BASIC CELL BIOLOGY

LEARNING OUTCOMES

The following topics are covered in this chapter:

• Cellular structure and function;

• Chromosomes;

• The cell cycle;

• Deoxyribonucleic acid (DNA);

• Protein synthesis;

• Mitochondrial DNA.

INTRODUCTION

The activities that occur within cells give us an understanding of how human traits are inherited. Knowledge of cellular function gives rise to the understanding of how the body works. The human body is made up of trillions of cells, many of which have specialised functions. Despite this, all cells share certain features:

• cells arise from the division of pre-existing cells;

• cells interact, they send and receive information;

• cells produce proteins for growth repair and normal body functioning;

• cells contain all the genetic instructions for the body.

All cells in the body behave in this way apart from red blood cells. Red blood cells are not considered to be true cells by the time they reach the blood stream as they do not contain a nucleus. Cells are the basic building blocks of all living matter.

CELL STRUCTURE

Cells have many parts, each with a specialised function. Any structure within the cell that has a characteristic shape and function is termed an organelle. Most organelles are too small to be seen through a light microscope but can be seen with an electron microscope (see Figure 1.1).

Plasma membrane

This is the outer lining of the cell. It is composed of a bilipid layer through which certain molecules can enter the cell (endocytosis) and wastes can exit (exocytosis).

Nucleus

The nucleus functions as the control centre of the cell (Figure 1.3). It contains DNA (Deoxyribonucleic Acid) which is the cell's genetic material. A double membrane separates the contents of the nucleus from the rest of the cell. This nuclear membrane (also called the nuclear envelope) is perforated by nuclear pores.

Nucleolus

The nucleolus (Figure 1.4) is a morphologically distinct area within the nucleus which is involved in the production of Ribonucleic Acid (RNA).

Cytoplasm

Cytoplasm is a gel-like fluid that contains all the organelles and the enzymatic systems which provide energy for the cell.

Cytoskeleton

The cytoskeleton is a network of fibres made from the protein tubulin (Figure 1.5). This provides the structural framework of the cell and functions in cellular shape, cell division and cell motility, as well as directing movement of the organelles within the cell.

Endoplasmic reticulum

The endoplasmic reticulum is an organelle that processes the molecules made by the cell (Figure 1.6). The endoplasmic reticulum transports these molecules to their specific destinations.

Ribosomes

Ribosomes are organelles that provide the sites for protein synthesis (Figure 1.7). Ribosomes are attached to the endoplasmic reticulum as well as freely floating in the cytoplasm.

Golgi body

The Golgi body is a structure that packages the molecules produced by the endoplasmic reticulum ready for transport out of the cell (Figure 1.8).

Mitochondria

Mitochondria are organelles that convert energy gained from food into a form that the cell can use (Figure 1.9). Adenosine triphosphate (ATP) is the main source of energy used by the cell. These organelles have their own genetic material and can make copies of themselves.

Lysosomes

Lysosomes are organelles that break down bacteria and other foreign bodies, as well as recycling worn out cell components (Figure 1.10).

Peroxisomes

Peroxisomes are responsible for the detoxification of foreign compounds and the oxidation of fatty acids (Figure 1.11).

CHROMOSOMES

Each of the trillions of cells in the body, with the exception of red blood cells, has a nucleus. Within each nucleus are structures called chromosomes. Chromosomes are not usually visible under a light microscope, but when a cell is about to divide, the chromosomes become denser and can be viewed at this stage.

Chromosome structure

A chromosome is composed of DNA and proteins and includes structures that enable it to replicate and remain intact (see Figure 1.12). During cell division, chromosomes have a constriction point termed a centromere. The centromere divides each chromosome into two sections or 'arms'. The long arm is referred to as the q arm and the short arm as the p arm (p for petite).

The location of the centromere gives the chromosome its characteristic shape and can be used to describe the location of specific genes.

Telomeres

Telomeres are distinctive structures found on the end of each arm of the chromosome. They are made up of the same short sequence of DNA, which is replicated about three thousand times. The function of the telomeres appears to be twofold.

1. They protect the chromosome by 'capping' off the ends to prevent them from sticking or joining onto other chromosomes.

2. Due to the way that chromosomes are replicated, the ends of the chromosomes are not copied. Telomeres shorten during every cell replication, but the loss of DNA within the telomeres protects against loss of essential DNA within the chromosome itself.

Chromosome numbers

Chromosomes exist in pairs. Although not actually joined together, each pair has a characteristic length. The human cell nucleus has 23 pairs of chromosomes; in other words, 46 individual chromosomes. One chromosome from each pair is inherited from the father and one from the mother. Twenty-three individual chromosomes are inherited from each parent. The total number of chromosomes in each cell is called the diploid number (diploid 46) while the number of pairs is called the haploid number (haploid 23). Of the 23 pairs of chromosomes, 22 pairs are termed autosomes and do not differ between the sexes. For ease of identification, these autosomes are numbered from 1 to 22. The chromosomes are numbered according to length, with chromosome number 1 being the longest and chromosome 22 being the shortest. The remaining two chromosomes are known as the sex chromosomes. These two chromosomes are not numbered but are known as the X chromosome and the Y chromosome. The Y chromosome determines maleness. A female will have two X chromosomes while a male will have one X and one Y chromosome.

Karyotype

The chromosome complement within the nucleus is called a karyotype. Charts called karyographs (see Figure 1.13) display chromosomes in pairs in size order. The 22 paired autosome chromosomes are displayed first, ranging from number 1 to 22 (largest to the smallest). The sex chromosomes, X and Y (male) or X and X (female) are always placed at the end of the chart. Karyographs can be a useful clinical tool to help confirm diagnosis through the identification of chromosomal aberrations, abnormalities or anomalies.

The centromere

Another physical characteristic of the chromosome, the centromere, also helps identification, as the position of the centromere varies in different chromosomes (see Figure 1.14).

ACTIVITY 1.1

What are the haploid and diploid numbers of chromosomes in humans?

CHROMOSOMAL INHERITANCE

The human cell has two sets of chromosomes, one set inherited from each parent. The complete genetic makeup within the cell is termed the genome. The total number of chromosomes within the cell has to be kept constant from one generation to the next. Each individual has a total of 46 chromosomes in each cell nucleus, 23 of which are inherited from their mother and 23 from their father.

For normal cell division two daughter cells are formed, both of which have the full 46 chromosome complement. This type of cell division is called mitosis and results in new cells that are genetically identical to the parent cell. Mitosis is cell division that is used by the body for growth and repair. Meiosis, on the other hand, is a type of cell division that produces new cells with only half the chromosomal complement (a total of 23 chromosomes). These 23 chromosomes are half the set of the original cell. Meiosis only occurs in the germ line cells, i.e. the ova in women and the sperm in men. If fertilisation occurs, the resulting offspring will inherit 23 chromosomes from the mother and 23 chromosomes from the father, resulting in a full 46 chromosomal complement. Meiotic division prevents the doubling of chromosomal numbers from one generation to the next.

Mitosis

Mitosis occurs rapidly during growth and tissue repair. It is a well-controlled process and consists of two major steps — the division of the nucleus followed by the division of the cytoplasm. Although mitosis is a continuous process it can be described as a series of four stages followed by a resting period where there is no cellular division (Table 1.1).

With mitosis each daughter cell is an exact copy of the previous cell. All cells receive identical chromosomal material.

The cycle of events during mitosis usually lasts several hours. The mitotic division of the chromosomal material during prophase, metaphase, anaphase and telophase takes a relatively short period of time and the resting phase (interphase) takes up most of the time within the cell cycle (see Figure 1.15).

The whole cell cycle takes approximately 24 hours, although this depends on which type of cell is involved. Mitosis usually only accounts for about an hour. Interphase is when no cellular division takes place. However, even during interphase, the cell needs to get ready for division so it increases in size. This stage is known as Gap 2 or G2. After division the cell needs to continue to grow so that it can achieve its optimum size; this is known as Gap 1 or G1.

Normally cells can undergo a total of 80 mitotic divisions before the cell dies, although this is dependent on the age of the individual.

Meiosis

Each cell contains two sets of chromosomes which exist in pairs. Meiosis results in cell division that produces new cells with only half the chromosomal complement. This is needed for the formation of germ cells (sperm in men, ova in women) so that two germ cells can fuse to form a full chromosomal complement.

Halving the full complement is achieved in two steps called meiosis I and meiosis II. Meiosis I is very similar to mitotic division in that two daughter cells are produced, both with 46 chromosomes. The main difference is that meiosis I takes much longer in comparison to mitosis and results in the 'crossing over' of chromosomal material (see Figure 1.16). Chromosomes 'swap' or exchange pieces of their structure with their partner chromosome before separating. This results in the daughter cells not having identical genetic material. This is the cause of genetic variability between individuals.

Meiosis II does not involve chromosomal replication but does involve the stages of prophase, metaphase, anaphase and telophase where chromosomes separate, new nuclei are formed and the cell splits into two. At the end of meiosis II the cells contain 23 individual chromosomes.

Differences between mitosis and meiosis

The differences between mitosis and meiosis are illustrated in Figure 1.17 and Table 1.2.

ACTIVITIES 1.2, 1.3 AND 1.4

1.2. Explain why there is significant genetic variation as a result of meiosis but not of mitosis.

1.3. Describe the phases of the cell cycle.

1.4. Explain the reason why germ cells have to undergo meiotic division.

GENETIC INFORMATION

Chromosomes are made up of long chains of DNA (Deoxyribonucleic Acid) and protein molecules. It is the DNA within the chromosomes that holds all genetic information. The total length of the DNA within each cell is over 2m (6 feet) and, in order to fit within the cell's nucleus, it has to exist in a tightly packaged form. This is achieved by the DNA being coiled around protein structures called histones (see Figure 1.18). The DNA wraps around eight histones to form a structure called a nucleosome. Thousands of nucleosomes are formed, which gives the DNA molecule the appearance of a string of beads. Further coiling of these nucleosome beads results in a shortened structure called a chromatin fibre. It is these tightly packaged chromatin fibres that make up chromosomes.

The DNA within the chromosomes contains coded instructions for the production of protein. The coded area for the production of a specific protein is called a gene.

The structure of DNA

The structure of DNA was discovered through X-ray diffraction back in 1953 by the Nobel Prize-winning scientists James Watson and Francis Crick. DNA is composed of bases, sugars and phosphates that combine together to form a double helix. The double helix shape looks like a twisted ladder. The 'sides' of the ladder are made of phosphates and sugars, while the 'rungs' of the ladder are made of bases. Only four different types of bases exist within the DNA:

• Adenine (A);

• Guanine (G);

• Cytosine (C);

• Thymine (T).

DNA bases pair up with each other to form the 'ladder rungs' (see Figure 1.19). Adenine always pairs with Thymine, and Guanine always pairs with Cytosine. Only these two types of base pairing exist in DNA. The order of the base 'rungs' along the DNA ladder varies but the base pairings are always complementary.

The sequences of bases on one DNA strand can be deduced from the sequence on the opposite strand, because base pairing is always complementary. Each strand independently carries the information required to form a double helix. Therefore, to describe a DNA sequence, only the sequence of the bases in one strand is needed, for example ATTGCAAT, as the other strand is always complementary, i.e. TAACGTTA. Human DNA consists of about 3 billion bases, of which over 99 per cent of the sequence is identical in all people. These bases, within the DNA, form the code for the production of proteins.

PROTEIN

All the functions of the cell depend on protein. Protein maintains cell structure, acts as both intracellular and extracellular messengers, binds and transports molecules and acts as enzymes.

Some proteins exist in every cell, such as the enzymes involved in glucose metabolism. Other proteins are highly specialised and are only found in specialised cells, such as the protein myosin, found only in muscle cells, or the protein insulin that is only produced in pancreatic islet cells.

What are proteins?

Proteins are made up of long chains of amino acids. There are only 20 different types of amino acids but, by varying the order and amount of amino acids in the chain, thousands of different proteins can be produced.

Links within the chain of amino acids are called peptide bonds, while the chain itself is known as a polypeptide. A protein can contain one or more polypeptides. Both the structure and function of the protein depend on the sequence of the amino acids making up the polypeptide chains.

In order to function, cells need information to produce proteins and the ability to pass this information on to new cells during cell division. This important information is provided by the DNA.

How are proteins made?

Proteins are not made in the cell nucleus but by the ribosomes in the cell's cytoplasm. The coded information in the DNA has to be transferred out of the nucleus. This is done by the use of ribonucleic acid (RNA).

Step 1: Copying the code

Segments of the DNA within the chromosomes separate at specific points and the DNA code is copied. This copy is called the messenger RNA (mRNA). During this process Guanine pairs with Cytosine and Adenine pairs with Uracil. RNA does not have Thymine but this is replaced with Uracil. Once a copy has been made, the DNA reattaches and the mRNA makes its way out of the nucleus into the cytoplasm (see Figure 1.20).