Gene Structure and Gene Expression Each chromosome contains

Gene Structure and Gene Expression

Each chromosome contains one DNA molecule, which consists of roughly 80 million bases. How these extremely long molecules fit in a nucleus? They are tightly associated with proteins to form nucleoprotein complex Nucleosome is the unit of nucleoprotein complex

Chromosomes contain DNA, which contains genes, which are vital for life Gene 2 Gene 5 Gene 3 Gene 1 Gene 4 Gene 7 Gene 6

Gene Expression A gene is a unit of inheritance. It is a defined segment of DNA on a chromosome that codes for a specific protein (or RNA), which performs biological functions. DNA m. RNA Central dogma Protein

Structure of a gene 5’ Upstream ORF Downstream 3’ 5’ m. RNA AUG……CCAGUCAUU……UAG Protein Met-Ile-- ---- Val-------- 3’

Genetic Code A set of three nucleotides constitute a genetic code AUG= Methionine UUU UUC AGA AGG Phenylalanine Arginine

ORF is also called coding region because it contains genetic codes Start Codon ATG Stop Codon TAA TAG TGA

Regulatory sequences 5’ Upstream ORF Regulator Downstream Regulator Where? Leaf, Seed, Root When? seedling, adult, senescing stage Day, night, cold, hot. How much? Abundant, rare, medium 3’

Upstream regulatory sequence 5’ Upstream Downstream ORF Promoter= facilitates gene expression. Gene expression level Enhancer= enhances or boosts gene expression. minus enhancer plus enhancer 3’

Downstream regulatory sequences 5’ Upstream ORF Downstream 3’ Transcription stop signal

Transcription Formation of m. RNA from the coding region or ORF is called transcription 5’ A T G T A C T T G C A C G T AT G 3’ T A C A T G A A C G T G C A T A C 5’ 3’ Sense 5’ Anti-sense A U G U A C U U G C A C G U A U G 3’ m. RNA

Translation is the process by which the information in the m. RNA is decoded into a polypeptide chain There are 3 steps in the process of translation: 1. Chain initiation 2. Chain elongation 3. Chain termination

Chain Initiation 1. First m. RNA binds to a ribosome. (Ribosomes are located in the cytoplasm, therefore translation takes place in cytoplasm and not in the nucleus). Nucleus Ribosome 2. The initiation codon of m. RNA binds with the respective t. RNA Met AUG

m. RNA Ribosome Met

Chain Elongation

Chain Termination When a stop codon such as UAG is encountered, the m. RNA and polypetide chain detach from ribosome and the process comes to end.

Gene Expression in a Nutshell First m. RNA is generated from the gene. m. RNA moves to the ribosome in cytoplasm. The first t. RNA loaded with methionine moves to the ribosome complex. The second t. RNA loaded with a new amino acid (determined by the nucleotide sequence of m. RNA) moves to the ribosome complex. The first amino acid (methionine) is joined to the second amino acid. t. RNAs leave. Third t. RNA loaded with a new amino acid moves in, and this new amino acid is joined with the elongating chain (peptide chain). The chain keeps elongating till the stop codon of the m. RNA is encountered. At this point the peptide chain leaves ribosome complex to become a functional protein.