BBC Bitesize - GCSE Combined Science - Cell division - AQA - Revision 1
What percentage of the human genome is identical between individuals? % of the What is the relationship between chromosomes, genes, and DNA?. By David N. Finegold, MD, Professor of Human Genetics, Department of Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a. Genes are made of a chemical called DNA, which is short for 'deoxyribonucleic Human beings have 23 pairs of chromosomes in every cell, which makes
A chromosome contains many genes. A gene is a segment of DNA that provides the code to construct a protein.
The DNA molecule is a long, coiled double helix that resembles a spiral staircase. In it, two strands, composed of sugar deoxyribose and phosphate molecules, are connected by pairs of four molecules called bases, which form the steps of the staircase. In the steps, adenine is paired with thymine and guanine is paired with cytosine. Each pair of bases is held together by a hydrogen bond. A gene consists of a sequence of bases. Sequences of three bases code for an amino acid amino acids are the building blocks of proteins or other information.
Synthesizing proteins Proteins are composed of a long chain of amino acids linked together one after another. There are 20 different amino acids that can be used in protein synthesis—some must come from the diet essential amino acidsand some are made by enzymes in the body.
As a chain of amino acids is put together, it folds upon itself to create a complex three-dimensional structure. It is the shape of the folded structure that determines its function in the body. Because the folding is determined by the precise sequence of amino acids, each different sequence results in a different protein.
Some proteins such as hemoglobin contain several different folded chains. Instructions for synthesizing proteins are coded within the DNA. The code is written in triplets. That is, the bases are arranged in groups of three.
Particular sequences of three bases in DNA code for specific instructions, such as the addition of one amino acid to a chain. For example, GCT guanine, cytosine, thymine codes for the addition of the amino acid alanine, and GTT guanine, thymine, thymine codes for the addition of the amino acid valine.
Thus, the sequence of amino acids in a protein is determined by the order of triplet base pairs in the gene for that protein on the DNA molecule. The process of turning coded genetic information into a protein involves transcription and translation. 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.
Genes and Chromosomes - Fundamentals - MSD Manual Consumer Version
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.
Cell division - AQA
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.
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.
BBC - GCSE Bitesize: DNA, genes and chromosomes
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. You are different from everyone alive now and everyone who has ever lived. DNA But your genes also mean that you probably look a bit like other members of your family.
For example, have you been told that you have 'your mother's eyes' or 'your grandmother's nose'? Genes influence what we look like on the outside and how we work on the inside. They contain the information our bodies need to make chemicals called proteins. 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 is short for 'deoxyribonucleic acid'.
The DNA molecule is a double helix: The DNA double helix showing base pairs The sides are sugar and phosphate molecules. 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.