# 29 DNA structure

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

• Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. 

• Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

• A chromatid is 1 of the 2 identical strands of DNA that make uo a chromosome. 2 chromatids are joined by a centromere. Before replication, 1 chromosome is composed of 1 DNA molecule. Following S phase of interphase, each chromosome now composed of 2 DNA molecules (DNA replication ↑ the amount of DNA but does not ↑ the number of chromosomes.) The 2 identical copies are called chromatids. They are normaly identical (homozygous) but may have slight differences due to mutations (heterozygous). 

Telomere is molecular protective cap of chromosomes. 

• Telomere is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighbouring chromosomes. It is essential for maintaining the integrity and stability of linear eukaryotic genomes.
• During chromosome replication, the enzymes that duplicate DNA cannot continue their duplication all the way to the end of a chromosome, so in each duplication the end of the chromosome is shortened. The telomeres are disposable buffers at the ends of chromosomes which are truncated (shortened) during cell division; their presence protects the genes on the chromosome from being truncated instead.

• Telomere length regulation and maintenance contribute to normal human cellular aging and human diseases. 

Syllabus 2016  - 2018 5.1 Replication and division of nuclei and cells  a) describe the structure of a chromosome, limited to DNA, histone proteins, chromatids, centromere and telomeres d) outline the significance of telomeres in permitting continued replication and preventing the loss of genes