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What is the approximate total length of DNA in a single human cell?
What is the approximate total length of DNA in a single human cell?
Why must DNA be contained within cells despite its large length?
Why must DNA be contained within cells despite its large length?
What is a possible challenge regarding the size of cellular DNA?
What is a possible challenge regarding the size of cellular DNA?
How does the diameter of a cell relate to the containment of DNA?
How does the diameter of a cell relate to the containment of DNA?
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What does the length of DNA in a human cell indicate about genomic organization?
What does the length of DNA in a human cell indicate about genomic organization?
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What is a key factor that limits the storage of DNA within human cells?
What is a key factor that limits the storage of DNA within human cells?
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Which of the following describes the length of DNA within a single human cell?
Which of the following describes the length of DNA within a single human cell?
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What challenge does the large length of cellular DNA present?
What challenge does the large length of cellular DNA present?
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Why is containing DNA within cells biologically important?
Why is containing DNA within cells biologically important?
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What must be considered when studying the length of DNA in human cells?
What must be considered when studying the length of DNA in human cells?
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What is a consequence of DNA's large length in relation to cell structure?
What is a consequence of DNA's large length in relation to cell structure?
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Which factor primarily determines how DNA fits within cells?
Which factor primarily determines how DNA fits within cells?
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How does the structure of human cells accommodate the length of DNA?
How does the structure of human cells accommodate the length of DNA?
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What challenge might arise from the length of DNA in human cells?
What challenge might arise from the length of DNA in human cells?
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Study Notes
Genes, Genomics & Chromosomes
- DNA in a single human cell is approximately 2 meters long.
- Cells must contain this DNA within a much smaller space, less than 10µm in diameter.
- Eukaryotic proteins associate with nuclear DNA.
- DNA and protein form complex structures, visualized as chromosomes during mitosis.
Overview: Genes & Chromosome Structure
- Chromosomes consist of a single, long DNA molecule.
- DNA is organized into increasing levels of condensation by histone and non-histone proteins.
- Smaller DNA molecules reside in mitochondria and chloroplasts.
- DNA sequences include single-copy genes, gene families, tandemly repeated genes, introns, and other DNA sequences.
Molecular Definition of a Gene
- A gene is the entire nucleic acid sequence needed to synthesize a functional gene product (protein or RNA).
- Genes are more than just protein-coding regions; some DNA sequences are transcribed into functional RNA (rRNA, tRNA).
- The definition of a gene also includes all DNA necessary to create a particular RNA transcript, such as enhancer sequences.
- Enhancers are regulatory sequences in eukaryotic DNA that may be distant from or within the gene they control, modulating transcription rates.
- Other critical non-coding regions of a gene include sequences that specify 3' cleavage & poly(A) sites and exon/intron splice sites.
Molecular Definition of a Gene (continued)
- Most genes are transcribed into mRNA that encodes proteins.
- Other DNA sequences are transcribed into RNA but do not encode proteins (tRNA, rRNA).
- Eukaryotic genes contain exons (coding regions) and introns (longer non-coding regions).
- A cistron is the DNA region that codes for a single polypeptide.
- Prokaryotic genes are often polycistronic, meaning a single mRNA can code for several proteins.
- Eukaryotic genes are monocistronic; one mRNA codes for one protein.
Comparing Prokaryotic & Eukaryotic Gene Organization
- Prokaryotic ribosome binding sites are near the start site of protein-coding regions (cistron) in mRNA.
- Prokaryotic translation initiation may start at internal sites within mRNA.
- Eukaryotic translation initiation starts at the closest AUG start codon.
- A 5' methyl cap directs ribosome binding, and translation begins at this point.
Operon
- Operons are a group of genes that work together.
- A promoter region is where RNA polymerase binds to transcribe genes.
- The operon contains several genes that are transcribed together into a single mRNA molecule.
The lac operon of E. Coli
- The lac operon in E. coli regulates lactose metabolism.
- The regulator gene controls the operon's activity.
- Genes in the operon include Z (B-galactosidase), Y (B-galactoside permease), and A (B-galactoside transacetylase).
Trp operon
- The trp operon regulates tryptophan synthesis in E. coli.
- Genes in the operon include trp E, D, C, B, and A.
- The promoter region is where RNA polymerase binds for transcription.
- The operator region is where the repressor binds.
- Transcription is regulated by the availability of tryptophan.
Transcription Units
- In eukaryotes, transcription units are the same as genes.
- Simple units produce a single mRNA that creates one protein.
- Complex units can produce multiple mRNAs (from alternative splicing) that come from a single gene and generate multiple proteins.
- Alternative splicing involves different combinations of introns/exons included in the final mRNA.
Simple Transcription Unit
- A region encoding a single protein, from the 5' cap site to the 3' poly(A) tail, and associated control regions (typically 50 kb).
- Mutations in the cap site or poly(A) site can disrupt transcription.
- Deletions and substitutions can result in abnormal protein with diminished activity.
Complex TS Units
- Primary transcripts can be processed into multiple mRNA variants through alternative splicing, creating different exons included in the mature mRNA.
- Different mRNA isoforms lead to different protein isoforms with different functions.
- Alternate promoters (active under different cell types) can lead to different 5' exons, while 3' exons are often common.
Chromosomal Organization of Genes and Noncoding DNA
- Most eukaryotic DNA doesn't code for RNA or have apparent regulatory functions.
- Approximately 1/3 of human genomic DNA is transcribed into premature mRNA.
- Most of this (~95%) is removed as introns.
- Only approximately 1.5% of total DNA encodes proteins or functional RNAs.
- Remaining (~98.5%) is noncoding DNA, of unknown function.
- Human introns vary in length, from a few dozen base pairs to over 10,000.
Known Nonprotein-Coding RNAs
- Table of known non-protein coding RNAs in the human genome and their functions.
Comparing Gene Density in Human & Yeast
- Higher eukaryotes have significant non-coding DNA, as opposed to yeast.
- The ẞ-globin gene cluster in humans has a lower density of protein-coding regions than protein-coding regions of the yeast genome.
- Differences in non-coding DNA likely occur because energy for synthesis is trivial in vertebrates whereas significant in micro-organisms.
Major Classes of Nuclear Eukaryotic DNA
- Table showing the lengths, copy numbers, and fractions of the human genome of different DNA classes (e.g., protein-coding genes, simple-sequence DNA, retrotransposons, processed pseudogenes).
Eukaryotic Protein Coding Genes
- Protein-coding genes can be solitary, found only once in the haploid genome.
- Approximately 25-50% of protein-coding genes are solitary.
Duplicated Genes
- Genes with close, but non-identical sequences are often located within a gene family, usually clustered close together (e.g., 5-50 kb).
- These genes encode closely related proteins (protein families), such as protein kinases and immunoglobulins.
Tandemly Repeated Genes
- Invertebrates and vertebrates, rRNAs and snRNAs are often encoded by multiple copies in tandem arrays.
- These copies are identical or nearly identical, allowing for high production of these essential molecules.
- Non-transcribed spacers may vary.
- The function is to meet high cellular demands for their transcripts.
Tandemly Repeated Genes (continued)
- Short tandem repeats (STRs) can be used for DNA typing.
- Short tandem repeats are highly variable between individuals and are used as genetic markers to identify individuals.
Repetitious DNA
- Eukaryotic cells also have multiple copies of DNA sequences not involved in coding proteins (repetitious DNA) beyond duplicated protein-coding genes and tandemly repeated genes.
- Two main types of repetitive DNA: simple-sequence and interspersed repeats.
Simple-Sequence DNA
- Simple-sequence DNA consists of short, repeated units.
- Satellite DNA, microsatellites, and minisatellites are different subtypes.
DNA Fingerprinting/Profiling
- Within a species, DNA nucleotide sequences of simple-sequence tandem repeats are generally conserved.
- Number of repeats (Variable Number of Tandem Repeats - VNTR) and total length of arrays vary widely between individuals.
- DNA fingerprinting relies on variations in the length of minisatellite simple-sequence DNA to differentiate individuals in paternity cases and criminal investigations.
Generating Different Lengths of Simple-Sequence DNA
- Unequal crossing-over (during meiosis) can create unusual lengths of tandem arrays during germ cell formation.
- This generates unique lengths of tandem arrays in each person.
Probing for Minisatellite DNA
- Differences in mini-satellite DNA lengths are detected by cutting DNA with restriction enzymes, separating the fragments, probing with labeled sequences, and visualizing the fragments on a gel.
Uses of DNA Fingerprinting
- DNA fingerprinting is used to determine paternity and in criminal identifications.
Interspersed Repeats
- Interspersed repeats (intermediate-repeat DNA) comprise 25-50% of mammalian DNA.
- They consist of transposable mobile DNA elements (e.g., DNA transposons, LTR retrotransposons, non-LTR retrotransposons like LINEs and SINES).
2 Classes of Mobile DNA Elements
- DNA transposons move via a DNA intermediate.
- Retrotransposons are transcribed into RNA, which is reverse-transcribed into DNA.
Eukaryotic Chromosome Structural Organization
- Chromosomes are packaged with histone proteins.
- Histones are small, basic proteins found in eukaryotic nuclei.
- Different types of histones include H1, H2A, H2B, H3, and H4.
- An octamer of histones (two copies of H2A, H2B, H3, and H4) forms a core around which DNA wraps, forming a nucleosome.
- Each nucleosome is associated with one H1 molecule.
- Chromatin packaging has multiple levels.
Extended & Condensed Forms of Extracted Chromatin
- Chromatin exists in extended (beads-on-a-string) form and condensed (30-nm fiber) form.
DNA Susceptibility to DNase Digestion
- Decondensed chromatin is more accessible to DNase I digestion than condensed chromatin.
Packing of chromatin & chromosome scaffold
- Shows the different levels and structures/organization of chromatin.
Histone-depleted Metaphase Chromosome
- Non-histone proteins are scaffold proteins important in the structure of chromosomes.
- The scaffold proteins are important in providing a physical structure for extended loops of DNA.
Types of repetitive sequences of DNA
- Includes highly repetitive DNA (satellite DNA) and moderately repetitive DNA (various types of repeats including dispersed retrotransposons).
- Example sequences/markers discussed include mini-satellites, micro-satellites, SINES, LINES, and others.
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Description
Explore the fascinating topic of DNA length within a single human cell and its implications for genomic organization. This quiz covers why DNA must be contained in cells, challenges related to its size, and how cell diameter affects DNA containment. Test your knowledge on the intricate relationship between cell structure and genetic material.
