BIO356Genome(2025)LMS

Topic 1: The Eukaryotic Genome

Overview

  • Understanding the composition and complexity of eukaryotic genomes in relation to prokaryotic genomes by comparing gene structure, sequence types, and mechanisms of evolution.

Objectives

  • After completing this topic, you should:

    1. Define the term ‘genome’.

    2. Describe patterns in genome size in eukaryotes.

    3. Discuss the concept of a gene.

    4. Describe main types of sequence found in eukaryotic genomes and their relative abundances.

    5. Discuss mechanisms of genome evolution.

Significance of the Genome

  • Acts as a blueprint for all life and biodiversity.

What is the Genome?

  • Definition: A complete set of genetic material in a cell, individual, or species.

    • In prokaryotes: Comprised of a singular circular chromosome without a nuclear envelope, mainly consisting of genes and promoters.

    • In eukaryotes: Most of the genome is housed in a membrane-bound nucleus (nDNA); small fractions exist in mitochondria (mtDNA) and chloroplasts (cpDNA).

Genome Size

Characteristic Genome Size

  • Each species has a characteristic genome size, measured as the amount of DNA in a haploid cell (C-value) as pg or base pairs.

  • **Genome Size Examples:

    • Arabidopsis thaliana:** 135 Mb

    • Drosophila melanogaster: 180 Mb

    • Chimpanzee: 2,733 Mb

    • Human: 3,223 Mb

    • Locust: 9,300 Mb

    • Polychaos dubium: 670,000 Mb.

Variability in Genome Size

  • Genome sizes of eukaryotes are usually larger and highly variable compared to prokaryotes, with significant size differences among species.

    • Example: Eukaryotes can have genome sizes ranging from the smallest to up to 100,000 times larger.

What is a Gene?

Traditional Definition

  • A gene is traditionally defined as a sequence of DNA occupying a specific position (locus) that codes for a protein or RNA product.

Modern Definition

  • A gene is now considered a DNA segment that specifies one or more sequence-related RNAs or proteins, where segments do not necessarily need to be physically contiguous.

  • The complexity of gene structure includes:

    • Regulatory elements can be distantly located.

    • Alternative splicing allows one gene to produce multiple protein products.

Gene Structure

  • **Key components:

    • Introns:** Transcribed into mRNA but spliced out before translation.

    • Exons: Sequences retained in mature mRNA that code for proteins.

    • Promoter: Region of DNA that initiates transcription.

Types of DNA Sequence in Eukaryotic Genomes

  1. Single Copy Genes: Most protein-coding genes that have unique sequences within the genome.

  2. Multiple Copy Genes: Structural genes like rRNA, tRNA, where multiple copies are necessary for function (e.g., ~50 - 70 repeats of rRNA genes).

  3. Mobile Genetic Elements (MGEs): Transposable elements within genomes that can move and affect the function of genes. Two main types include:

    • DNA Transposons (cut and paste method).

    • Retroelements (transpose via an RNA intermediate).

  4. Simple Sequence Repeats (SSRs): Short repeated sequences found in intergenic regions or within genes.

Genome Evolution Mechanisms

  1. Gene Duplication: Leads to the creation of additional gene copies, which can undergo different fates such as mutation, loss, or regulation variance, resulting in divergent functions and gene families.

  2. Chimeric Genes: Fusion of adjacent genes or exons from different genes leading to new functional genes.

  3. Horizontal Gene Transfer (HGT): Transfer of genetic material between organisms unrelated by descent, significantly noted in prokaryotes and sometimes in eukaryotes, influencing adaptability and innovation (e.g., carotenoid production in pea aphids).

  4. Whole Genome Duplication: Important for evolutionary change, especially in plants (polyploidy).

Conclusion

  • Eukaryotic genomes are complex and variable, comprising a mix of coding and non-coding sequences, with mechanisms of evolution such as gene duplication, chimeric genes, and HGT driving innovation and diversity in life forms.


**1. Most Abundant Sequence Type in the Human Genome:** The most abundant type of sequence in the human genome is b) introns. While exons (which code for proteins) make up only a small fraction of the genome, introns are non-coding regions that are present between exons and are more numerous overall. **2. Horizontal Gene Transfer:** Horizontal gene transfer refers to c) a gene is transferred from one organism to another other than by reproduction. This is a significant mechanism in prokaryotes and occasionally in eukaryotes, facilitating adaptations. **3. Retroelements:** Retroelements are described by d) require the enzyme reverse transcriptase for transposition. They replicate through an RNA intermediate, which is converted back into DNA by reverse transcriptase. **4. True or False - Retroelements Duplication:** True. Retroelements duplicate themselves via the enzyme reverse transcriptase, which catalyses the synthesis of DNA from mRNA. **5. True or False - Chimeric Gene:** True. A chimeric gene is a new hybrid gene that arises from gene fusion or exon shuffling, leading to new functional capabilities.