1. Dr. Israa ayoub alwan Lec -6-
AL-Ma’moon University College Medical Laberatory techniques Department Molecular biology/ Second stage
Dr. Israa ayoub alwan
Lec -6-

2. Transcription

3. Transcription
Watson and Crick described the relationship between nucleic acids and proteins as a directional flow of information called the “central dogma”, “The specificity of a piece of nucleic acid is expressed solely by the sequence of its bases, and this sequence is a code for the amino acid sequence of a particular protein. ”
The process centers around RNA. RNA is the bridge between gene and protein. RNA and DNA share an intimate relationship,. The bases of an RNA sequence are complementary to those of one strand of the double helix, which is called the template strand. An enzyme, RNA polymerase, assists the construction of an RNA molecule. The other strand of the DNA double helix is called the coding strand.

4. The Central Dogma
DNA RNA protein
Transcription Translation
transcription = transfer of genetic information from DNA to RNA
translation = transfer of genetic information from RNA to functional protein molecules, in which the amino acid sequence is encoded in the structure of the DNA.
replication = transfer of genetic information from mother- cell to daugther-cells through synthesis of a two identical copies of DNA

6. RNA Structure and Types
RNA Structure and Types

7. RNA Structure and Types
RNA and DNA have similarities and differences . Both are nucleic acids, consisting of sequences of nitrogen-containing bases joined by sugar-phosphate backbones.
RNA is usually single-stranded, whereas DNA is double-stranded.
RNA has the pyrimidine base uracil where DNA has thymine. As their names imply,
RNA nucleotides include the sugar ribose, rather than DNA’s deoxyribose.

8. 4. Functionally, DNA stores genetic information, whereas RNA controls how that information is used.
5. The presence of the —OH at the 5 ′ position of ribose makes RNA much less stable than DNA, which is critical in its function as a short-lived carrier of genetic information.
6. As RNA is synthesized along DNA, it folds into a threedimensional shape, or conformation, that is determined by complementary base pairing within the same RNA molecule. For example, a sequence of AAUUUCC might hydrogen bond to a sequence of UUAAAGG—its complement—elsewhere in the same molecule, a little like touching elbows to knees. These shapes are very important for RNA’s functioning.
7. The three major types of RNA are messenger RNA, ribosomal RNA, and transfer RNA and Many of other classes of RNA control which genes are expressed (transcribed and translated)

9. Messenger RNA (m RNA)

10. Messenger RNA (mRNA) carries the information that specifies a particular protein. Each three mRNA bases in a row form a genetic code word, or codon, that specifies a certain amino acid. Because genes vary in length, so do mature mRNA molecules. Most mRNAs are 500 to 4,500 bases long. Differentiated cells can carry out specialized functions because they express certain subsets of genes—that is, they produce certain mRNA molecules, also called transcripts. The information in the transcripts is then used to manufacture the encoded proteins. A muscle cell, for example, has many mRNAs that specify the contractile proteins actin and myosin, whereas a skin cell contains many mRNAs that specify the scaly keratin proteins. To use the information in an mRNA sequence, a cell requires the two other major classes of RNA.

12. Ribosomal RNA (rRNA)

13. Ribosomal RNA (rRNA) molecules range from 100 to nearly 3,000 nucleotides long. Ribosomal RNAs associate with certain proteins to form a ribosome. a ribosome is an organelle made up of many different protein and RNA subunits. Overall, a ribosome functions as a machine to attach amino acids to form proteins . A ribosome has two subunits that are separate in the cytoplasm but join at the site of initiation of protein synthesis. The larger ribosomal subunit has three types of rRNA molecules, and the small subunit has one. Ribosomal RNA, however, is more than a structural support. Certain rRNAs catalyze the formation of the peptide bonds between amino acids. Such an RNA with enzymatic function is called a ribozyme. Other rRNAs help to align the ribosome and mRNA.

15. Transfer RNA (tRNA)

16. transfer RNA (tRNA), binds an mRNA codon at one end and a specific amino acid at the other. A tRNA molecule is only 75 to 80 nucleotides long. Some of its bases form weak chemical bonds with each other, folding the tRNA into loops in a characteristic cloverleaf shape . One loop of the tRNA has three bases in a row that form the anticodon, which
is complementary to an mRNA codon. The end of the tRNA opposite the anticodon strongly bonds to a specific amino acid. A tRNA with a particular anticodon sequence always carries the same amino acid. (Organisms have 20 types of amino acids.) For example, a tRNA with the anticodon sequence GAA always picks up the amino acid phenylalanine. Enzymes attach amino acids to tRNAs that bear the appropriate anticodons, where they form chemical bonds .

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