Prokaryotes and Eukaryotes Flashcards

Key Concepts

• Prokaryotes: These are simple, single-celled organisms that lack a true nucleus and membrane-bound organelles. They are generally smaller than eukaryotic cells and include domains such as Bacteria and Archaea. Prokaryotes reproduce asexually through binary fission, and they can exhibit various metabolic pathways, allowing them to thrive in diverse and extreme environments.

• Eukaryotes: These organisms possess more complex cellular structures, characterized by the presence of a nucleus that houses their genomic DNA, and various membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Eukaryotes can be unicellular or multicellular and represent kingdoms such as animals, plants, fungi, and protists.

Genome Organization in Prokaryotes vs. Eukaryotes

Prokaryotic Genomes:

• Typically consist of a single, circular chromosome located in a region called the nucleoid. Unlike eukaryotic genomes, prokaryotic DNA is less complex and does not contain introns.

• Have plasmids, which are small, circular extra-chromosomal DNA elements that can carry genes advantageous for survival, such as antibiotic resistance. These plasmids can be transferred between bacteria via horizontal gene transfer, promoting genetic diversity.

Eukaryotic Genomes:

• Composed of multiple linear chromosomes, organized within a true nucleus. Each chromosome consists of a long strand of DNA wrapped around histone proteins, making up chromatin.

• The complex structure includes introns in protein-coding genes, which are non-coding regions that must be spliced out during RNA processing. Eukaryotic genomes exhibit varying ploidy levels, such as haploid, diploid, and polyploid, adding layers of complexity to genetic regulation.

Gene Clusters and Operons (Prokaryotes)

• Operons are a special feature of prokaryotic gene organization that facilitates the coordinated expression of related genes through a single promoter, resulting in polycistronic mRNA. This efficiency allows prokaryotes to quickly adapt to changing environments by simultaneously regulating the expression of multiple genes involved in a particular pathway.

Eukaryotic Genomes: Complexity

Eukaryotic genomes contain several types of sequences:

• Protein-coding Genes: These genes are responsible for encoding proteins, and their regulation can involve complex interactions between transcription factors and various cis-acting elements.

• Non-coding RNA Genes: Including rRNA and tRNA, which are crucial for protein synthesis, along with other regulatory RNAs like microRNAs.

• Origins of Replication: Sites in the genome where DNA replication begins, facilitating accurate and complete DNA duplication during cell division.

• Repetitive Sequences: Many eukaryotic genomes contain large amounts of repetitive DNA, which can play roles in genome architecture and regulation.

• Specialized Structures: Eukaryotic chromosomal features, such as centromeres (which play a key role during cell division) and telomeres (protecting chromosome ends), are critical for genomic stability.

Ploidy in Eukaryotes

• Ploidy refers to the number of chromosome sets in a cell:

  • Diploid: Two sets of chromosomes per cell (humans and many plants), one set inherited from each parent.

  • Polyploid: More than two sets of chromosomes, which is common in plants and some fungi, contributing to their genetic diversity and adaptability.

Packaging of DNA

• In eukaryotes, DNA is tightly packaged into chromatin, with nucleosomes (DNA wrapped around histone proteins) serving as the fundamental unit. This packaging is essential for fitting long DNA molecules into the nucleus and regulating gene expression.

Euchromatin vs. Heterochromatin

• Euchromatin: Represents transcriptionally active regions of DNA where genes are expressed and accessible for transcription machinery.

• Heterochromatin: Represents transcriptionally silent regions that are tightly packed, often serving structural roles or regulating gene expression indirectly.

From Genes to Proteins

Gene Structure:

The arrangement of genes within the genome plays a pivotal role in controlling gene expression and is vital for the proper functioning of cells. Key components of gene structure include:

• Promoter Regions: These are specific DNA sequences located upstream of a gene that serve as the binding site for RNA polymerase and general transcription factors. They are essential for initiating transcription and determine when and how much RNA is produced.

• Enhancers: Enhancers are regulatory DNA sequences that can be located far from the gene they regulate. They serve to increase the rate of transcription by interacting with transcription factors, which can facilitate the recruitment of RNA polymerase. Enhancers can function in a tissue-specific manner, ensuring that certain genes are expressed only in specific cell types or developmental stages.

• Silencers: Opposite to enhancers, silencers are elements that can repress gene expression. They bind repressor proteins which prevent transcription factors and RNA polymerase from accessing the promoter, thus decreasing the likelihood of transcription.

• Transcription Terminators: These sequences signal the end of transcription for a gene. Once RNA polymerase reaches a terminator, it halts transcription and releases the newly synthesized RNA molecule. Proper identification of terminators is crucial for ensuring that RNA is produced to correct lengths and is processed for function in protein synthesis.

Transcription:

The process of transcription depends on RNA polymerase binding to promoter regions and goes through several distinct stages:

• Initiation: RNA polymerase assembles at the promoter to start transcription.

• Elongation: The polymerase moves along the DNA template, synthesizing RNA.

• Termination: The transcription process ends when the polymerase reaches a termination signal, leading to the release of RNA.