Lecture 22: Gene Complexity and Gene Structure

How is biological complexity defined at the cellular and tissue level?

The result of the hierarchical organization of nested levels of cells, tissues, and higher-order interacting part types.

What is the primary concept behind the C-value paradox?

The total amount of DNA in an organism's haploid genome (C-value) does not correlate with its biological or organismal complexity.

How did Benjamin Lewin summarize the C-value paradox in 1983?

As an inability to account for the total content of the genome in terms of known function, due to excess non-essential DNA.

What does T. Ryan Gregory's "Onion Test" (2007) demonstrate regarding genome size?

An onion has roughly 5 times more DNA per haploid genome than a human, proving genome size does not track functional complexity.

What percentage of the human genome consists of protein-coding DNA sequences?

Approximately 2% to 5%

What is the estimated total number of protein-coding genes in the human genome?

Approximately 20,000 protein-coding genes

What is the G-value paradox?

The observation that the total number of protein-coding genes (G-value) does not correlate with organismal complexity.

Which simple organisms share a similar gene count (~20,000 genes) with humans, exemplifying the G-value paradox?

Watercress (Arabidopsis thaliana) and nematodes (Caenorhabditis elegans)

Which molecular mechanisms allow humans to achieve higher complexity than simpler organisms despite having similar gene counts?

Alternative splicing and expanded networks/functions of transcription factors.

What are the three main sequence classes of genomic DNA based on repeat frequency?

Highly Repetitive (HR) DNA, Moderately Repetitive (MR) DNA, and Single Copy (Unique) DNA.

What percentage of the human genome is made up of Highly Repetitive (HR) DNA?

Approximately 10%

Where is Highly Repetitive (HR) DNA primarily localized in human chromosomes?

In heterochromatic regions around centromeres and telomeres.

What is the typical motif size and tract length for Highly Repetitive (HR) DNA sequences?

Motifs of 5-100 bp occurring in long continuous tracts up to 100 Mb.

What copy number threshold defines Highly Repetitive (HR) DNA?

Greater than 10^6 copies per genome.

What monomer size and arrangement characterize alpha satellite DNA?

Tandem repeats of a 171 bp monomer unit arranged in higher-order repeat arrays.

What are the postulated physiological functions of Highly Repetitive (HR) DNA?

Providing structural integrity and organization for centromeres and telomeres.

What percentage of the human genome is composed of Moderately Repetitive (MR) DNA?

Approximately 30%

In which chromatin region is Moderately Repetitive (MR) DNA mainly located?

Scattered throughout euchromatin (and facultative heterochromatin).

What is the average sequence length and copy number range for Moderately Repetitive (MR) DNA?

Average length of 300 bp and copy number ranging from 10 to 10^5 copies per genome.

What cellular regulatory processes are influenced by Moderately Repetitive (MR) DNA elements?

Modulation of transcription factor binding, promoter spacing, cytosine methylation, alternative splicing, and mRNA stability.

Which high-demand functional gene families are categorized as Moderately Repetitive (MR) DNA?

Redundant genes encoding histones, ribosomal RNA (rRNA), and ribosomal proteins.

What are the two primary structural types of Moderately Repetitive (MR) DNA?

Microsatellite DNA (tandem repeats) and interspersed repetitive DNA (transposable elements).

What is the repeat unit size range of microsatellite DNA?

Repeating sequences of 2 to 30 bp of DNA.

Through what mutational mechanism do microsatellite sequences alter their repeat length?

Slippage recognition (replication slippage).

Why are microsatellites highly useful as genetic markers in forensic science and disease mapping?

They display high degree of sequence polymorphism (variable repeat counts) across individuals.

What percentage of the human genome consists of Single Copy (Unique) DNA sequences?

Approximately 1% to 5%

In which chromatin region is Single Copy (Unique) DNA located?

Throughout euchromatin.

How do heterochromatin and euchromatin differ in coiling and appearance after Giemsa staining?

Heterochromatin is tightly coiled and dark-staining, whereas euchromatin is uncoiled/less condensed and light-staining.

What proportion of genomic DNA sequence is identical across all human individuals?

Greater than 99.5%

What percentage of total human genes contributes to an individual's unique physical traits?

Less than 1%

How is a gene defined at the molecular functional level?

A DNA sequence that gives rise to a functional RNA molecule (a transcriptional unit).

What is the functional difference between exons and introns within a transcribed region?

Exons are coding sequences retained in mRNA that determine phenotype; introns are intervening sequences that generally do not code for phenotype.

What is the location and main function of the 5' Untranslated Region (5' UTR) of mRNA?

Located directly upstream from the initiation codon; regulates translation initiation and recruits ribosomal subunits.

What is the location and main function of the 3' Untranslated Region (3' UTR) of mRNA?

Located immediately after the termination codon; regulates mRNA localization, stability, nuclear export, and translation efficiency.

What is a gene promoter?

A DNA sequence onto which transcription machinery binds to initiate RNA synthesis.

What is the consensus sequence and genomic position of the core TATA Box element?

Consensus sequence 5'-TATAAA-3', located at approximately -25 relative to the transcription start site.

What level of transcription is controlled by the TATA Box?

It acts as an ON/OFF switch establishing the basal level of transcription.

What is the consensus sequence and position of the CAAT Box in eukaryotic promoters?

Consensus sequence 5'-GGCCAATCT-3', located 60 to 100 bases upstream (-80) from the initial transcription site.

Why is the CAAT Box required in inducible promoter regions?

It is required for inducible genes to be transcribed in sufficient functional quantities.

What is the regulatory function of the promoter GC Box element (-100)?

It binds protein activators to ramp up the transcription rate of a gene.

How does the RE1 regulatory sequence prevent ectopic gene expression?

It binds repressor proteins to silence neuronal gene expression in non-neuronal tissues.

What is the function of a transcription terminator?

A sequence of nucleic acids that marks the end of a gene or operon and signals transcription termination.

What is a solitary gene?

A single-copy gene represented once per haploid genome that forms the bulk of euchromatic coding regions.

What is a paralog gene?

An additional gene copy produced by gene duplication that can acquire new functional mutations.

During which cellular process do gene duplication errors most commonly occur?

Errors during meiotic recombination (Meiosis).

What defines a multigene family?

A set of similar genes formed by duplication of a single ancestral gene, possessing related biochemical functions and similar genomic locations.

How are members of a multigene family typically expressed across an organism's lifecycle?

They are synthesized or utilized at different developmental times or in different tissues.

What are pseudogenes?

Dysfunctional gene relatives that have lost protein-coding ability due to multiple accumulated mutations.

What structural configuration characterizes repeated genes?

Multiple copies of small genes clustered in high copy numbers, often arranged in a tandem head-to-tail orientation.

Which non-coding RNA molecules are encoded by classic repeated genes?

Transfer RNA (tRNA) and ribosomal RNA (rRNA).