How is DNA put together into chromosomes?
The process of how DNA is organized into chromosomes involves several steps. Here's an explanation:
1. DNA Replication: Before cells divide, the DNA within the chromosomes needs to make copies of itself in a process called DNA replication. During replication, the double-stranded DNA molecule unwinds, and each strand serves as a template for the synthesis of a new complementary strand. This results in two identical DNA molecules, each consisting of one old strand and one newly synthesized strand.
2. Chromatin Packaging: After replication, the DNA is in a loosely coiled form called chromatin. Chromatin consists of DNA wrapped around proteins called histones. This helps to condense the DNA and protect it from damage. The DNA-histone complex forms a repeating unit called a nucleosome.
3. Nucleosome Formation: The nucleosome consists of a core histone octamer (eight histone proteins) with DNA wrapped around it. The DNA wraps around the histones in a left-handed coiled structure, forming around 147 base pairs of DNA. The nucleosomes are connected by linker DNA, resulting in a "beads-on-a-string" structure.
4. Higher Order Packaging: The nucleosomes further compact through additional folding and coiling. Additional proteins, such as condensins, help to organize and compact the nucleosomes into a more condensed structure.
5. Chromosome Formation: The highly compacted and organized chromatin structure eventually condenses even further to form distinct structures called chromosomes. As the cell prepares to divide, the chromosomes become highly condensed, allowing them to be easily segregated into daughter cells during cell division.
So, in summary, DNA is first replicated, then packed into nucleosomes by wrapping around histone proteins. The nucleosomes further compact into higher-order structures, eventually forming the condensed and organized chromosomes we observe during cell division.