Chromosomes (article) | Khan Academy
Genes are part of chromosomes, which are long strands of a chemical substance called deoxyribonucleic acid (DNA). Therefore, genes are made up of DNA. . than unrelated parents to have children with health problems or genetic disorders . The closer the genetic relationship between the parents, the greater the risk of . Genes are made of a chemical called DNA or deoxyribonucleic acid and are organised in chromosomes. Although individuals that have this mutation have problems when these sickle-shaped red blood cells block the flow. Which comes first: imagination or fantasy, and what's the difference Think of it this way: DNA is in genes, genes are on chromosomes.
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.
Genes and genetics explained
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. 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.
Genes and Chromosomes - Fundamentals - MSD Manual Consumer Version
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. Each amino acid can be coded for by more than one codon. A codon table sets out how the triplet codons code for specific amino acids.
DNA replication The enzyme helicase breaks the hydrogen bonds holding the two strands together, and both strands can then act as templates for the production of the opposite strand. The process is catalysed by the enzyme DNA polymerase, and includes a proofreading mechanism. Genes The gene is the basic physical and functional unit of heredity.
It consists of a specific sequence of nucleotides at a given position on a given chromosome that codes for a specific protein or, in some cases, an RNA molecule. Genes consist of three types of nucleotide sequence: These genes are known, collectively, as the human genome.
Chromosomes Eukaryotic chromosomes The label eukaryote is taken from the Greek for 'true nucleus', and eukaryotes all organisms except viruses, Eubacteria and Archaea are defined by the possession of a nucleus and other membrane-bound cell organelles. The nucleus of each cell in our bodies contains approximately 1.
This DNA is tightly packed into structures called chromosomes, which consist of long chains of DNA and associated proteins. In eukaryotes, DNA molecules are tightly wound around proteins - called histone proteins - which provide structural support and play a role in controlling the activities of the genes.
A strand to nucleotides long is wrapped twice around a core of eight histone proteins to form a structure called a nucleosome. The chains of histones are coiled in turn to form a solenoid, which is stabilised by the histone H1.
Further coiling of the solenoids forms the structure of the chromosome proper. Another way of saying this is that the O group is recessive — a person needs two O alleles to have the blood group O. So a child may have blood group A because the blood group A gene inherited from their mother is dominant over the blood group O gene inherited from their father. The father has two O alleles OOso he has the blood group O.
Each one of their children has a 50 per cent chance of having blood group A AO and a 50 per cent chance of having blood group O OOdepending on which alleles they inherit. Co-dominant genes Not all genes are either dominant or recessive. Sometimes, each allele in the gene pair carries equal weight and will show up as a combined physical characteristic.
So someone with one copy of A and one copy of B has the blood group AB. Continuing the example of blood groups, a person with the alleles AO will have the blood group A.
The observable trait — blood group — is known as the phenotype. The genotype is the genes that produce the observable trait. Chemical communication Although every cell has two copies of the 23, genes, each cell needs only some specific genes to be switched on in order to perform its particular functions. The unnecessary genes are switched off. Genes communicate with the cell in chemical code, known as the genetic code. The cell carries out its instructions to the letter. A cell reproduces by copying its genetic information then splitting in half, forming two individual cells.
Occasionally, a mistake is made, causing a variation genetic mutation and the wrong chemical message is sent to the cell. Genetic mutations are permanent. Some of the causes of a spontaneous genetic mutation include exposure to radiation, chemicals and cigarette smoke.
Genetic mutations also build up in our cells as we age. For example, skin cancer can be caused by a build-up of spontaneous mutations in genes in the skin cells caused by damage from UV radiation. Sometimes, a parent may have one copy of a gene that is faulty and the other copy containing the correct information.
The correct copy of a gene overrides the faulty copy. For example, the gene controlling red—green colour recognition is located on the X chromosome. A mother who carries the faulty gene causing red—green colour blindness on one of her X chromosome copies will have perfectly normal vision, as she still has a functioning gene copy for red—green colour recognition on her other X chromosome.
However, her sons have a 50 per cent chance of being colourblind. This is because there is a 50 per cent chance that they will inherit the X chromosome from their mother that contains the faulty gene. There is also a 50 per cent chance that they will inherit the X chromosome containing the correct copy of the gene and so will have normal vision. Genetic conditions To date, scientists have identified around 1, conditions caused directly or indirectly by changes in the genes.
Around half of all miscarriages are caused by changes in the total number of genes in the developing baby. Similarly, about half of the Australian population will be affected at some point in their life by an illness that is at least partly genetic in origin. The three ways in which genetic conditions can happen are: The variation in the gene that makes it faulty a mutation happens spontaneously in the formation of the egg or sperm, or at conception.
The faulty gene is passed from parent to child and may directly cause a problem that affects the child at birth or later in life.
The faulty gene is passed from parent to child and may cause a genetic susceptibility. Environmental factors, such as diet and exposure to chemicals, combine with this susceptibility to trigger the onset of the disorder.