BBC - GCSE Bitesize: DNA, genes and chromosomes
Chromosomes carry genetic information in a molecule called DNA. A type of that contains the code for the production of a particular protein is called a gene. 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. Proteins form the structure of our bodies, as well playing an important role in the processes that keep us alive. Genes are made of a chemical called DNA, which.
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. 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.
Cell division - AQA
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.
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. 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. A person may have an abnormal number of chromosomes or have abnormal areas on one or more chromosomes.
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Many such abnormalities can be diagnosed before birth see Testing for chromosome and gene abnormalities. Abnormal numbers of nonsex chromosomes usually result in severe abnormalities. For example, receiving an extra nonsex chromosome can be fatal to a fetus or can lead to abnormalities such as Down syndromewhich commonly results from a person having three copies of chromosome Absence of a nonsex chromosome is fatal to the fetus.
Large areas on a chromosome may be abnormal, usually because a whole section was left out called a deletion or mistakenly placed in another chromosome called translocation.
For example, chronic myelogenous leukemia is sometimes caused by translocation of part of chromosome 9 onto chromosome This abnormality can be inherited or be the result of a new mutation. Proteins are chains of chemical building blocks called amino acids. A protein may contain a few amino acids or it could have several thousands.
The size of a protein is an important physical characteristic that provides useful information including changes in conformation, aggregation state and denaturation. Protein scientists often use particle size analysers in their studies to discuss protein size or molecular weight.
Archibald Garrod Archibald Garrod was one of the first scientists to propose that genes controlled the function of proteins. Inhe published his observations regarding patients whose urine turned black.
This condition known as alkaptonuria happens when there is a buildup of the chemical homogentisate, which causes the darkening of urine.
In most situations, excess amounts of amino acid phenylalanine are metabolised by the body. This led Garrod to surmise that the enzyme responsible for its breakdown must be defective in these patients. In addition, since the black urine phenotype was passed from generation to generation in a regular pattern, Garrod reasoned that a gene had to be responsible for the production of the defective enzyme.
He attributed a defective enzyme to a defective gene, suggesting a direct link between genes and proteins. The Relationship Between Genes and Proteins Most genes contain the information require to make proteins.