Chromosomes and DNA

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* Most cells have a nucleus with a certain number of chromosomes.

* There are twenty-three different pairs of chromosomes in human cells.

* In females, each chromosome in the twenty-three pairs, appears to be identical to its partner.

* In males, one pair is different, one chromosome is shorter than its partner. These two chromosomes are the sex chromosomes.

* In females both the sex chromosomes are the same length.

* We use an X to represent the longer sex chromosome and a Y to represent the shorter one.

Fertilisation A woman has two X chromosomes, XX, while a man has one of each, XY. The two Xs or the X and Y are separated from each other during meiosis – the cell divisions that form sex cells or gametes. All healthy eggs produced by a human ovary have an X chromosome. However, with sperm, half carry an X and half carry a Y. A human baby’s gender is determined when an egg X is fertilised by a sperm. The baby will be a girl if the sperm is an X sperm.

The baby will be a boy if the sperm is a Y sperm. Chance is involved in gender. For example, a family with four children will not necessarily include two boys and two girls. It’s a bit like spinning a coin. The family is as likely to have two of each as it is to get two ‘heads’ and two ‘tails’ in four spins of a coin. If you were to spin a coin, millions of times, you get roughly equal numbers of ‘heads’ and ‘tails’. The many millions of fertilisations that made the population of the world tend to produce roughly equal numbers of boys and girls.

* 22 of the 23 pairs of chromosomes in human body cells appear to be the same in both sexes. The 23rd pair (sex chromosomes) differ. Females have XX sex chromosomes and males have XY.

* Two chromosomes of each pair are separated from each other during meiosis. Meiosis is a type of cell division used to make sperms or eggs.

* Gametes have single chromosomes not pairs. All eggs have an X. Sperms have either an X or a Y.

* X and Y sperms are released during intercourse. Chance plays a part in fertilisation – here the male sex chromosome joins the X chromosome found in an egg.

* The expected 50:50 ratio of males and females only shows up when large numbers of fertilisations are involved.

The composition of DNA

DNA stands for deoxyribonucleic acid. Chromosomes are made of DNA. DNA is a very large molecule (polymer) made from smaller molecules (monomers) called nucleotides. There are different kinds of nucleotide in a DNA molecule.

A nucleotide is made of a nitrogenous base, a pentose sugar (a 5-carbon sugar) and a phosphate group

The structure of DNA

The DNA molecule has two chains of nucleotides. The chains are weakly linked together by bonds called hydrogen bonds. These bonds link a base in one chain to a base in the other. They hold the molecule together as a double helix. The way that the one type of nitrogenous base links to another type of base is called complementary base pairing.

The DNA bases form the genetic code. Three bases form one bit of the code – a triplet code. Cells use the triplet code to make proteins in cell organelles called ribosomes. Some of the proteins work as enzymes controlling chemistry in the cell. In this way, the DNA tells cells what to do.

DNA replication A living cell can make a copy of its DNA. To do so it needs nucleotides, special carrier molecules, some energy and particular enzymes. This happens during cell divisions such as mitosis. The DNA of a chromosome unwinds and is replicated.Knowing how DNA works also allows us to tell cells what to do. So for example, if we change a bacterial cell’s DNA by genetic engineering, we can get it to make human insulin for us.

DNA is the genetic material in the nucleus of a cell which makes up the chromosomes. Genes are short sections of DNA each acting as a code for a particular characteristic. Since we inherit particular chromosomes through the egg and sperm, we also inherit the particular characteristics they code for. For your biology exam you will need to know the difference between a gene and an allele, and why it’s important whether alleles are dominant or recessive.

Genes Each gene acts as a code or a set of instructions for a cell to make a particular protein. The many different proteins give a person particular characteristics, such as blue or brown eye colour. Some characteristics, such as whether we make a particular enzyme or a hormone, are not visible but they affect the way our body works. A human egg cell and a human sperm cell both contain 23 single chromosomes. Fertilisation brings these two sets of single chromosomes to make 23 pairs of chromosomes in the embryo. A gene is a short section of DNA which acts as a code for a particular characteristic.

After fertilisation the embryo contains chromosomes in pairs. One set of chromosomes were inherited from the father and contains his form of each gene. The other set of chromosomes was inherited from the mother and contains her form of each gene. This means that genes are in pairs. For example, you have two genes controlling eye colour. Different forms of the same gene are called alleles. The gene for eye colour has different forms, such as the allele for blue eye colour and the allele for brown eye colour. The alleles inherited by the zygote may be the same (called homozygous), or different (called heterozygous).

Purebred individuals are homozygous for a characteristic. Many human characteristics are controlled by just one pair of genes, for example eyelash length, tongue rolling, colour of iris of eye, shape of the upper eyelid, shape of the earlobe and the ability to taste phenylthiocarbamide (PTC). Other characteristics, such as hair colour and type, may be controlled by more than one set of genes. A genotype describes the alleles which someone has inherited. A phenotype is how the gene appears or expresses itself. A dominant allele always shows. A recessive allele only shows when it is on its own (because it’s on a sex chromosome) or when the alleles are the same (homozygous). Many diseases are caused by living organisms called pathogens which infect us. Some other diseases are inherited from our parents through our genes.

Diseases Many diseases are caused by infectious pathogens. Like a cold, these pass from one organism to another by air, water or by direct contact. Pathogens include organisms such as some bacteria, viruses, fungi, protozoans, round worms and flat worms. This table shows types of disease which are not caused by a pathogen.DiseaseExampleDegenerative (caused by a breakdown of tissues)Multiple sclerosisDietary (caused by something you eat or something lacking in your diet)ScurvyEnvironmental (caused by a condition in your environment)AsbestosisPhysiological (caused by a body system not working correctly)DiabetesGenetic (inherited)Sickle-cell anaemia, cystic fibrosis, haemophilia Sickle-cell anaemia

Sickle-cell anaemia is caused by a recessive allele for the haemoglobin gene. This results in the production of abnormal haemoglobin. Individuals homozygous for this allele suffer sickle-cell anaemia.

Red cells that contain the abnormal haemoglobin change shape and become sickled when the oxygen levels are low. The haemoglobin forms crystals that distort the membrane of the cells and may even destroy them. The loss of red blood cells makes the sufferer anaemic and feel weak. The irregularly shaped cells may block capillaries and cause clotting which could affect the blood supply to the heart and brain.

Some people are heterozygous because they only have one allele for the sickle cell condition. Here the red cells are affected but only some become sickle-shaped, so the effect is not so great. Such individuals are heterozygous for this allele and are called carriers.

Cystic fibrosis Cystic fibrosis is caused by a recessive allele that controls the way mucus is formed. Mucus produced by people with cystic fibrosis is abnormally thick and sticky. As a result they have congested lungs and are more likely to get respiratory infections. Daily physiotherapy helps to relieve congestion, while antibiotics are used to fight infection.

Cystic fibrosis also affects the gut and pancreas so that food is not digested as well

Sex-linked diseases The human X-chromosome carries some alleles for which there are no partners on the Y-chromosome. The result of inheriting an allele like this is different for males and females, and so they are called sex-linked characters. A sex-linked allele inherited by a male is expressed even if it is recessive because it is not hidden by another allele. Haemophilia is an example of a sex-linked disease which is caused by a recessive allele. The normal dominant allele causes the production of a blood clotting factor while the recessive allele does not. Males only inherit one allele because the Y-chromosome does not have one. So inheriting one recessive allele means that their blood does not clot and there is a danger of bleeding to death.

There is less chance of females inheriting two recessive alleles for haemophilia, but it is possible. In the past there was little chance of people with haemophilia surviving, but now blood clotting factors can be injected regularly to help control the condition. Females who are heterozygous for a sex-linked allele are called carriers. Their sons have a fifty-fifty chance of inheriting this type of disease.Predicting genetic disease People who have a history of a genetic disease in their family often seek advice about what will happen if they have children.

This is called genetic counselling and it gives a couple an idea of how likely they are to have children with the particular disease, so they can decide whether to have children or notVariations Inside the nucleus of all plant and animal cells are chromosomes. These chromosomes are like threads and carry the genes that control characteristics such as the colour of a flower, the shape of a leaf, height and hair colour. Inside your normal body cells there are 23 pairs of chromosomes. There are different numbers of chromosomes for different species of plants and animals. The gametes (for example sperm, pollen and egg cells) contain half the number of chromosomes. This halving in the number of chromosomes happens during meiosis (see the Revision Bite on Cell Division). So each new baby carries a unique set of chromosomes, half from the mother and half from the father.

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