Human cells except spermatozoa and ova have 23 pairs of chromosomes, giving a total of 46 chromosomes; this is known as the diploid number. During cell division mitosis , the diploid number is maintained. The chromosomes in the first 22 pairs — the autosomes — are the same in both sexes. The 23rd pair determines the gender of the individual; its two chromosomes are called the sex chromosomes VanPutte et al, In most people, sex chromosomes come in one of two combinations:.
As female cells only contain X chromosomes, ova will only ever contain X chromosomes. However, male cells always contain both X and Y chromosomes, so spermatozoa can have either an X or Y chromosome associated with them. Roughly equal numbers of X-bearing and Y-bearing spermatozoa are produced, so sex is determined by which type of spermatozoon fuses with the ovum.
This normally results in roughly half of all children being male and half female Fig 2. There are only two possibilities when determining sex X-bearing spermatozoon producing a girl or Y-bearing spermatozoon producing a boy so it is a bit like flipping a coin — it is possible to see four or five heads or more in a row and, in the same way, parents may have four or five children or more of the same sex in a row. However, when populations are examined as a whole, there is roughly a split between the sexes.
Not all males are XY and not all females are XX; indeed, some of the most common survivable chromosomal abnormalities affect the sex chromosomes. These will be discussed in part 4 in this series. Photographs of chromosomes can be taken in actively dividing cells and then arranged into pairs according to size using special computer software.
Karyotyping is often carried out during pregnancy. In a procedure known as amniocentesis, a sample of the amniotic fluid which surrounds the foetus in the amniotic sac is taken using a needle. The amniotic fluid contains cells from the foetus, shed during intrauterine movement and when the fluid is breathed in and out of the foetal lungs.
As the foetus is in a continual state of growth, most cells will be dividing and so chromosomes will be visible. Sometimes karyotyping will reveal extra or missing chromosomes: such deviations from the diploid number are referred to as aneuploidy. Each nucleated cell has 47, not 46, chromosomes. Having this extra copy results in affected individuals developing common physical and pathophysiological features Mundakel and Lal, , which include:.
Today, people tend to have children later in life and it is increasingly common for women to have their first child in their 40s. Karyotyping provides a useful prenatal screening tool, so parents, in conjunction with health professionals, can make informed decisions about the pregnancy and potentially prepare for the birth of children with specific medical or care needs.
The fact that the age of the mother affects the likelihood of chromosomal abnormalities is linked to a phenomenon called non-disjunction, which will be discussed in part 4. As karyotyping is a relatively simple procedure and can reveal the sex of the unborn child, it can be misused for gender selection.
In most circumstances, it is illegal in the UK to terminate a pregnancy based on the sex of the foetus; it is only allowed in very exceptional cases based on serious medical grounds, such as one parent carrying a sex-linked genetic disease. Genes are arranged linearly along the length of each chromosome like beads on a string , with each gene having its own unique position or locus.
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. When transcription is initiated, part of the DNA double helix opens and unwinds. The mRNA separates from the DNA, leaves the nucleus, and travels into the cell cytoplasm the part of the cell outside the nucleus—Home.
Inside a Cell Inside a Cell Often thought of as the smallest unit of a living organism, a cell is made up of many even smaller parts, each with its own function. Human cells vary in size, but all are quite small. 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 chemical markers to indicate where transcription should begin and end. Various chemical substances such as histones in and around the DNA block or permit transcription. Cells reproduce by dividing 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 may happen. Such mistakes may 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 harmful 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. Genes are segments of DNA that encode the order of amino acids in proteins.
Chromosomes are structures within the nucleus of eukaryotic cells that can be observed as distinct entities at a particular stage of a cell's cycle.
Usually if a cell is observed without treatment and at a non-specific stage we get to see chromatin. It looks like a tangled skein of fibre. Chromosomes are composed of DNA and histones. DNA is the complex biomolecule made up of sugars,phosphorus and nitrogen bases that carries in it the coded information by which amino acids are lined up to form proteins. Genes are stretches of DNA that make up the different codes for the different proteins. The chromosomes of malignant cells usually do not lose their telomeres, helping to fuel the uncontrolled growth that makes cancer so devastating.
In fact, each species of plants and animals has a set number of chromosomes. A fruit fly, for example, has four pairs of chromosomes, while a rice plant has 12 and a dog, In humans and most other complex organisms, one copy of each chromosome is inherited from the female parent and the other from the male parent.
This explains why children inherit some of their traits from their mother and others from their father. The pattern of inheritance is different for the small circular chromosome found in mitochondria. Only egg cells - and not sperm cells - keep their mitochondria during fertilization. So, mitochondrial DNA is always inherited from the female parent.
In humans, a few conditions, including some forms of hearing impairment and diabetes, have been associated with DNA found in the mitochondria. Yes, they differ in a pair of chromosomes known as the sex chromosomes. Females have two X chromosomes in their cells, while males have one X and one Y chromosome.
Inheriting too many or not enough copies of sex chromosomes can lead to serious problems. For example, females who have extra copies of the X chromosome are usually taller than average and some have mental retardation. Males with more than one X chromosome have Klinefelter syndrome, which is a condition characterized by tall stature and, often, impaired fertility.
Another syndrome caused by imbalance in the number of sex chromosomes is Turner syndrome. Women with Turner have one X chromosome only. They are very short, usually do not undergo puberty and some may have kidney or heart problems. Scientists looking at cells under the microscope first observed chromosomes in the late s.
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