Chromosomes and DNA packaging | Major Biology I (2023)

Understand the structure and organization of chromosomes in eukaryotic cells.

In this paper, we look at autosomes and sex chromosomes, the differences between the two, and how cells package DNA.

learning objective

  • Distinguish between two types of chromosomes: autosomes and sex chromosomes.
  • Learn how DNA is protected and compacted in cells

When a cell divides in two, one of its main functions is to ensure that each of the two new cells receives a complete and perfect copy of its genetic material. Errors in the replication process, or an uneven distribution of genetic material between cells, can cause cells to be unhealthy or non-functional (and even lead to diseases such as cancer). But what exactly is this genetic material? What about cell division?

DNA and genome

DNA (deoxyribonucleic acid)It is the genetic material of an organism. In humans, DNA is present in almost every cell in the body, providing the instructions they need to grow, function, and respond to their environment. When a cell in the body divides, it passes a copy of its DNA to each daughter cell. DNA is also transmitted at the organism level, and the DNA from sperm and egg cells is combined to form a new organism that possesses the genetic material of its parents. Physically, DNA is a long series of pairs of chemical units (nucleotides) of four different types that carry information organized into units called genes. Genes often provide instructions for making proteins that confer functional properties to cells and organisms.

Chromosomes and DNA packaging | Major Biology I (1)In eukaryotic organisms such as plants and animals, most of the DNA is found in the cell nucleus, called the cell nucleus.DNA of Kern. Mitochondria are organelles that generate energy for cells and contain their ownmitochondrial DNAand chloroplasts, the organelles of plant cells that carry out photosynthesis, are also presentChloroplastene-DNA. The DNA content in mitochondria and chloroplasts is much less than that of the nucleus. In bacteria and other prokaryotes, most of the DNA is found in what is called the central region of the cell.nucleoide, which function as a nucleus but are not surrounded by a membrane.

The group of DNA in a cell is called its own.By. Since all cells in an organism (with few exceptions) contain the same DNA, an organism can also be said to have its own genome, and since members of a species often have similar genomes, it can also be described a kind. Generally, when people refer to the human genome or any other eukaryotic genome, they are referring to the set of DNA that resides in the cell's nucleus (ie, the nuclear genome). Mitochondria and chloroplasts are believed to have their own separate genomes.


Each species has its own characteristic number of chromosomes. For example, humans have 46 chromosomes in a typical body cell, while dogs have 78. Like many plant and animal species, humans do too.diploid (2norte, which means that most of your chromosomes are in a matching set called "pair of homologues. Thus, the 46 chromosomes of a human cell are organized into 23 pairs, and the two members of each pair are named.counterpartThe other's chromosomes (with the small exception of the X and Y chromosomes; see below).

Each pair of human sperm and egg cells has only one homologous chromosome, i.e.Haploid (1norte. When a sperm and egg cell fuse, their genetic material combines to form a complete set of diploid chromosomes. So, for every pair of homologous chromosomes in your genome, one of the homologs is from your mother and the other is from your father.

Chromosomes and DNA packaging | Major Biology I (2)

Image changed from "karyotype“Courtesy of the National Institutes of Health (public domain).

The two chromosomes of a homologous pair are usually very similar to each other. They have the same size, shape, and pattern of light and dark bands that you would see on humans.karyotype(Images of chromosomes) are shown above. Bands appear when chromosomes are stained with a dye: darker bands mark more compact DNA (usually containing fewer genes), and lighter bands mark less compact DNA (usually containing more genes). Most importantly, the two homologues in a pair carry the same genetic information. For example, a gene that affects eye color has been found near the base of chromosome 15.[1]. you can have the blue version, ortongue, the gene is in one homologue but is a brown version in the other. Two homologues have the same type of gene at the same locus, but can (and often do!) have different versions of the gene.

In humans, the X and Y chromosomes determine a person's biological sex, with XX being female and XY being male. While the two X chromosomes in female cells are homologous, the X and Y chromosomes in male cells are not. They vary in size and shape, with X being much larger than Y and containing different genes, mostly different (although they share small areas of similarity). They are called X and Y chromosome, while the other 44 human chromosomes are calledautosomas

chromosome structure

The basis for the continuity of life from one cell to another is cell reproduction throughout the cell cycle. Hecell cycleIt is an ordered sequence of events that describes the stages in the life of a cell, from the division of a single mother cell to the production of two new daughter cells. The mechanisms involved in the cell cycle are strictly regulated.

Structure and compaction of eukaryotic chromosomes.

If the DNA of the 46 chromosomes in the nucleus of the human cell were connected end to end, it would be about two meters long; however, its diameter is only 2 nanometers. Considering that a typical human cell is about 10 µm in size (100,000 cells per meter in a row), DNA must be densely packed in order to fit into the nucleus. At the same time, it must be easy to express genes. At certain phases of the cell cycle, long strands of DNA condense into compact chromosomes. There are many methods of chromosome compaction.

In the first stage of compression, short stretches of the DNA double helix are wound around a core of eight histone proteins at regular intervals along the length of the chromosome (Figure 1). The DNA-histone complex is called chromatin. The globular histone-DNA complex is called a nucleosome, and the DNA that connects the nucleosomes is called adapter DNA. This form of DNA molecule is about seven times shorter than a non-histone double helix, and the beads are about 10 nm in diameter compared to 2 nm for a DNA double helix. The next stage of compaction occurs when the nucleosomes and the DNA connecting them are coiled into 30 nm chromatin fibers. This twist further shortens the chromosome, making it about 50 times shorter than the elongated form. In tertiary packaging various fibrins are used to package the chromatin. These fibrins also ensure that each chromosome in non-dividing cells occupies a specific region of the nucleus that does not overlap with other chromosomes.

Chromosomes and DNA packaging | Major Biology I (3)

Figure 1. Double-stranded DNA encapsulates histones, forming nucleosomes with a “pearl” appearance. Nucleosomes coil into 30 nm chromatin fibers. Chromosomes continue to condense as cells undergo mitosis.

DNA is replicated during the S phase of interphase. After replication, chromosomes consist of two connected sister chromatids. When fully compacted, the identically packed pairs of chromosomes are held together by cohesin. The connection between sister chromatids is strongest in a region called the centromere. Connected sister chromatids about 1 micron in diameter can be seen under a light microscope. The centromeric regions are highly condensed and therefore appear as narrow regions.

This animation illustrates the different levels of chromosome packaging:

Summary – Chromosome Structure

Eukaryotic DNA is highly structured and organized at all stages of the organism's life. Each diploid organism contains one pair of chromosomes; Humans have 23 pairs, for a total of 46 chromosomes. Pairs of chromosomes (also called homologous chromosomes) contain the same genes, although there may be differences in the versions of the genes in each member of the pair. The DNA in the nucleus of eukaryotic cells is normally densely packed by protein-DNA complexes, forming the characteristic condensed "chromosome" shape. In preparation for cell division, the DNA condenses further.

check your understanding

Answer the following questions to see how well you understand the topics covered in the previous section. This quiz doesNOIt counts toward your course grade and you can retake the exam as many times as you like.

Use this quiz to check your understanding and decide if you want to (1) continue reading the previous section or (2) go to the next section.

  1. Museum of Technological Innovation. (2013). Doesn't that make your brown eyes blue? existStanford Tech: Understanding Genetics. received from


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