الحمد هلل رب العالميه الذي هداوا لهذا وما كىا لىهتدي لىال أن هداوا اهلل والصالة والسالم على أشزف األوبياء. 222Cell Biolgy 1

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1 الحمد هلل رب العالميه الذي هداوا لهذا وما كىا لىهتدي لىال أن هداوا اهلل والصالة والسالم على أشزف األوبياء 222Cell Biolgy 1

2 Lecture Cell Biolgy 2

3 DNA replication DNA replication is a semi-conservative process One strand serves as the template for the second strand. DNA replication is initiated at a region on a chromosome called an origin of replication An enzyme called DNA Helicase binds to the origin and unwinds the DNA in both directions from the origin. As the DNA is unwound, specific single stranded DNA binding proteins prevent the strands from reannealing RNA primers are produced by primase, which bind to the DNA. The DNA polymerase then binds and begins to synthesize the DNA complementary to the parental strand DNA replication has two requirements that must be met: 1. A DNA template. 2. A free 3' -OH group. 222Cell Biolgy 3

4 DNA Replication Types (i) Conservative mode This hypothesis states that during replication both the strands of the DNA remains with the parent cell and the newly synthesized two strands of DNA is retained by the daughter cell. However this hypothesis is not accepted anymore. (ii) Dispersive mode Replication would involve fragmentation of the parent DNA strands and intermixing with the newly synthesized strands. However this model is discarded since it not proved experimentally. (iii) Semi conservative mode Here the replication process of a parent DNA double helix results in two hybrid DNAs. Here each of the daughter DNA double helices retains a parent strand and a newly synthesized strand. The experiments of Mathew Messelson and Franklin W. Stall has proved the semi conservative mode of replication in the DNA. 222Cell Biolgy 4

5 Semiconservative replication In a cell, DNA replication begins at specific locations in the genome, called "origins". Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork In addition to DNA polymerase, the enzyme that synthesizes the new DNA by adding nucleotides matched to the template strand, a number of other proteins are associated with the fork and assist in the initiation and continuation of DNA synthesis. Because DNA polymerase can only synthesize a new DNA strand in a 5' to 3' manner, the process of replication goes differently for the two strands comprising the DNA double helix Nucleic acids can only be synthesized in vivo in a 5'- to 3'-direction, as the polymerase used to assemble new strands must attach a new nucleotide to the 3'-hydroxyl (-OH) group via a phosphodiester bond 222Cell Biolgy 5

6 The replication fork The leading strand is that strand of the DNA double helix that is oriented in a 5' to 3' manner. On the leading strand, a polymerase "reads" the DNA and adds nucleotides to it continuously. This polymerase is DNA polymerase III (DNA Pol III) in prokaryotes The lagging strand is that strand of the DNA double helix that is orientated in a 3' to 5' manner. Because of its orientation, opposite to the working orientation of DNA polymerase III which is in a 3' to 5' manner, replication of the lagging strand is more complicated than that of the leading strand On the lagging strand, primase "reads" the DNA and adds RNA to it in short, separated segments, forming Okazaki fragments DNA polymerase I "reads" the fragments, removes the RNA and replaces the RNA nucleotides with DNA nucleotides. DNA ligase joins the fragments together 222Cell Biolgy 6

7 222Cell Biolgy 7

8 Ribonucleic acid (RNA) Crick's central dogma. Ribonucleic acid (RNA) was discovered after DNA. DNA, with exceptions in chloroplasts and mitochondria, is restricted to the nucleus (in eukaryotes, the nucleoid region in prokaryotes). RNA occurs in the nucleus as well as in the cytoplasm (also remember that it occurs as part of the ribosomes that line the rough endoplasmic reticulum). Ribosomes are 2/3 RNA (a type of RNA known as ribosomal RNA or rrna) and 1/3 protein.rna was involved in protein synthesis. Information flow is from DNA to RNA via the process of transcription, and thence to protein via translation 222Cell Biolgy 8

9 The central dogma 222Cell Biolgy 9

10 Ribonucleic acid (RNA) Transcription is the making of an RNA molecule off a DNA template. Translation is the construction of an amino acid sequence (polypeptide) from an RNA molecule. RNA has ribose sugar instead of deoxyribose sugar. The base uracil (U) replaces thymine (T) in RNA. Most RNA is single stranded, although trna will form a "cloverleaf" structure due to complementary base pairing. 222Cell Biolgy 10

11 RNA types 1. messenger RNA (mrna) "Blueprint" for protein synthesis that is transcribed from one strand of the DNA (gene) and which is translated at the ribosome into a polypeptide sequence. ribosomal RNA One of the three types of RNA; rrna is a structural component in ribosomes 2. Ribosomal RNA (rrna) is the construction site where the protein is made 3. transfer RNAs (trnas) Small, single-stranded RNA molecules that bind to amino acids and deliver them to the proper codon on messenger RNA The trucks of protein synthesis that carry the specified amino acid to the ribosome. Abbreviated trna 222Cell Biolgy 11

12 Transcription Transcription The synthesis of RNA from a DNA template. The making of RNA from one strand of the DNA molecule RNA polymerase opens the part of the DNA to be transcribed. Only one strand of DNA (the template strand) is transcribed. RNA nucleotides are available in the region of the chromatin (this process only occurs during Interphase) and are linked together similar to the DNA process. During transcription, an enzyme that attaches to the promoter region of the DNA template, joins nucleotides to form the synthesized strand of RNA and detaches from the template when it reaches the terminator region. template strand The strand of DNA that is transcribed to make RNA 222Cell Biolgy 12

13 222Cell Biolgy 13

14 The Genetic Code: Translation of RNA code into protein The code consists of at least three bases. To code for the 20 essential amino acids a genetic code must consist of at least a 3-base set (triplet) of the 4 bases. If one considers the possibilities of arranging four things 3 at a time (4X4X4), we get 64 possible code words, or codons (a 3-base sequence on the mrna that codes for either a specific amino acid or a control word). The genetic code consists of 61 amino-acid coding codons and three termination codons, which stop the process of translation. The genetic code is thus redundant, for example, glycine coded for by GGU, GGC, GGA, and GGG codons. If a codon is mutated, say from GGU to CGU, is the same amino acid specified? 222Cell Biolgy 14

15 The Genetic Code 222Cell Biolgy 15

16 Protein Synthesis Prokaryotic gene regulation differs from eukaryotic regulation. Promoters are sequences of DNA that are the start signals for the transcription of mrna. Terminators are the stop signals. mrna molecules are long (500-10,000 nucleotides). Ribosomes are the organelle (in all cells) where proteins are synthesized. They consist of two-thirds rrna and one-third protein. Ribosomes consist of a small (in E. coli, 30S) and larger (50S) subunits.the length of rrna differs in each. The 30S unit has 16S rrna and 21 different proteins. The 50S subunit consists of 5S and 23S rrna and 34 different proteins. The smaller subunit has a binding site for the mrna. The larger subunit has two binding sites for trna. 222Cell Biolgy 16

17 Ribosome 222Cell Biolgy 17

18 Transfer RNA (trna) Transfer RNA (trna) is basically cloverleaf-shaped. trna carries the proper amino acid to the ribosome when the codons call for them. At the top of the large loop are three bases, the anticodon, which is the complement of the codon. There are 61 different trnas, each having a different binding site for the amino acid and a different anticodon. For the codon UUU, the complementary anticodon is AAA. Amino acid linkage to the proper trna is controlled by the aminoacyl-trna synthetases. Energy for binding the amino acid to trna comes from ATP conversion to adenosine monophosphate (AMP). 222Cell Biolgy 18

19 Transfer RNA (trna) 222Cell Biolgy 19

20 Translation Translation is the process of converting the mrna codon sequences into an amino acid sequence. The initiator codon (AUG) codes for the amino acid N-formylmethionine (f-met). No transcription occurs without the AUG codon. f-met is always the first amino acid in a polypeptide chain, although frequently it is removed after translation. The intitator trna/mrna/small ribosomal unit is called the initiation complex. The larger subunit attaches to the initiation complex. After the initiation phase the message gets longer during the elongation phase 222Cell Biolgy 20

21 222Cell Biolgy 21

22 Translation New trnas bring their amino acids to the open binding site on the ribosome/mrna complex, forming a peptide bond between the amino acids. The complex then shifts along the mrna to the next triplet, opening the A site. The new trna enters at the A site. When the codon in the A site is a termination codon, a releasing factor binds to the site, stopping translation and releasing the ribosomal complex and mrna. 222Cell Biolgy 22

23 Termination 222Cell Biolgy 23

24 Often many ribosomes will read the same message, a structure known as a polysome forms. In this way a cell may rapidly make many proteins. 222Cell Biolgy 24

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