Genome how many chromosomes

Our genes provide cells with information on how to make proteins. Scientists have estimated that humans may produce up to , proteins, so they thought there were about as many human genes. Today, they know that some genes contain the code for making multiple proteins. That's the number of genes in our "other" genome—the mitochondrial genome. Mitochondria are the cell's power plants, and many of their genes are involved in production of cellular energy.

They have their own set of genes because they are thought to have evolved from bacteria that were engulfed by eukaryotic cells cells containing a nucleus some 1. That's how many feet long the DNA from one of your cells would be if you uncoiled each strand and placed them end to end. Do this for all your DNA, and the resulting strand would be 67 billion miles long—the same as about , round trips to the Moon.

That's the length in inches across a cell's nucleus, which holds your DNA. If you sliced human hair into tenths lengthwise, each slice would be about that big around. To keep the space tidy, DNA spools around a group of proteins called histones.

The resulting taut package of wound-up DNA is called chromatin, which winds up even tighter to form your chromosomes. The DNA of any two people on Earth is Still, mistakes do occur on rare occasions.

Changes in the number or structure of chromosomes in new cells may lead to serious problems. For example, in humans, one type of leukemia and some other cancers are caused by defective chromosomes made up of joined pieces of broken chromosomes. It is also crucial that reproductive cells, such as eggs and sperm, contain the right number of chromosomes and that those chromosomes have the correct structure.

If not, the resulting offspring may fail to develop properly. For example, people with Down syndrome have three copies of chromosome 21, instead of the two copies found in other people. Chromosomes vary in number and shape among living things. Most bacteria have one or two circular chromosomes.

Humans, along with other animals and plants, have linear chromosomes that are arranged in pairs within the nucleus of the cell. The only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome. When two reproductive cells unite, they become a single cell that contains two copies of each chromosome. This cell then divides and its successors divide numerous times, eventually producing a mature individual with a full set of paired chromosomes in virtually all of its cells.

Besides the linear chromosomes found in the nucleus, the cells of humans and other complex organisms carry a much smaller type of chromosome similar to those seen in bacteria.

This circular chromosome is found in mitochondria, which are structures located outside the nucleus that serve as the cell's powerhouses. Scientists think that, in the past, mitochondria were free-living bacteria with the ability to convert oxygen into energy.

When these bacteria invaded cells lacking the power to tap into oxygen's power, the cells retained them, and, over time, the bacteria evolved into modern-day mitochondria. The constricted region of linear chromosomes is known as the centromere.

Although this constriction is called the centromere, it usually is not located exactly in the center of the chromosome and, in some cases, is located almost at the chromosome's end. Somatic cells usually have one copy of chromosomes from each parent, plus an X chromosome from the mother, and either an X or Y chromosome from the father, for a total of The estimate of the number of human genes has been repeatedly revised down from initial predictions of , or more as genome sequence quality and gene finding methods have improved, and could continue to drop further.

Reference Terms. The human genome is the genome of Homo sapiens. It is made up of 23 chromosome pairs with a total of about 3 billion DNA base pairs. Chromosomes are numbered roughly in order of decreasing size. The largest chromosome of an organism is generally referred to as chromosome 1, the next largest as chromosome 2, and so on.

Different chromosomes contain different genes. That is, each chromosome contains a specific chunk of the genome. For example, in humans the gene for alpha globin, a part of the hemoglobin protein that carries oxygen in red blood cells, is found on chromosome The gene for beta globin, the other part of the hemoglobin protein, is found on chromosome Stained chromosomes can be photographed and arranged in order of size to produce a karyotype, a chart scientists use to study chromosomes.

A karyotype isn't detailed enough to tell you about the individual genes on a chromosome, but it can tell you whether the chromosomes as a whole are in working order and it may help doctors diagnose and understand diseases.

For example, people with Down's syndrome have too many chromosomes, and chromosome rearrangements when a part of a chromosome breaks off and reattaches to a different chromosome are associated with certain cancers. A quick glance at any karyotype will tell you one of the most important facts about chromosomes: They come in pairs. The members of a pair are the same size and shape, and they have the same banding patterns. In other words, each person actually possesses two copies of chromosome 1, two copies of chromosome 2, and so on.

Human cells contain 23 pairs of chromosomes. Humans have 22 pairs of autosomes.