DNA organization

Learning objectives • Outline the properties and functions of Histone proteins

• Describe the packing of DNA into higher order structure – chromosomes

• Describe the principle of karyotyping and its interpretation

• List the parts of a Chromosome and outline the principles of banding techniques

• Differentiate Coding and non-coding regions of DNA

• Compare the functions of Introns and Exons

DNA of a human cell is 2.3 m (7.5 ft) in length if placed end to end while the nucleus is a few micrometers; packaging/folding of DNA is necessary

3 *10 9 nucleotide pairs divided into 23 chromosomes

DNA folding• 2 main groups of proteins involved in DNA folding:

– Histones = positively charged proteins filled with amino acids Lysine and Arginine.

– Nonhistones = less positive

• DNA + histones = nucleosomes

• DNA + histones + non-histone protein = chromatin

Histones • Histone proteins

• H2A• H2B• H3• H4

Two copies in each nucleosome ‘histone octomer’;

10nm fibril

Linker DNA

Histone (H1)

Chromosome packing Metaphase chromosome – shortest and thickest

Histone packing

Non histone protein packing

Naked DNA – thinnest and longest

Interphase chromosome – Transcriptionally active

Cell division – attachment of mitotic spindles from centromeres

Solenoid fibres

Lamp brush appearance

Beads on a string appearance

Karyotype: The representation of entire metaphase chromosomes in a cell, arranged in order of size and other characteristics

Examples of abnormal karyotype

Cont..

Ideogram:Diagrammatic representation of a karyotype:

Individual chromosomes are recognized based on:

1. Length of the arm: p –arm = short arm q –arm = long arm

2. Position of centromere :

3. Staining patterns :

Centromeric position and arm length

• Q (quinicrine) & G (Giemsa) banding - stain AT rich regions

• R (reverse banding) - stains GC-rich regions

• C-banding (denaturation & staining) - constitutive heterochromatin, found in the centromere regions and distal Yq

Chromosome banding

Higher order structure of chromosome

• Heterochromatin – tightly packed, inactive region of chromosome

1. Constitutive - Telomere, Centromere.2. Facultative - x chromosome (barr body)

• Euchromatin – loosely packed active region

HeterochromatinCentromere

• Primary constriction

• AT rich region

• Kinetochore – spindle attachment

Telomere

• End structure of chromosomes

• TTAGGG repeats

• Helps in replicating chromosome ends

• Helps to stabilize chromosome ends

Human genome• Codes for approximately 30,000 proteins – 2% of

DNA. • 98% is non-coding regions

Coding and non coding regions of the gene :

• Protein coding regions of DNA are interrupted by non coding sequences in eukaryotes.

• Coding sequences – Exons • Noncoding sequences – introns

Introns Introns

Gene after removal of introns

Non coding regions of the DNA • Does not yield any gene product.• They are repetitive sequences.

• Uses :1. Evolution (new mutations)2. Genetic rearrangement 3. Regulatory regions of the gene4. These sequences are specific for individuals

– Used for DNA fingerprinting, paternity disputes etc.

Types of non-coding regions• Satellite DNA : variable number tandem repeats

(VNTRs) (100kb – 1mb)

• Minisatellite DNA : Telomeres (0.1 -20kb)

• Microsatellite repeat sequence (<100bp)

• Transposons – jumping DNA (size of mini satellite)

a. SINEs – alu repeats (most abundant sequence)b. LINEs -

Types of transposition:• Conservative : piece of double stranded DNA

detaches from one chromosome to a different chromosome

• Retro transposition : produce m-RNA which later produces double stranded DNA

• Enzymes involved are :1. Transposase 2. Integrase 3. Reverse transcriptase – for retro transposition

Crossing over in Meiosis

•Mitosis •Meiosis

• Somatic cell reproduction, general growth & repair

• Sexual reproduction

• Daughter cells are genetically identical • Genetically different

• No crossing over or mixing of chromosomes (-ve)

• Crossing over occurs (+ve)

• Number of divisions = 1• Number of daughter cells = 2

• Number of divisions = 2• Number of daughter cells = 4

• Chromosome number = remains the same as the parent

• Chromosome number = reduced by half of the parent

• Karyokinesis = occurs in Interphase • Karyokinesis = occurs in Interphase I

• Cytokinesis = occurs in Telophase • Cytokinesis = occurs in Telophase I & II

• Centromeres split during Anaphase • Centromeres split during Anaphase II

Random combination of gametes

References

• Kaplan Biochemistry Notes• Online resources retrieved from:

waynesword.palomar.edu