present in ribosomes as their structural component (in cytoplasm.)

2) It forms 75 – 85 % of total RNA content of cell.

3) It is single stranded and may show coiling or folding to form loops.

4) The loops show base pairing.

5) It is most stable type of RNA

6) It is synthesized from DNA.

7) Functions:

a) Provides site for protein synthesis.

b) Helps to arrange m-RNA properly.

c) Provides binding site for m-RNA strand.

t– RNA or s RNA

1) It is smallest type of RNA

2) It is present in cytoplasm in soluble form, hence the name s – RNA,

3) It is relatively stable and forms 12 – 18 % of total RNA contents of cell.

4) Two structural models were proposed to explain t – RNA Structure.

  a)   Hair pin Model, b) Clover leaf Model.

5) According to clover leaf model t – RNA shows presence of three major arms, one mini arm or variable arm, two free ends such as 3´ and 5´ and loop at the end of each major arm.

6) The nitrogen base in arm shows base pairing while that of loop remains unpaired.

7) The central loop shows presence of sequence of three consecutive nucleotides known as anticodon or NODOC.

8) At 3’ end it shows presence of CCA sequence and that at 5’ end a single unpaired guanine is present.

9) The important feature of t – RNA is that it shows presence of unusual nitrogen bases like hydroxyuracil and methylated purines.

10)  Functions:

a) It identifies codon on m – RNA strand.

b) It brings about transfer of amino acid from cellular pool to site of protein synthesis i.e. r – RNA