1. RNA (Ribonucleic acid)
Structure: Similar to that of DNA except:
1- it is single stranded polyunucleotide chain.
2- Sugar is ribose
3- Uracil is instead of thymine
There are 3 types of RNA:
1- Ribosomal RNA (rRNA)
2- Messenger RNA (mRNA)
3- Transfer RNA (tRNA)
RNA are copies from DNA sequences formed by a process called “ transcription”. After transcription some modifications occur to obtain the three types of RNA.

2. 1- Ribosomal RNA (rRNA): 80 % of total RNA in the cells are rRNA.
rRNA are found in combination with several proteins
(about 82 proteins) as component of the ribosome Which is the site of protein synthesis.
In Eucaryotic ( mammals),there are 4 size types of rRNA (5S, 5.8S, 18Ss and 28S)
representing 2/3 particle mass of the ribosome.
18S rRNA. One of these molecules, along with some 30 different protein molecules, is used to make the small subunit of the ribosome.
28S, 5.8S, and 5S rRNA. One each of these molecules, along with some 45 different proteins, are used to make the large subunit of the ribosome.
N.B In procaryotics ( bacteria),
there are 3 size species of rRNA.

3. 2- Messenger RNA (mRNA):
comprised only 5% of total cellular RNA.
Function: Carry genetic information from DNA in the nucleus to ribosomes (in cystol) where it is used as template for protein biosynthesis.

4. 3- Transfer RNA (tRNA): tRNA represents 15% of total RNA in the cell.
Structure:
1- amino acid attachment site or amino acid acceptor: at 3´ end which terminates with the triplet CCA.
2- Anticodon loop or anticodon triplet
Functions of tRNA:
1- transport amino acids to ribosome for protein synthesis. Each tRNA carry only one amino acid. The specific amino acid is attached enzymatically to 3' end of tRNA.
2- recognize the specified codon on mRNA to ensure the insertion of the correct amino acid in the growing polypeptide chain. This function is due to anticodon triplet which binds to codon on mRNA by base pairing.
NB: Three nucleotide bases on mRNA form a codon which is then translated into specific amino acid.

5. Structure of tRNA
Funtions of tRNA

8. Gene expression:
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins.
Gene expression in two major steps
1-Transcription: Synthesis of mRNA from DNA. By this process, the gene carried on DNA will transferred into RNA.
2-Translation: The gene on mRNA is translated into a functional protein
Transcription = DNA → RNA
Translation = RNA → protein

9. Transcription:
The enzyme responsible for transcription is RNA polymerase II
Steps in RNA synthesis: (Watch Movie)
Initiation:
The transcription is initiated by the binding of RNA polymerase to a specific region of DNA double helix. This site is called promoter site or promoter region. This region is recognized by sigma factor (subunit) of RNA polymerase.
When RNA polymerase recognizes this region, it binds to it leading to a local unwinding (separation) of the promoter region into 2 single strands:

10. a- DNA strand that is transcripted into mRNA and called template strand or antisense strand.
b- The other strand is coding strand or sense strand that contains gene to be translated (This strand not transcripted, not used)
Direction of transcription:
RNA polymerase will read the information sequence on DNA template from 3′ → 5′ direction, so RNA is synthesized antiparallel to DNA template i.e. from 5′ → 3′ direction.
2) RNA elongation:
Once RNA polymerase recognizes promoter region, it begins to synthesize a transcript (copy) of DNA template.
3) Termination: Process of elongation of RNA continues until reach what is called : termination region which is recognized by rho factor (subunit in RNA polymerase) resulting in release of the enzyme, and the synthesized RNA

12. Notes:
1- The synthesized RNA will have the sequence of the sense strand except for U instead of T.
2- In prokaryotic (bacteria) all types of RNA are synthesized by only one species of RNA polymerase.
3- In Eukaryotic ( mammaians), there are 3 classes of RNA polymerase
a- RNA polymerase I: synthesizes the precursor of rRNA named : pre rRNA
b- RNA polymerase II: synthesizes pre mRNA
c- RNA polymerase III: synthesizes pre tRNA
All these enzymes synthesize what is called primary transcript or immature RNAs (pre form) which by some modifications occur after transcription, will give the mature rRNA, mRNA and tRNA.

13. Post-transcriptional modifications of mRNA:
OR called: mRNA processing
After transcription, the formed immature mRNA will undergo the following modifications to
be mature and functioning:
1) 5′-capping:
5′- end in the first nucleotide
is blocked by 7-methyl
guanosine triphosphate
(7 methyl-GTP).
Role of cap:
a- help to stabilize and
Protect mRNA in cytoplasm (how)
b- Permit initiation of translation
(specifies, where translation
should begin).

15. 2) Poly A tail: chain of about 50-250 AMP is attached to 3′- end is added after transcription
Role of poly A: Help to stabilize mRNA and facilitate its exit from the nucleus. Also protect mRNA from degradation in cytoplasm when move from nucleus (cytoplasm contains 3´exonuclease that degrades mRNA).

16. mRNA ready for splicing (what is splicing? , see next slides)

17. 3- RNA splicing:
The gene carried now on primary mRNA contains two types of regions:
a- coding regions called (exons) which will be translated into a functional protein.
b- non coding regions (not code for amino acids). These regions called introns.
The gene or mRNA to be translated, it must contain only coding sequence ( i.e. exons), for this reason RNA splicing is essential to produce a correct protein by translation.
RNA splicing (or called gene splicing) is the process by which introns are removed and exons are joined (spliced) to produce a typical mRNA for translation. Splicing is catalyzed by enzyme called: spliceosome which is a group of nucleoproteins.
Splicing is a complex process and involve certain mechanism. Briefly, after a two-step enzymatic reaction, the intron is removed and two neighboring exons are joined together (movie for explanation only)

19. RNA splicing (I)

20. RNA splicing (II)

21. RNA splicing (III)