what do genes and chromosomes have to do with inherited traits

Genes are the basic units of inheritance in nature. This article is the kickoff in a iv-part serial exploring the role of genes and chromosomes in inheritance, health and disease

Abstract

Genes are passed down the generations in a predictable manner and we receive roughly one-half of our genetic textile from each parent. This article explains the nature, construction and role of genes, deoxyribonucleic acrid and chromosomes, describes how chromosomes determine gender, and touches on chromosomal abnormalities. Information technology is the first article in a four-function series exploring the role of genes and chromosomes in inheritance, health and disease.

Citation: Knight J, Andrade M (2018) Genes and chromosomes 1: basic principles of genetics. Nursing Times [online]; 114: 7, 42-45.

Authors: John Knight and Maria Andrade, both senior lecturers in biomedical scientific discipline at the College of Human Health and Science, Swansea University.

This commodity has been double-blind peer reviewed
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Introduction

With the exception of identical twins, all humans are genetically unique. During conception, a paternal sperm cell (spermatozoon) fuses with a maternal egg cell (ovum). The resulting embryo will have received approximately half of its genetic material from the father and half from the mother. Therefore, although all children are genetically unique, they typically showroom a mix of characteristics inherited from both parents. The basic unit of inheritance in nature is the factor and genes are passed down the generations in a predictable manner. Each gene stores data in the form of deoxyribonucleic acrid (DNA).

Cells and DNA

The boilerplate adult human trunk is composed of approximately 50 trillion (50 million million) cells. Near Deoxyribonucleic acid is located in the nuclei of these cells; a much smaller amount is in the mitochondria – these are tiny edible bean-shaped organelles present in the cytoplasm of nucleated cells, which release the energy these need to survive.

The set of instructions – or genetic pattern – used to construct the proteins that provide construction in all living organisms is encoded in the genes in the form of nucleic acids, the most common of which is DNA. This is true of the most simple viruses and bacteria all the way through to circuitous multicellular animals, including humans.

Composition and shape

The edifice blocks of Deoxyribonucleic acid are called nucleotides (VanPutte et al, 2017). Each nucleotide is constructed from a:

Five-carbon saccharide (deoxyribose);
Phosphate grouping;
Nitrogenous base of operations.

In full, in that location are 4 nitrogenous bases, as follows:

Adenine;
Cytosine;
Guanine;
Thymine.

Molecules of DNA adopt the characteristic shape of a double helix. It is helpful to visualise them as a long ladder that has been twisted into a spiral shape, with the sugar and phosphate groups forming the rail and the nitrogenous bases forming the rungs (Fig i). DNA has infrequent storage capacity (Box one).

fig1

Box one. Dna and bioinformatics

DNA tin can store information in a very stable and compact course. It has been calculated that all the data in the world (including every book, photograph and video) could be encoded into approximately 1kg of DNA (Extance, 2016). This has opened up a whole new area of enquiry in the field of bioinformatics, which is currently exploring the utilise of DNA every bit a digital storage medium.

Complementary base pairing

The nitrogenous bases in DNA pair up in a predictable way:

Adenine always pairs with thymine (A-T);
Cytosine e'er pairs with guanine (C-G).

This 'complementary base pairing' occurs because the bases structurally complement each other, fitting together like two pieces of a jigsaw puzzle. Equally a consequence, if the sequence of ane strand of the DNA molecule is known, the other strand can be deduced (Box ii).

In that location are many online tools that apply the base of operations pairing rule to ascertain the complementary sequences for any given Deoxyribonucleic acid sequence. Complementary base pairing as well allows the genetic code to exist easily replicated during cell partitioning and translated into proteins.

Box two. Deducing a complementary DNA sequence

If the sequence on one strand of Deoxyribonucleic acid is ATCGATGCC, then the complementary sequence will necessarily exist TAGCTACGG. This will requite the following complementary base pairing sequence:

ATCGATGCC

TAGCTACGG

Purines and pyramidines

In addition to forming the base messages of the genetic code, adenine, cytosine, guanine and thymine are intimately involved in cell signalling, regulating enzyme activity and cellular metabolism. Co-ordinate to their chemical structure, the four nitrogenous bases are divided into ii subgroups:

Purines – this includes denine and guanine.
Pyramidines – this includes cytosine and thymine.

Problems with metabolising purines tin can result in the common joint disorder, gout (Box three).

Box 3. How gout develops

In later life, some people develop bug with metabolising purines (adenine and guanine), which can lead to a build-up of uric acid crystals in tissues and joints. This results in gout, which is a common disorder. As all foods are derived from plants or animals, which contain their ain Dna, the human diet is naturally rich in purines. However, certain foods – notably offal, oily fish, shellfish and fermented grain products such as beer – are particularly rich in purine and tin, therefore, exacerbate gout. Limiting the intake of purine-rich foods ofttimes relieves the symptoms of gout.

Mitochondrial Deoxyribonucleic acid

Although most human genes are encoded in the DNA located in the nuclei of cells, mitochondria contain some of our DNA. Mitochondrial Dna (mtDNA) has 37 genes, many of which code for the production of enzymes involved in cellular metabolism (Chial and Craig, 2008). During fecundation, spermatozoa usually just deliver nuclear DNA, so mtDNA is inherited solely along maternal lines via the ova.

Like bacteria, mitochondria tin replicate by binary fission – a procedure that allows the mitochondrial genes to be passed on to 'girl' mitochondria. Many of the genes carried by mitochondria are like to those constitute in bacteria, which has led to the theory that mitochondria were once complimentary-living bacteria that archaic cells incorporated into their cytoplasm.

DNA and chromosomes

The average human being jail cell is around 20µm in diameter (a micron is one-thousandth of a millimetre). Nearly of a cell's Dna is stored in its nucleus, which is fifty-fifty smaller: around 2-10µm in diameter (Milo and Phillips, 2016). Despite their small size, each nucleated man cell manages to pack in around 3m (x feet) of Dna.

To make this possible, the DNA is neatly wrapped around histone proteins that function like a spool or bobbin. Dna and histone proteins form a material called chromatin, which is visible in the cell nucleus in the course of microscopic granules (VanPutte et al, 2017).

When a cell is preparing to divide, the Deoxyribonucleic acid is wound up tighter and tighter and folds upon itself into coils. This tightly wound DNA is denser and thicker, and appears in the nucleus in the form of thread-like structures chosen chromosomes.

Chromosomes and gender

Human being cells (except spermatozoa and ova) have 23 pairs of chromosomes, giving a full of 46 chromosomes; this is known as the diploid number. During prison cell sectionalisation (mitosis), the diploid number is maintained. The chromosomes in the get-go 22 pairs – the autosomes – are the aforementioned in both sexes. The 23rd pair determines the gender of the private; its two chromosomes are called the sex chromosomes (VanPutte et al, 2017). In most people, sexual activity chromosomes come up in one of two combinations:

XX = female;
XY = male person.

Every bit female cells just incorporate X chromosomes, ova will merely ever contain X chromosomes. Withal, male person cells e'er contain both X and Y chromosomes, so spermatozoa can have either an X or Y chromosome associated with them. Roughly equal numbers of 10-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 existence male and half female (Fig two).

fig2

At that place are only ii possibilities when determining sex (Ten-begetting spermatozoon producing a girl or Y-bearing spermatozoon producing a boy) so information technology is a scrap like flipping a coin – it is possible to see iv or v heads (or more than) in a row and, in the same way, parents may have four or five children (or more than) of the same sex in a row. Yet, when populations are examined equally a whole, there is roughly a 50:fifty divide between the sexes.

Non all males are XY and not all females are Xx; indeed, some of the virtually common survivable chromosomal abnormalities affect the sex chromosomes. These will exist discussed in part 4 in this serial.

Karyographs and karyotyping

Photographs of chromosomes can be taken in actively dividing cells and then arranged into pairs co-ordinate to size using special reckoner software. These photographs, known as karyographs, reveal an private's chromosomal make-up or karyotype.

Karyotyping is often carried out during pregnancy. In a process known every bit 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, near cells will exist dividing and so chromosomes will be visible.

Karyotyping tin can reveal many things about an unborn kid, including:

Its gender;
Whether the diploid number of chromosomes is nowadays;
Whether there are whatsoever extra chromosomes and which pair has the extra copy;
Whether at that place are missing chromosomes and which pair has the missing copy;
Whether a particular chromosome is too long;
Whether a particular chromosome is too short.

Aneuploidy and Down syndrome

Sometimes karyotyping will reveal extra or missing chromosomes: such deviations from the diploid number are referred to as aneuploidy. The World Wellness Organization estimates that aneuploidy is present in at least 5% of pregnancies.

The most common aneuploidy is Down's syndrome, or trisomy 21, in which people inherit an actress copy of chromosome 21. Each nucleated cell has 47, not 46, chromosomes. Having this actress copy results in affected individuals developing common concrete and pathophysiological features (Mundakel and Lal, 2018), which include:

Macroglossia (enlarged and often protruding natural language);
Pronounced epicanthal folds (giving the optics an oriental appearance);
Learning difficulties and reduced intelligence quotient;
Increased risk of heart defects, particularly septal defects;
Increased risk of age-related diseases, including arthritis and dementia, earlier in life;
Reduced lifespan.

Today, people tend to have children later in life and it is increasingly common for women to take their first child in their 40s. Every bit a issue, more than children are built-in with chromosomal abnormalities such equally Down's syndrome. Karyotyping provides a useful prenatal screening tool, so parents, in conjunction with health professionals, can brand informed decisions about the pregnancy and potentially fix for the birth of children with specific medical or care needs. The fact that the historic period of the mother affects the likelihood of chromosomal abnormalities is linked to a phenomenon called non-disjunction, which will exist discussed in part 4.

Gender choice

As karyotyping is a relatively simple procedure and can reveal the sex of the unborn kid, it can be misused for gender selection. In almost circumstances, it is illegal in the UK to terminate a pregnancy based on the sexual practice of the foetus; it is just allowed in very exceptional cases based on serious medical grounds, such equally 1 parent carrying a sex-linked genetic illness.

Chromosomes and genes

Genes are arranged linearly along the length of each chromosome (like chaplet on a string), with each factor having its own unique position or locus. In a pair of chromosomes, ane chromosome is always inherited from the female parent and ane from the father. This means that, with the exception of genes on the sex chromosomes of males, we have two copies of each gene, one inherited from our mother and one from our father. These pairs of genes, which control many of our characteristics, are called alleles.

Genotype and phenotype

Each pair of genes that is inherited for a particular trait is referred to equally the genotype for that trait; the physical characteristics that result from possessing those genes is referred to as the phenotype (VanPutte et al, 2017). It can be said that:

The genotype is an individual's genetic brand-upwardly;
The phenotype is the physical appearance of the individual, which is adamant largely past their genes but besides affected by environmental factors.

Ascendant and recessive genes

Some genes exert more than influence than others: the sometime are called dominant and the latter are called recessive. For example, individuals may inherit genes for both blood group A and blood group O, in which case the genotype for that item trait would be AO. In the case of claret groups, the A cistron is dominant and the O gene is recessive, so an individual with the genotype AO for that trait would e'er have the phenotype claret grouping A. Ascendant and recessive genes will be explored in more than item in part 4.

Structural and command genes

Genes tin be divided into structural genes and control (or regulatory) genes.

Structural genes contain sequences of Dna coding for proteins such every bit:

Actin and myosin (used to build musculus), keratin (constitute in peel, pilus and nails) and haemoglobin (found in red blood cells);
Enzymes driving the metabolic processes of the body;
Hormones maintaining homoeostasis (for example, insulin);
Neurotransmitters that assist to co-ordinate physiological processes (for example, substance P).

Control or regulatory genes, every bit their proper noun implies, control or regulate the activity of structural genes – including when these are switched on or off – and how much poly peptide is produced. Command genes ensure that only certain genes are turned on in certain tissues; for example, the pancreas does non crave the genes coding for keratin or haemoglobin to be switched on, but information technology is crucial to take the genes coding for insulin or glucagon switched on. Control genes are often themselves controlled, directly or indirectly, by other chemical signals including hormones, growth factors and neuro-transmitters.

The switching on and off of genes is also influenced by environmental factors including diet, exercise and pollutants. Epigenetics, a relatively new branch of genetics, examines the factors that can influence gene action without actually changing any of the DNA sequences in the genome (Weinhold, 2006).

A revolution in the making

In 1990, the laborious process of sequencing the entire human genetic sequence, or genome, started. The human genome project ended ahead of schedule in 2003 and its results have been fine-tuned since then. In 2014 the human genome was thought to consist of 60,155 genes of which 19,881 were structural genes coding for proteins.

The original human genome project took over 12 years to complete and cost $2.7bn. Today, the entire genome of an individual tin can be sequenced in a few hours for around $1,000. That toll is predicted to drib to around $100 in the adjacent few years (Herper, 2017). Rapid genomic sequencing will likely revolutionise medicine in the coming decades and targeted therapies tailored to an individual'due south genetic code could become commonplace.

Primal points

Genes store information in the form of deoxyribonucleic acid (DNA) located in the cells
When a jail cell is preparing to divide, DNA takes the grade of thread-similar structures called chromosomes
Most human cells take 23 pairs of chromosomes, giving a total of 46 – the diploid number
Having extra or missing chromosomes is called aneuploidy, the almost mutual condition of which is Downwards's syndrome
Structural genes, which incorporate sequences of Dna coding for proteins, are controlled by regulatory genes

Chial H, Craig J (2008) mtDNA and mitochondrial diseases. Nature Education; i: 1, 217.

Extance A (2016) How Dna could store all the globe's data. Nature; 537: 7618, 22-24

Herper M (2017) Illumina promises to sequence human genome for $100 – just not quite even so. Forbes; 9 Jan.

Milo R, Phillips R (2016) Cell Biology past the Numbers. Abingdon: Garland Science.

Mundakel GT, Lal P (2018) Down's syndrome. Medscape.

VanPutte CL et al (2017) Seeley's Anatomy and Physiology. New York, NY: McGraw-Hill.

Weinhold B (2006) Epigenetics: the science of change. Environmental Wellness Perspectives; 114: 3, A160-A167.

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Source: https://www.nursingtimes.net/clinical-archive/genetics/genes-and-chromosomes-1-basic-principles-of-genetics-25-06-2018/