Genoma 1: definiciones

Introduction to Human Genome Structure

Overview of the Presentation

• José Tort introduces the topic of human genome structure, outlining a three-part structure for the discussion.

• The presentation will cover definitions and concepts related to genes and genomes, the structure of the human genome focusing on genes and gene families, and an exploration of repeated sequences.

Defining the Genome

• The genome is defined as the total DNA content within a cell; eukaryotic organisms possess two types: nuclear genomes organized into chromosomes and organelle genomes (e.g., mitochondria).

• Genomic studies encompass not only coding regions (genes) but also intergenic regions and repetitive sequences.

Historical Context of Genomics

Key Figures in Genomics

• Margaret Dayhoff is recognized for her pioneering work in protein sequence databases starting in 1965, earning her the title "mother of bioinformatics."

• Fred Sanger contributed significantly to DNA sequencing methods, winning two Nobel Prizes for his work on protein sequencing and DNA sequencing.

Technological Advances

• The development of automated tools has accelerated genomic analysis, enabling rapid completion of various organismal genomes.

Objectives and Insights into Genome Size

Goals of Genomic Research

• The primary objectives include obtaining complete gene sequences, comparing genomic structures across organisms, determining chromosomal locations of genes, and analyzing gene functions.

Genome Size vs. Complexity

• There is no direct correlation between an organism's genome size and its complexity; simpler organisms may have smaller genomes while more complex ones can have larger amounts of DNA.

• For example, humans have less DNA than some plants like onions or pines despite being more complex.

Gene Definition and Structure

Understanding Genes

• A gene is operationally defined as a nucleotide sequence that encodes information necessary for synthesizing a polypeptide or functional RNA.

Differences Between Prokaryotic and Eukaryotic Genes

• In prokaryotes, coding regions are contiguous; however, eukaryotic genes consist of exons (coding regions) separated by introns (non-coding regions).

Genomic Organization in Prokaryotes and Eukaryotes

Functional Genes and Operons

• Functional genes are physically linked in operons, which serve as regulatory units with a single regulatory region formed by adjacent structural genes transcribed into a polycistronic mRNA.

Differences Between Prokaryotic and Eukaryotic Gene Regulation

• In eukaryotes, genes are regulated independently, producing monocistronic mRNA. Coding sequences are interrupted by introns that must be removed during maturation, increasing genomic complexity.

Genomic Structure Variations

• The organization of genomes differs between prokaryotes and eukaryotes; prokaryotic genomes tend to be compact with minimal space between genes, while eukaryotic genomes can have larger amounts of dispersed DNA.

Complexity of Eukaryotic Genomes

• As organisms become more complex (e.g., plants), their genomes increase in size with significant non-coding DNA interspersed among coding regions, including introns within genes.

Example: Escherichia coli Genome

• The bacterial chromosome is a circular double-stranded molecule located in the nucleoid region. It contains approximately 4,600 genes represented as contiguous segments without gaps.

Eukaryotic Genomes: Structure and Characteristics

Nuclear vs. Mitochondrial Genomes

• Humans possess a large nuclear genome consisting of 46 linear chromosomes and a smaller mitochondrial genome (16,000 base pairs), which is circular due to its bacterial origin.

Chromosomal Variation Among Species

• Different species exhibit distinct chromosomal characteristics despite having similar chromosome counts; for example, humans have 23 pairs while some primates have variations due to evolutionary changes.

Genetic Density in Eukaryotes

• Eukaryotic genomes are characterized by low gene density; for instance, human chromosome one shows varying frequencies of gene distribution interspersed with repetitive elements across the genome.

Repetitive Elements Within Genomes

Understanding Genome Complexity

Mechanisms of Studying Genomic Complexity

• The study of DNA kinetic reassociation emerged before genome sequencing, focusing on the physical and chemical properties of DNA.

• Kinetic association studies involve breaking DNA into small fragments (around 300 base pairs), mechanically disrupting it, and denaturing it by increasing temperature to separate strands.

• As separated DNA strands are incubated at various temperatures, they seek complementary sequences, leading to a faster reassociation process as more bases find their matches.

Analyzing Reassociation Rates

• The rate of reassociation is influenced by the frequency of sequence repetition; highly repeated sequences have a higher probability of finding their complementary strand quickly compared to unique sequences.

• This method allows for graphical representation of reassociation rates, showing distinct fractions in eukaryotic genomes based on how quickly different types of DNA associate.

Classifying Eukaryotic Genomes

• In a sample from a calf's DNA, three major fractions were identified: highly repeated DNA (10-15%), moderately repeated DNA (15-40%), and unique copy DNA (50% or up to 75% in some species).

• Unique copy sequences include coding regions for proteins but also contain introns and non-coding regions that do not constitute genes.

Evolutionary Insights into Repeated Sequences

• Within unique sequences, there are portions related to coding genes; however, these coding regions represent only about 1.5% of the human genome.

• Repeated sequences can be classified into various categories such as LINEs, retroviral elements, transposons from DNA origins, and simple repeat elements.

Summary of Human Genome Composition