Biomed 08

Nucleic acids DNA and RNA are the main types of nucleic acid. They are composed of repeated units called nucleotides.

A nucleotide

Each nucleotide is composed of: 1- A nitrogenous base, 2- A pentose sugar and 3- A phosphate group. The nitrogenous base may be: i- Pyrimidines: They comprise Cytosine (C),

i) Pyrimidine

Thymine (T) and Uracil (U). ii- Purines: They comprise Adenine (A) and Guanine (G).

ii) Purine

The nitrogenous bases of DNA & RNA

Both DNA and RNA contain adenine and guanine (purine bases) and cytosine (pyrimidine bases). Thymine is found in DNA while uracil is found in RNA. There are two major pentoses in nucleic acids: deoxyribose in DNA and ribose in RNA. The phosphate group is found in the nucleotide of both DNA and RNA. Nucleotides are linked together in both DNA and RNA via covalent bonds that found between phosphate group and pentose sugar. Nitrogenous bases (purine or pyrimidine) are joined by glycosidic bonds to pentose sugar of a repeating sugar-phosphate backbone. RNA is usually a single-stranded, whereas DNA is usually a doublestranded helix. In DNA, the nitrogenous bases of the two strands are connected together via hydrogen bonds. Adenine binds to thymine through two hydrogen bonds while cytosine binds to guanine by three hydrogen bonds. The sequence of a nucleic acid is usually read from 5' (the end that has the phosphate group) to 3' (the end which has not phosphate group). The two strands of DNA run in opposite directions i.e. 5' end of one strand is opposite 3' end of the other strand.

A single strand of DNA

A single strand of RNA

A double strand of DNA

3 Nucle otide s

DNA is found mainly in the nucleus. Very small amount is found in the mitochodria. RNA is formed in the nucleus and pass to the cytoplasm carrying the informations about the structure of protein which will synthesized in the ribosomes. There are different types of RNA; the most famous of them are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).

NOTES

Genes

DNA

Enzymes

Transcription

DNA sequence Triplet sequence in DNA

(TAC)

RNA

Metabolism

Translation

RNA sequence Codon in mRNA

(AUG)

Protein

amino acid sequence Amino acid in protein

(Met.)

Replication is the copying of DNA into DNA.

Transcription is

the copying of DNA sequence into RNA.

Translation is the copying of RNA sequence into protein.

Triplet sequence in DNA is the genetic word called codon. A trilet sequence of nucleotides (CAT) in a polynucleotide (i.e. 3 nucleotides) equal to 1 codon which again equal to 1 amino acid. The Size of human genome is ≈ 3,000,000,000 base pairs ≈ 500,000,000 possible codons (words or amino acids). Humans, mice and indeed all mammals have roughly the same number of nucleotides in their genomes (about 3 billion base pairs).

CYTOGENETICS Cell division Cell division in prokaryotes Prokaryotes such as bacteria use a relatively simple form of cell division called binary fission. Typically bacterial chromosomes consist of a single loop of DNA often called circular DNA but eukaryotes have a linear DNA molecule. When the prokaryote reaches to a level to be dividing, the circular chromosome attaches to the cell membrane at a certain point. Bacterial chromosome replicates leading to two identical chromosomes which are attached to separate points. The cell begins to divide giving two daughter cells which are identical to the parent cell. Bacteria can divide every 20 -30 minutes. This gives bacteria a remarkable power of multiplication where each cell gives 2.81 x 1014 bacteria after one day.

Cell division in eukaryotes There are two types of cell divisions which are mitosis and meiosis.

The cell cycle There are two main stages in the cell cycle:

I) Interphase: It is the part of the cell cycle when the cell is doing its normal job. Generally, there are one or more nucleoli in each cell which are the sites of ribosomal RNA synthesis. Interphase has three big phases which are:

1) G1 phase ◙ In this phase, the cell is doing its normal (everyday) job. ◙ ◙ At this time, chromosome (2n) are called unduplicated or unreplicated chromosomes. $ Usually, G1 is the longest period of the cell cycle. $ However, in some embryonic cells that are rapidly divided, G1 might only last a few minutes i.e. very short. $ Some cells, like nerve cells never leave G1 and this is sometimes called a G0 state (phase). $ G1 prepares the cell to undergo the next stage (S phase).

2) S phase ◙ All chromosomes are duplicated where DNA is replicated. ◙ ◙ New proteins are synthesized to assemble with new DNA forming new chromosomes. The time necessary to complete S phase varies between different life stages and between species. During S phase, the entire cell's DNA is duplicated resulting in 4 copies of each gene instead of the normal 2 in a diploid cell.

3) G2 phase ◙ Cell prepares itself for mitosis by synthesizing needed components. ◙ ◙ Some cells remain in interphase (G1 + S + G2) their whole life because they do not divide e.g. nerve cells and adult muscle cells. The result of cell cycle is the cell proliferation (division) while any uncontrolled proliferation leads to cancer.

Notes: ☼ Cells spend most of their time in this intermediate non-mitotic state (interphase). ☼ ☼ Interphase is not a part of mitosis but it is stage between two successive mitotic divisions.

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