DNA exist in 2 places in the cell The nucleus & the Mitochondria

Nucleus → Chromosome → Gene → DNA

The Watson-Crick Model of the Structure of DNA DNA consists of two chains of nucleotides in a ladder-like structure which is twisted called (Double Helix)

One Strand of DNA One strand of DNA is a polymer of nucleotides. One strand of DNA has many millions of nucleotides. 6 nucleotide

The Watson-Crick Model of the Structure of DNA A.The sides of the ladder are made up of alternating molecules of phosphate and deoxyribose (Backbone) B.The bases make up the rungs of the ladder C.The bases that make up the rungs of the ladder are attracted by a weak chemical bonds called hydrogen bonds

Nucleic acids are polymers of nucleotides The Nucleotide consists of : Nitrogenous bases Adenine A Thymine T Cytocine C Guanine G Uracile U Sugar Ribose Deoxyribose Phosphates

RNA onlyDNA only

BASE In RNA In DNA AdenineAdenosinedeoxyadenosine GuanineGuanosinedeoxyguanosine CytosineCytodinedeoxycytodine UracilUridine Not found Thymine deoxythymidine

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a polypeptide or for an RNA chain that has a function in the organism. Transcription of a gene produce mRN, which translated via ribosomes into functional protein Each gene composed of (Introns & Exons )  Introns : do not contain a functional code (noncoding for proteins). Exons : contain functional code (Coding for proteins)

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Transcription  performed by RNA polymerase enzyme and produce Premature mRNA Splicing  produce mature mRNA by removal of intronic sequences Translation  performed by Ribosome's and produce proteins

Genetic code : The ribosome read mRNA as a code of 3 nucleotides, Each 3 nucleotides called (Codon) that add one amino acid to the polypeptide chain (protein) tRNA Transfer one amino acid to the polypeptide chain for each codon

read the gene sequence from 5’  3’

The upper strand sequence : 5’-3’ : CGTACT The lower strand sequence : 5’-3’ : AGTACG

The sense strand has the same base sequence as the transcribed mRNA except that the base thymine is replaced by the base uracil. So the sense strand contain the gene code The anti-sense strand acts as the template for the transcription of mRNA so it is also called the Template strand The RNA nucleotides are added along the sugar phosphate backbone by RNA polymerase

Promoter : determine the start of the gene and so the start of transcription Terminator : determine the end of the gene and so the end of the transcription

The mRNA builds up with the RNA polymerase moving along the anti-sense strand joining the nucleotides. And so it make a copy of the sense strand

The RNA polymerase reaches the terminator and the RNA polymerase stops.

Pre-mRNA has been produced through transcription of the anti-sense strand as described for prokaryotic transcription. (a) The non coding introns are spliced out of the mRNA. (b) The introns are broken down in the nucleus. (c) The remaining mRNA is called mature mRNA and is exported from the nucleus to the cytoplasm for translation into the polypeptide in the ribosomes