Submicroscopic and Microscopic Structure of Chromosomes

Describe the genes in prokaryotes

- Uninterrupted {without introns} and are strictly complementary to RNA

- Are not amplified{no more than 1 copy}

- Have only coding regions

Explain highly repetitive DNA sequence in eukaryotes

- Up to 1 million copies {10-15% of genomes}

- Localised in the heterochromatin and around centromeres and telomeres

How much DNA in the human genome is used for protein synthesis?

2-3% is used for protein synthesis and the rest is "Junk DNA".

What is "Junk DNA"?

Nucleotides sequence with unknown function

How many DNA molecules are in a chromosome?

1 DNA molecule

What is chromatin?

A complex of DNA + protein{RNA} packaging DNA in the eukaryotic nucleus

- DNA molecule tightly coiled around histones

Explain DNA length

1 chromosome = 1 DNA molecule = 12 cm

- The length of DNA shortens 50,000 times due to its packaging into chromatin

What is a nucleosome?

- Basic, repetitive unit of chromatin {DNA wrapped 2x around histone protein}

- Made up of DNA and 4 pairs {octamers} of histones

- Core histones: H2A, H2B, H3 and H4

- Linker histones: H1 binds to the linker DNA between 2 nucleosomes

What are the characteristics of nucleosome?

- Packages DNA into chromosomes

- Controls gene expression by regulating transcription

- 'beads on string of DNA' electron microscope

- Basic carrier of epigenetic information

Histones are the most what proteins in eukaryotes?

The most conservative proteins in eukaryotes

What does acetylation do to histones?

- Increase gene activity

What do de-acetylation do to histones?

De-acetylation of histones decreases the gene activity {methylation of DNA does the same thing}

What are the levels of chromatin organisation {folding}?

Consecutive spiralization and condensation: 2nm DNA {diameter}

1. 10nm chromatin fibre: DNA is shortened 7x

2. 30nm chromatin fibre: DNA is shortened another 6x

3. 100-300nm loop, depending on gene activity: DNA is shortened 200x

4. Chromatid-700nm: the highest level of compaction

Chromosomes

Chromosomes before and after replication

- Before: interphase, 1 chromatid

- After: metaphase, 2 chromatids bound by a centromere

What is the centromere?

-It holds together the two sister chromatids

The strongest and thinnest region of the chromosome.

What does the centromere function in?

- sister chromatid adhesion

- kinetochore formation

- pairing of homologous chromosomes

What is a kinetochore?

A protein structure on the centromeres that links chromosomes to the microtubules of the mitotic and meiotic spindle

What is a telomere?

A region of highly repetitive DNA at the end of a linear chromosome

What happens to telomeres each time a cell divides?

Telomeres shorten every time a cell divides

Metacentric

Chromosome in which the two chromosome arms are approximately the same length.

Submetacentric

Lengths of the arms are unequal- long arm "q", short arm "p"

Acrocentric

The short hand 'p' is so short that it's hard to observe but still present

Telocentric

centromere is located at the terminal end of the chromosome

Euchromatin

The less condensed form of eukaryotic chromatin that is available for transcription.

- Lightly packed; lightly coloured

- Contains unique and moderately repetitive DNA sequence

- Transcriptionally active

- Rich in G-C base pairs

- Replicates in the early s-phase

Heterochromatin

- Tightly packed {highly condensed}; dark colour

- Transcriptionally inactive

- Contains highly repetitive DNA sequence

- Rich in A-T base pairs

- Replicates in the late s-shape

Concentrated around centromeres and telomeres

What are the 2 types of heterochromatin?

constitutive and facultative

Describe constitutive heterochromatin

- Highly condensed and functionally inactive

- Contains tandemly repeated DNA seq.

- Localised at centromeres and near telomeres

Describe facultative heterochromatin

- Temporarily inactivated euchromatin; contains temporarily repressed genes: regulates gene activity

Where are tissue specific genes found?

- In the heterochromatic zone when repressed

- In euchromatic zones when active

What is a gene?

Nucleotide sequence {DNA,RNA} which determines the amino acid sequence of a protein or RNA or controls where and when a protein or RNA molecule is to be produced {regulatory sequence}

What is the coding region?

Nucleotide coding for the amino acid sequence of a protein

What are non-coding regions?

regulatory sites, introns, sites binding RNA-polymerase, sites binding transcription factors.

Describe the genes in Eukaryotes

- Are interrupted {split } {with introns} and are not complementary to the mature mRNA transcribed on them

- Are amplified

- Have non-coding regions

What are the 3 DNA sequence in eukaryotic genes?

- Unique

- Moderately repetitive

- Highly repetitive

Explain unique DNA sequence in eukaryotes

- 1-2 copies{1-3% of genomes}

- Localised in particular chromosomes

- Codes for all proteins except histones

Explain moderately repetitive DNA sequence in eukaryotes

- Up to 100,000 copies

- Genes for: histone proteins {scattered within the genome}, actin, myosin, haemoglobin, rRNA and tRNA

- Non coding and unknown function