Chromosome how many genes

After splitting, bases on each strand bind to complementary bases A with T, and G with C floating nearby. When this process is complete, two identical double-strand DNA molecules exist. There are also chemical mechanisms to repair DNA that was not copied properly. However, because of the billions of base pairs involved in, and the complexity of, the protein synthesis process, mistakes may happen. Such mistakes may occur for numerous reasons including exposure to radiation, drugs, or viruses or for no apparent reason.

Minor variations in DNA are very common and occur in most people. Most variations do not affect subsequent copies of the gene. Mistakes that are duplicated in subsequent copies are called mutations. Inherited mutations are those that may be passed on to offspring. Mutations can be inherited only when they affect the reproductive cells sperm or egg.

Mutations that do not affect reproductive cells affect the descendants of the mutated cell for example, becoming a cancer but are not passed on to offspring. Mutations may be unique to an individual or family, and most harmful mutations are rare. Mutations may involve small or large segments of DNA. Depending on its size and location, the mutation may have no apparent effect or it may alter the amino acid sequence in a protein or decrease the amount of protein produced.

If the protein has a different amino acid sequence, it may function differently or not at all. An absent or nonfunctioning protein is often harmful or fatal. For example, in phenylketonuria Phenylketonuria PKU Phenylketonuria is a disorder of amino acid metabolism that occurs in infants born without the ability to normally break down an amino acid called phenylalanine.

Phenylalanine, which is toxic This deficiency allows the amino acid phenylalanine absorbed from the diet to accumulate in the body, ultimately causing severe intellectual disability. In rare cases, a mutation introduces a change that is advantageous. For example, in the case of the sickle cell gene, when a person inherits two copies of the abnormal gene, the person will develop sickle cell disease Sickle Cell Disease Sickle cell disease is an inherited genetic abnormality of hemoglobin the oxygen-carrying protein found in red blood cells characterized by sickle crescent -shaped red blood cells and chronic However, when a person inherits only one copy of the sickle cell gene called a carrier , the person develops some protection against malaria Malaria Malaria is infection of red blood cells with one of five species of Plasmodium, a protozoan.

Malaria causes fever, chills, sweating, a general feeling of illness malaise , and sometimes diarrhea Although the protection against malaria can help a carrier survive, sickle cell disease in a person who has two copies of the gene causes symptoms and complications that may shorten life span.

Natural selection refers to the concept that mutations that impair survival in a given environment are less likely to be passed on to offspring and thus become less common in the population , whereas mutations that improve survival progressively become more common.

Thus, beneficial mutations, although initially rare, eventually become common. The slow changes that occur over time caused by mutations and natural selection in an interbreeding population collectively are called evolution.

Not all gene abnormalities are harmful. For example, the gene that causes sickle cell disease also provides protection against malaria. A chromosome is made of a very long strand of DNA and contains many genes Genes Genes are segments of deoxyribonucleic acid DNA that contain the code for a specific protein that functions in one or more types of cells in the body.

The genes on each chromosome are arranged in a particular sequence, and each gene has a particular location on the chromosome called its locus. In addition to DNA, chromosomes contain other chemical components that influence gene function. Except for certain cells for example, sperm and egg cells or red blood cells , the nucleus of every normal human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes.

Normally, each pair consists of one chromosome from the mother and one from the father. There are 22 pairs of nonsex autosomal chromosomes and one pair of sex chromosomes. Paired nonsex chromosomes are, for practical purposes, identical in size, shape, and position and number of genes.

Because each member of a pair of nonsex chromosomes contains one of each corresponding gene, there is in a sense a backup for the genes on those chromosomes. The pair of sex chromosomes determines whether a fetus becomes male or female. Males have one X and one Y chromosome. Females have two X chromosomes, one from the mother and one from the father. In certain ways, sex chromosomes function differently than nonsex chromosomes.

The smaller Y chromosome carries the genes that determine male sex as well as a few other genes. The X chromosome contains many more genes than the Y chromosome, many of which have functions besides determining sex and have no counterpart on the Y chromosome. In males, because there is no second X chromosome, these extra genes on the X chromosome are not paired and virtually all of them are expressed.

Genes on the X chromosome are referred to as sex-linked, or X-linked, genes. Normally, in the nonsex chromosomes, the genes on both of the pairs of chromosomes are capable of being fully expressed. However, in females, most of the genes on one of the two X chromosomes are turned off through a process called X inactivation except in the eggs in the ovaries. X inactivation occurs early in the life of the fetus.

In some cells, the X from the father becomes inactive, and in other cells, the X from the mother becomes inactive. Because of X inactivation, the absence of one X chromosome usually results in relatively minor abnormalities such as Turner syndrome Turner Syndrome Turner syndrome is a sex chromosome abnormality in which girls are born with one of their two X chromosomes partially or completely missing. Turner syndrome is caused by the deletion of part Thus, missing an X chromosome is far less harmful than missing a nonsex chromosome see Overview of Sex Chromosome Abnormalities Overview of Sex Chromosome Abnormalities Sex chromosome abnormalities may be caused by full or partial deletions or duplications of sex chromosomes.

Chromosomes are structures within cells that contain DNA and many genes. A gene is If a female has a disorder in which she has more than two X chromosomes, the extra chromosomes tend to be inactive. Thus, having one or more extra X chromosomes causes far fewer developmental abnormalities than having one or more extra nonsex chromosomes. Genes contain Males who have more than one Y chromosome Home. There are several types of chromosome abnormalities Overview of Chromosome and Gene Disorders Chromosomes are structures within cells that contain a person's genes.

The regions on either side of the centromere are referred to as the chromosome's arms. Centromeres help to keep chromosomes properly aligned during the complex process of cell division. As chromosomes are copied in preparation for production of a new cell, the centromere serves as an attachment site for the two halves of each replicated chromosome, known as sister chromatids.

Telomeres are repetitive stretches of DNA located at the ends of linear chromosomes. They protect the ends of chromosomes in a manner similar to the way the tips of shoelaces keep them from unraveling. In many types of cells, telomeres lose a bit of their DNA every time a cell divides.

Eventually, when all of the telomere DNA is gone, the cell cannot replicate and dies. White blood cells and other cell types with the capacity to divide very frequently have a special enzyme that prevents their chromosomes from losing their telomeres.

Because they retain their telomeres, such cells generally live longer than other cells. Telomeres also play a role in cancer. The chromosomes of malignant cells usually do not lose their telomeres, helping to fuel the uncontrolled growth that makes cancer so devastating. In fact, each species of plants and animals has a set number of chromosomes.

A fruit fly, for example, has four pairs of chromosomes, while a rice plant has 12 and a dog, In humans and most other complex organisms, one copy of each chromosome is inherited from the female parent and the other from the male parent. This explains why children inherit some of their traits from their mother and others from their father.

The pattern of inheritance is different for the small circular chromosome found in mitochondria. Only egg cells - and not sperm cells - keep their mitochondria during fertilization.

So, mitochondrial DNA is always inherited from the female parent. In humans, a few conditions, including some forms of hearing impairment and diabetes, have been associated with DNA found in the mitochondria.

Yes, they differ in a pair of chromosomes known as the sex chromosomes. Females have two X chromosomes in their cells, while males have one X and one Y chromosome. Inheriting too many or not enough copies of sex chromosomes can lead to serious problems.

For example, females who have extra copies of the X chromosome are usually taller than average and some have mental retardation. Males with more than one X chromosome have Klinefelter syndrome, which is a condition characterized by tall stature and, often, impaired fertility.

A genetic disorder happens when a gene or genes has a problem with its code, and this causes a health problem. Sometimes a genetic disorder happens when a child inherits it from one or both parents. Other times, it happens only in the child and the parents do not have the genetic disorder. Scientists are learning more and more about genetics. A worldwide research project called The Human Genome Project created a map of all human genes.

It shows where the genes are located on the chromosomes. Doctors can use this map to find and treat or cure some kinds of genetic disorders. There is hope that treatments for many genetic disorders will be developed in the future. Reviewed by: Amy W. Anzilotti, MD. Larger text size Large text size Regular text size.