Genetics Chapter 12&19 - DNA Organization in Chromosomes (Exam 3)

Bacterial Chromosome Structure:

• Consist of a Single Nucleic Acid Molecule

• Mostly Lack Associated Proteins

• Much Smaller Than Eukaryotic Chromosomes

• Contain Less Genetic Information, Sufficient for Bacterial Functions

Viral Chromosome Structure:

• Genetic Material Can be DNA or RNA, Single or Double Stranded

• Can be Circular or Linear Molecules

• Remain Inert Until Entering a Host Cell

• Efficiently Pack Long DNA Into a Very Small Volume

Steps Leading to DNA Supercoiling

1. Closed Circular DNA

2. Helicase Unwinds DNA

3. DNA Gyrase Introduces Negative Supercoils

4. Type I Topoisomerases Cut One Strand, Relax Twists, and Rejoin

5. DNA Becomes Compact and Supercoiled

DNA Organization into Chromatin in Eukaryotes Steps:

1. DNA Wraps Around Histone Proteins → Forms Nucleosomes

2. Nucleosomes Coil → Form 30nm Chromatin Fiber

3. Chromatin Fibers Fold and Loop

4. Cell Division: Chromatin Condenses → Visible Chromosomes

What are Histones?

A Family of Small, Positively Charged Proteins (H1, H2A, H2B, H3 & H4) That Bind to DNA (Because DNA is Negatively Charged)

What are Nucleosomes?

The Repeating Unit of Chromatin

How are Nucleosomes Formed?

By Wrapping ~145 bp of DNA Around a Histone Octamer Core and H1 Histone Holding the Cores Together

What is Linker DNA?

Short DNA Segments That Connect Nucleosomes

What is the Superhelix?

The Principle Packaging Unit of DNA in Eukaryotic Nucleus: Twists and Turns of DNA That Encircle Histones

What is Chromatin Remodeling?

The Remodeling of Chromatin to Accommodate DNA-Protein Interactions

To Allow Replication and Gene Expression, Chromatin Must:

1. Relax Compact Structure

2. Expose Regions of DNA to Regulatory Proteins

3. Have a Reversal Mechanism For Inactivity

What is Epigenetics?

How Cells Control Gene Activity Without Changing the DNA Sequence

What is Histone Acetylation?

Addition of an Acetyl Group to Histone Proteins. Leads to De-Condensation of the Nucleosome Leading to the Activation of Transcription

What is Histone Methylation?

Adding Methyl Groups to Histone Proteins. Can Activate Genes to Loosen Chromatin to Promote Transcription

What is the Enzyme for Histone Acetylation?

Histone Acetyltransferase (HAT)

What is the Enzyme for Histone Methylation?

Histone Methyltransferases (HMTs)

What is the Enzyme for Methylation of DNA?

DNA Methyltransferases (DNMTs)

What is the Enzyme for Histone Phosphorylation?

Kinases

What is the Methylation of DNA?

A Methyl Group is Added to DNA. Targets Promoters (CpG Islands) and Represses Transcription

What are CpG Islands?

Regions Rich in CpG Sites, Often in Promoters, Critical For Gene Regulation

What is Histone Phosphorylation?

Addition of a Phosphate Group to a Protein. Can Activate or Inhibit Protein Function.

What is Euchromatin?

Uncoiled and Active

What is Heterochromatin?

Condensed Areas That are Mostly Inactive

What are Repetitive DNA Sequences?

DNA Sequences Repeated Many Times Within Eukaryotic Chromosomes

What is Satellite DNA?

Highly Repetitive, Non-Coding DNA, Found in Heterochromatin

What is Moderately Repetitive DNA?

Repetitive Sequences Present in Multiple Copies, Less Clustered, Found in Euchromatin and Heterochromatin

What is the Function of Satellite DNA?

Chromosome Structure, Centromere and Telomere Stability

What is the Function of Moderately Repetitive DNA?

Functional Genes (rRNA, Histones) and Transposable Elements

What is a Simple Transcription Unit?

One mRNA → One Protein

What is a Complex Transcription Unit?

A Gene That Can Produce More Than One Type of mRNA

What are the Mechanisms of Complex Transcription Unit?

1. Alternative Splicing

2. Alternative poly(A) Sites

3. Alternative Promoters

What is Alternative Splicing?

A Post-Transcriptional Process in Eukaryotes That Splices Pre-mRNA to Produce Different Mature mRNA Transcripts. This Allows One Gene to Express Multiple Proteins.

What is Alternative Poly(A) Sites?

The Transcripts Contains More Than One Site For Adding the Poly(A) Tail. Choosing Different Sites Creates mRNA With Different 3’ Exons

What is Alternative Promoters?

Different Promoters Can Be Used in Different Cell Types. Promoter f Produces mRNA1 With Exon 1A, While Promoter g Produces mRNA2 With Exon 1B

What are Pseudogenes in Eukaryotes?

Nonfunctional Segments of DNA

What are Genetic Markers?

DNA Sequences With Known Genome Positions, Act Like Signposts for Tracking Inheritance and Studying Genes

What are VNTRs (Variable Number Tandem Repeats)?

A Type of Genetic Marker: Long Repeats (10-100bp), Highly Variable. Used in Paternity Testing, Forensic ID, Population Genetics.

What are STRs/Microsatellites?

A Type of Genetic Marker: Short Repeats (2-6bp). Used in Genetic Fingerprinting, Forensic Analysis, Studying Diversity.

What are Transposable Elements?

AKA Jumping Gene: A Segment of DNA That Can Move From One Location to Another Within a Genome

What are Retrotransposons?

A Type of Transposable Element: Copy & Paste, Move Via a RNA Intermediate. Uses Reverse Transcriptase to Convert RNA Back Into DNA

What are DNA Transposons?

A Type of Transposable Element: Cut and Paste, Move Directly as DNA. Use an Enzyme Called Transposase