Introduction aux gènes et génomes

Introduction and Definitions

• Human genome contains 20-25,000 genes, a highly diverse genome.
• Y chromosome has a low gene density, with a complex genome due to alternative splicing.
• 2% of the genome codes for proteins.
• Other regions are transcribed but do not code for proteins (spliced); these make up 43% of the genome.
• 55% of the genome is non-coding.
• Pseudogenes are inactive genes due to mutations.
• Telomeres are located at chromosome ends. They are single-stranded nucleic acid fibers connected to the nuclear lamina, impacting cellular aging through shortening during cell division.
• 90% of non-coding sequences have unknown functions.
• MicroRNAs are regulatory genes responsible for cellular function.

Gene Structure

• A gene is a DNA sequence composed of regulatory regions and coding regions.
• Coding regions (exons) dictate amino acid sequences, while non-coding regions (introns) are excluded during protein synthesis.
• Genes dictate protein production.
• Proteins either form structures or regulate gene function.

Gene Expression Regulation in Eukaryotes

• Gene expression is primarily controlled at three levels: transcription, RNA processing, and translation.
• All cells in an organism contain the complete genetic information.
• Mammalian cells synthesize thousands of proteins from fewer genes.
• Transcription regulation involves specific transcription initiation factors at the RNA polymerase II site, and additional control elements some considerable distance from the transcribed region.

Genome Organisation Eukaryotes

• The human genome is representative of complex mammalian genomes.
• Repetitive sequences are crucial structural elements.
• The genome contains ~3 billion base pairs (in the haploid state).
• Only 1.5% of this is coding DNA (exons).
• Repetitive sequences dispersed throughout the genome include transposable elements.

Different types of transposable Elements

• Long Interspersed Nuclear Elements (LINEs): These are longer sequences, often coding for proteins to support their replication and insertion within the genome.
• Short Interspersed Nuclear Elements (SINEs): Shorter sequences that do not code for proteins and depend on LINEs for insertion.

Eukaryotic Genome Organisation (continued)

• Long Terminal Repeat (LTR) retrotransposons are similar in structure to retroviruses and are found across numerous genomes.
• Repetitive dispersed sequences contribute to genomic variation. However, they might also cause mutations if inserted within coding/regulatory regions.

Chloroplast Genome

• Chloroplasts are plant cell organelles crucial for photosynthesis.
• Their genome is circular and contains 120-160 kbp in length, coding for approximately 120 genes.
• Roughly half of these genes are involved in DNA metabolism.

Mitochondrial Genome

• Mitochondria contain circular DNA.
• The genome is small (16kbp) and compact, without introns.
• Mitochondrial genomes are maternally inherited.
• Active in the majority in oocytes.
• Contain 13 protein-coding genes, 22 tRNAs, and 2 rRNAs, concerning 4 major respiratory metabolic pathways in the cell.

Regulation of Gene Expression in Eukaryotes

• Gene expression is governed at the transcriptional level to control the timing and locations where proteins are produced.
• This level includes transcription, RNA processing (including RNA splicing), and translation.
• Cells utilize mechanisms for controlling how and when proteins are generated to ensure they carry out specific functions.

Transcription and Classification of Genes in Eukaryotes

• Classification can be based on the specific RNA polymerase involved or the number of copies in the genome. Types I, II, and III are specified by the RNA polymerase.
• Gene I transcribed by RNA polymerase I, genes II by RNA polymerase II, and genes III by RNA polymerase III.
• RNA Polymerase I transcribes ribosomal RNA genes. RNA Polymerase II transcribes messenger RNA genes and some small nuclear RNA genes. RNA polymerase III transcribes transfer RNA genes, 5S rRNA genes and other small RNA genes.

Transcription Regulation

• Genes use promoters and other regulatory sequences.
• Sequences may be near the initiation or further away on the DNA.
• Proteins bind to these regions altering gene expression to activate or inhibit.
• Gene expression can be modulated at several levels, including transcription initiation, RNA processing, and translation.