BBC - GCSE Bitesize: DNA, genes and chromosomes
Find information, videos, and activities about DNA, genes, chromosomes, the Experiment with the forces involved and measure the relationship between the. Chromosomes carry genetic information in a molecule called DNA. A type of cell division called mitosis ensures that when a cell divides each new cell produced. Genes are smaller sections of DNA still, and there are many on each chromosome. A gene is a set of instructions on how to make one protein ( molecular.
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.
DNA, genes and chromosomes — University of Leicester
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.Genes vs. DNA vs. Chromosomes - Instant Egghead #19
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.
Cell division - AQA
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.
Genes and Chromosomes - Fundamentals - MSD Manual Consumer Version
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. 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.
DNA, genes and chromosomes
Mitochondrial chromosomes Mitochondria are tiny structures inside cells that synthesize molecules used for energy. Unlike other structures inside cells, each mitochondrion contains its own circular chromosome. This chromosome contains DNA mitochondrial DNA that codes for some, but not all, of the proteins that make up that mitochondrion. Mitochondria from the sperm usually do not become part of the developing embryo. Traits A trait is any gene-determined characteristic. Many traits are determined by the function of more than one gene.
For example, a person's height is likely to be determined by many genes, including those affecting growth, appetite, muscle mass, and activity level. However, some traits are determined by the function of a single gene. Variation in some traits, such as eye color or blood type, is considered normal. Other variations, such as albinismMarfan syndromeand Huntington diseaseharm body structure or function and are considered disorders.
However, not all such gene abnormalities are uniformly harmful. The rungs are pairs of chemicals called 'nitrogenous bases', or 'bases' for short. There are four types of base: These bases link in a very specific way: A always pairs with T, and C always pairs with G.
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.
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.