structure and organisation of eu chromosomes

lecture 12

How is the enormously long double-stranded DNA in eukaryotic cells packaged?

It is packaged into chromosomes by specialised proteins that bind to and fold the DNA

How is the packaged DNA organised in eukaryotic cells?

• DNA is folded into a very organised series of coils and loops.

• This structure is still accessible to all enzymes and proteins that replicate it, repair it, and transcribe it.

What is the complex of DNA and protein in the nucleus called?

Chromatin.

What is the total length of DNA if placed end to end from all cells in the human body?

If DNA from each cell (50 to 75 trillion cells) were placed end to end, it would be 90 to 135 trillion meters long

What is chromatin?

• DNA + protein

• histone and non-histone proteins with DNA

• Euchromatin and heterochromatin2

How is DNA packaged during different phases of the cell cycle?

◦Eukaryotic cells whether in interphase or mitosis package DNA tightly into chromosomes.

◦The compression of DNA is performed by proteins that coil and fold the DNA into higher and higher levels of organisation

which are more condensed chromosomes mitotic or interphase

mitotic

Why must chromosome packaging be flexible?

to allow rapid, localised, on-demand access to DNA

Describe the compaction of human chromosome 22.

• Human chromosome 22 contains 48 million nucleotide pairs.

◦Stretched out end-to-end, its DNA would extend 1.5 cm.

◦Yet, during mitosis chromosome 22 measures only 2 mm in length

◦This means it is nearly 10,000 times more compact than its extended form during mitosis

What is a nucleosome?

• basic structural unit of DNA packing in Eukaryotic cells

• It consists of DNA and proteins

• refers to 1 nucleosome core particle + one adjacent DNA linker

How are nucleosomes formed?

• formed by the binding of negatively charged DNA to a unique set of basic proteins called Histones.

What are Histones and their role?

• set of basic proteins that bind to DNA to form nucleosomes

• comprised of many lysine and arginine amino acid residues

• big amounts

• responsible for the first and most fundamental level of chromatin packaging: the nucleosome

What is a nucleosome core particle?

• DNA wound around a core of proteins formed from histones

• 8 histone proteins called histone octamer

• A double-stranded DNA of ~146 nucleotide pairs winds around the histone octamer

What is linker DNA?

• Each nucleosome core particle is separated from the next by a region of linker DNA.

What is the result of nucleosome formation?

The formation of nucleosomes converts the DNA molecule into a chromatin thread approximately one-third of its initial length and provides the first level of DNA packaging

Why do histones bind tightly to DNA?

◦The 4 histones that make up the nucleosome core particle are relatively small proteins with a high proportion of positively charged amino acids (lysine and arginine)

◦The positive charges help the histones bind tightly to the negatively charged sugar-phosphate backbone of DNA

How can histones be chemically modified, and why is this important?

1.Acetylation

2.Methylation

3.Phosphorylation

• These modifications facilitate the compaction and organisation of DNA during molecular processes of DNA replication and Transcription

What is the next level of DNA compaction after nucleosomes?

Nucleosomes are further packed upon one another to generate a more compact structure called the 30nm fiber

How is the 30nm fiber formed?

• The packaging of nucleosomes into the 30nm fiber depends on a 5th histone called histone H1.

• Histone H1 is thought to pull the nucleosomes together into a regular repeating array

How is the 30nm fiber further compacted?

organised into loops

What is the final level of packaging to form the mitotic chromosome?

The string of loops undergoes at least one more level of packaging to form the mitotic chromosome

Is chromatin structure static or dynamic, and why?

◦Chromatin structure is dynamic.

◦Its structure is temporarily altered so that proteins involved in transcription, replication, and repair have rapid localised access to the necessary DNA

What does each chromosome consist of?

linear DNA molecule plus associated proteins

What are the key parts of a chromosome?

◦The p arm: the short arm of the chromosome.

◦The q arm: the long arm of the chromosome.

◦The centromere: the midpoint separating the two arms of the chromosome

◦The telomere: the ends of the chromosome

How are chromosomes classified?

according to the position of the centromere

What are the types of chromosomes based on centromere position?

1. telocentric

2. acrocentric

3. submetacentric

4. metacentric

What is a karyotype?

• display of the full set of 46 human chromosomes.

• Human metaphase chromosomes are organised by size and type into a karyotype

How many chromosomes do human somatic cells have, and where do they come from?

• 46 chromosomes

• 22 pairs of chromosomes and 2 sex chromosomes

• Within each pair of chromosomes, 1 copy is maternal one copy is paternal.

What are homologous chromosomes and sex chromosomes in humans?

• The pairs of chromosomes (one maternal, one paternal) ARE HOMO

• The sex chromosomes are the Y chromosome (inherited from father) and the X chromosome (inherited from mother)

• Females are XX (homologous); Males are XY (non-homologous)

How are chromosomes stained for analysis, and what is the result?

• stained with a Giemsa stain

• produces a dark banding pattern at regions rich in A-T nucleotides

How is a karyotype produced? (Steps 1-3)

1.Culture cells from the organism (e.g., blood) to allow cells to go through the cell cycle (DNA duplication in S-phase) and mitosis.

2.A spindle inhibitor is used in the cell culture to prevent the progression of mitosis (metaphase to anaphase).

3.Slide preparations of cells are made and then G-banded (stained with Giemsa).

How is a karyotype produced? (Steps 4-6)

• 4 :Microscopic analysis of interphase vs metaphase cells on slides is performed.

• 5. Metaphase analysis is done, first checking the cell number (n=46 for human samples.

• 6. A karyotype is prepared from metaphase analysis to organise each chromosome

What criteria are used to organise chromosomes in a karyotype?

1.Size.

2.Position of centromere (type of chromosome).

3.Banding pattern (Giemsa stained bands