https://en.wikipedia.org/wiki/Chromatin

Folded chromatin –> Chromosome

During Cell division

Unfolded chromatin

The human genome is made of 23 chromosome pair which are contained within the cell nucleus. During mitosis and meiosis the chromosome condense into supercoiled chromatids attached into pairs at the centromere. While during normal cellular growth, known as interphase, the chromosomes need to unfold in order for the DNA to be either replicated or for gene transcription to occur. The total size of the human genome is approximately 3 thousands Mega-basepairs (3 billion BPs), which needs to be contained inside the cells nucleus, only ~10 micron (0.01mm) in diameter. The DNA is wrapped around nucleosomes consisting of an octamer of histone proteins which can be up to 80bp appart. The linker DNA between histones contains cis-regulatory elements. They are non-coding DNA sequences such as core and proximal promoter regions, enhancers, silencers and other type of sites that can bind transcription factors and activators, and thus, regulate gene expression. ~ Nikolai Drouzine