Genes and Chromosomes - Fundamentals - MSD Manual Consumer Version
Genes and Chromosomes and Fundamentals - Learn about from the MSD Manuals - Medical Consumer Version. What is the relationship between genes and chromosomes? Chromosomes are paired long chains within a cell nucleus that are composed of genes, about. Find information, videos, and activities about DNA, genes, chromosomes, the Experiment with the forces involved and measure the relationship between the.
Transcription and translation Transcription is the process in which information coded in DNA is transferred transcribed to ribonucleic acid RNA. When transcription is initiated, part of the DNA double helix splits open and unwinds. The mRNA separates from the DNA, leaves the nucleus, and travels into the cell cytoplasm the part of the cell outside the nucleus—see Figure: There, the mRNA attaches to a ribosome, which is a tiny structure in the cell where protein synthesis occurs. Each molecule of tRNA brings one amino acid to be incorporated into the growing chain of protein, which is folded into a complex three-dimensional structure under the influence of nearby molecules called chaperone molecules.
These cells look and act differently and produce very different chemical substances. However, every cell is the descendant of a single fertilized egg cell and as such contains essentially the same DNA. Cells acquire their very different appearances and functions because different genes are expressed in different cells and at different times in the same cell. The information about when a gene should be expressed is also coded in the DNA.
Gene expression depends on the type of tissue, the age of the person, the presence of specific chemical signals, and numerous other factors and mechanisms. Knowledge of these other factors and mechanisms that control gene expression is growing rapidly, but many of these factors and mechanisms are still poorly understood.
The mechanisms by which genes control each other are very complicated. Genes have markers to indicate where transcription should begin and end. Various chemical substances such as histones in and around the DNA block or permit transcription. Replication Cells reproduce by splitting in two.
Because each new cell requires a complete set of DNA molecules, the DNA molecules in the original cell must reproduce replicate themselves during cell division. Replication happens in a manner similar to transcription, except that the entire double-strand DNA molecule unwinds and splits in two. 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.
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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 can happen. Such mistakes can 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 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 phenylketonuriaa mutation results in the deficiency or absence of the enzyme phenylalanine hydroxylase. 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. However, when a person inherits only one copy of the sickle cell gene called a carrierthe person develops some protection against malaria a blood infection.
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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 populationwhereas 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.
Chromosomes A chromosome is made of a very long strand of DNA and contains many genes hundreds to thousands. 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. Pairing Except for certain cells for example, sperm and egg cells or red blood cellsthe nucleus of every 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.
DNA, genes and chromosomes
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 23rd pair is the sex chromosomes X and Y. Sex 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. Thus, missing an X chromosome is far less harmful than missing a nonsex chromosome see Overview of Sex Chromosome Abnormalities.
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. For example, women with three X chromosomes triple X syndrome are often physically and mentally normal. Males who have more than one Y chromosome see XYY Syndrome may have physical and mental abnormalities.
Chromosome abnormalities There are several types of chromosome abnormalities. The DNA molecule has two important properties. It can make copies of itself. If you pull the two strands apart, each can be used to make the other one and a new DNA molecule. It can carry information. The order of the bases along a strand is a code - a code for making proteins.
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Genes A gene is a length of DNA that codes for a specific protein. So, for example, one gene will code for the protein insulin, which is important role in helping your body to control the amount of sugar in your blood. Genes are the basic unit of genetics.
Human beings have 20, to 25, genes. These genes account for only about 3 per cent of our DNA. The function of the remaining 97 per cent is still not clear, although scientists think it may have something to do with controlling the genes.
Chromosomes If you took the DNA from all the cells in your body and lined it up, end to end, it would form a strand million miles long but very, very thin! To store this important material, DNA molecules are tightly packed around proteins called histones to make structures called chromosomes.
The packaging of DNA into chromosomes Human beings have 23 pairs of chromosomes in every cell, which makes 46 chromosomes in total. A photograph of a person's chromosomes, arranged according to size, is called a karyotype.What is DNA and Genes?
The sex chromosomes determine whether you are a boy XY or a girl XX. The other chromosomes are called autosomes. The karyotype of a male human being The largest chromosome, chromosome 1, contains about genes.