Bases of geneticBases of genetic

Nucleus & Related Structures Nucleus & Related Structures

A nucleus is present in all eukaryotic cells that A nucleus is present in all eukaryotic cells that divide. If a cell is cut in half, the anucleate portion divide. If a cell is cut in half, the anucleate portion eventually dies without dividing. The nucleus is eventually dies without dividing. The nucleus is made up in large part of the chromosomes, the made up in large part of the chromosomes, the structures in the nucleus that carry a complete structures in the nucleus that carry a complete blueprint for all the heritable species and blueprint for all the heritable species and individual characteristics of the animal. individual characteristics of the animal.

Except in germ cells, the chromosomes occur in Except in germ cells, the chromosomes occur in pairs, one originally from each parent. Each pairs, one originally from each parent. Each chromosome is made up of a giant molecule of chromosome is made up of a giant molecule of deoxyribonucleic acid (DNA). deoxyribonucleic acid (DNA).

Endoplasmic ReticulumEndoplasmic Reticulum

The endoplasmic reticulum is a complex series of The endoplasmic reticulum is a complex series of tubules in the cytoplasm of the cell. The outer limb of tubules in the cytoplasm of the cell. The outer limb of its membrane is continuous with a segment of the its membrane is continuous with a segment of the nuclear membrane, so in effect this part of the nuclear membrane, so in effect this part of the nuclear membrane is a cistern of the endoplasmic nuclear membrane is a cistern of the endoplasmic reticulum. reticulum.

The tubule walls are made up of membrane. In rough The tubule walls are made up of membrane. In rough or granular endoplasmic reticulum, granules called or granular endoplasmic reticulum, granules called ribosomes are attached to the cytoplasmic side of the ribosomes are attached to the cytoplasmic side of the membrane, whereas in smooth or agranular membrane, whereas in smooth or agranular endoplasmic reticulum, the granules are absent. endoplasmic reticulum, the granules are absent.

RNARNA

The nucleus of most cells contains a The nucleus of most cells contains a nucleolus, a patchwork of granules rich in nucleolus, a patchwork of granules rich in ribonucleic acid (RNA). In some cells, the ribonucleic acid (RNA). In some cells, the nucleus contains several of these structures. nucleus contains several of these structures.

Nucleoli are most prominent and numerous Nucleoli are most prominent and numerous in growing cells. They are the site of in growing cells. They are the site of synthesis of ribosomes, the structures in the synthesis of ribosomes, the structures in the cytoplasm in which proteins are synthesized cytoplasm in which proteins are synthesized

DNA3DNA3

DNA4DNA4

DNA5DNA5

DNA6DNA6

ribosomesribosomes

RibosomesRibosomes

The ribosomes that become attached to the The ribosomes that become attached to the endoplasmic reticulum synthesize all transmembrane endoplasmic reticulum synthesize all transmembrane proteins, most secreted proteins, and most proteins proteins, most secreted proteins, and most proteins that are stored in the Golgi apparatus, lysosomes, and that are stored in the Golgi apparatus, lysosomes, and endosomes. All these proteins have a hydrophobic endosomes. All these proteins have a hydrophobic signal peptide at one endsignal peptide at one end..

The Golgi apparatus, which is involved in processing The Golgi apparatus, which is involved in processing proteins formed in the ribosomes, and secretory proteins formed in the ribosomes, and secretory granules, vesicles, and endosomes are discussed granules, vesicles, and endosomes are discussed below in the context of protein synthesis and below in the context of protein synthesis and secretion. secretion.

The GenomeThe Genome

DNA is found in bacteria, in the nuclei of eukaryotic DNA is found in bacteria, in the nuclei of eukaryotic cells, and in mitochondria. It is made up of two cells, and in mitochondria. It is made up of two extremely long nucleotide chains containing the extremely long nucleotide chains containing the bases adenine (A), guanine (G), thymine (T), and bases adenine (A), guanine (G), thymine (T), and cytosine (C). cytosine (C).

The chains are bound together by hydrogen The chains are bound together by hydrogen bonding between the bases, with adenine bonding bonding between the bases, with adenine bonding to thymine and guanine to cytosine. An indication to thymine and guanine to cytosine. An indication of the complexity of the molecule is the fact that of the complexity of the molecule is the fact that the DNA in the human haploid genome (the total the DNA in the human haploid genome (the total genetic message) is made up of 3 × 109 base pairs. genetic message) is made up of 3 × 109 base pairs.

Formation of mRNA by Formation of mRNA by Transcription of DNATranscription of DNA

A segment of the DNA A segment of the DNA molecule is opened, and molecule is opened, and RNA polymerase (an RNA polymerase (an enzyme that is not enzyme that is not shown) assembles shown) assembles nucleotides into mRNA nucleotides into mRNA according to the base-according to the base-pair combinations shown pair combinations shown in the inset. in the inset.

Thus the sequence of Thus the sequence of nucleotides in DNA nucleotides in DNA determines the determines the sequence of nucleotides sequence of nucleotides in mRNA. As nucleotides in mRNA. As nucleotides are added, an mRNA are added, an mRNA molecule is formed.molecule is formed.

The Human GenomeThe Human Genome

When the human genome was finally mapped When the human genome was finally mapped several years ago, there was considerable several years ago, there was considerable surprise that it contained only about 30,000 surprise that it contained only about 30,000 genes and not the 50,000 or more that had been genes and not the 50,000 or more that had been expected. Yet humans differ quite markedly from expected. Yet humans differ quite markedly from their nearest simian relatives. their nearest simian relatives.

The explanation appears to be that rather than a The explanation appears to be that rather than a greater number of genes in humans, there is a greater number of genes in humans, there is a greater number of mRNAs—perhaps as many as greater number of mRNAs—perhaps as many as 85,000. The implications of this increase are 85,000. The implications of this increase are discussed below. discussed below.

Transcription & Transcription & Translation Translation

The strands of the DNA double helix not only replicate The strands of the DNA double helix not only replicate themselves, but also serve as templates by lining up themselves, but also serve as templates by lining up complementary bases for the formation in the nucleus complementary bases for the formation in the nucleus of messenger RNA (mRNA), transfer RNA (tRNA), the of messenger RNA (mRNA), transfer RNA (tRNA), the RNA in the ribosomes (rRNA), and various other RNAs. RNA in the ribosomes (rRNA), and various other RNAs.

The formation of mRNA is called transcription and is The formation of mRNA is called transcription and is catalyzed by various forms of RNA polymerase. Usually catalyzed by various forms of RNA polymerase. Usually after some posttranscriptional processing (see below), after some posttranscriptional processing (see below), mRNA moves to the cytoplasm and dictates the mRNA moves to the cytoplasm and dictates the formation of the polypeptide chain of a protein formation of the polypeptide chain of a protein (translation). This process occurs in the ribosomes. (translation). This process occurs in the ribosomes.

Posttranscriptional Posttranscriptional Change in mRNAChange in mRNA

An intron is An intron is cleaved from cleaved from between two between two exons and is exons and is discarded. The discarded. The exons are spliced exons are spliced together by together by spliceosomes to spliceosomes to make the make the functional mRNA.functional mRNA.

Translation of mRNA Translation of mRNA to Produce a Proteinto Produce a Protein

To start protein To start protein synthesis a synthesis a ribosome binds to ribosome binds to mRNA. The mRNA. The ribosome also has ribosome also has two binding sites two binding sites for tRNA, one of for tRNA, one of which is occupied which is occupied by a tRNA with its by a tRNA with its amino acid. Note amino acid. Note that the codon of that the codon of mRNA and the mRNA and the anticodon of tRNA anticodon of tRNA are aligned and are aligned and joined. The joined. The

other tRNA binding other tRNA binding site is open.site is open.

By occupying the By occupying the open tRNA open tRNA binding site the binding site the next tRNA is next tRNA is properly aligned properly aligned with mRNA and with mRNA and with the other with the other tRNA.tRNA.

Translation of mRNA to Produce a Translation of mRNA to Produce a ProteinProtein

An enzyme within An enzyme within the ribosome the ribosome catalyzes a catalyzes a synthesis reaction to synthesis reaction to form a peptide bond form a peptide bond between the amino between the amino acids. Note that the acids. Note that the amino acids are now amino acids are now associated with only associated with only one of the tRNAs.one of the tRNAs.

Translation of mRNA to Produce a Translation of mRNA to Produce a ProteinProtein

The ribosome shifts The ribosome shifts position by three position by three nucleotides. nucleotides.

The tRNA without the The tRNA without the amino acid is released from amino acid is released from the ribosome, and the tRNA the ribosome, and the tRNA with the amino acids takes with the amino acids takes its position. A tRNA binding its position. A tRNA binding site is left open by the shift. site is left open by the shift.

Additional amino acids can Additional amino acids can be added by repeating be added by repeating steps 2 through 4. steps 2 through 4. Eventually a stop codon in Eventually a stop codon in the mRNA ends the the mRNA ends the production of the protein, production of the protein, which is released from the which is released from the ribosome.ribosome.

Translation of Translation of mRNA to Produce a mRNA to Produce a

ProteinProtein

Multiple Multiple ribosomes attach ribosomes attach to a single mRNA. to a single mRNA.

As the ribosomes As the ribosomes move down the move down the mRNA, proteins mRNA, proteins attached to the attached to the ribosomes ribosomes lengthen and lengthen and eventually detach eventually detach from the mRNA.from the mRNA.

Translation of mRNA Translation of mRNA to Produce a Proteinto Produce a Protein

Cell CycleCell Cycle The cell cycle is divided The cell cycle is divided

into interphase and into interphase and mitosis. Interphase is mitosis. Interphase is divided into G1, S, and divided into G1, S, and G2 subphases. During G1 G2 subphases. During G1 and G2, the cell carries and G2, the cell carries out routine metabolic out routine metabolic activities. During the S activities. During the S phase DNA is replicated. phase DNA is replicated. (a) Following mitosis, two (a) Following mitosis, two cells are formed by the cells are formed by the process of cytokinesis. process of cytokinesis.

Each new cell begins a Each new cell begins a new cell cycle. (b) Many new cell cycle. (b) Many cells exit the cell cycle cells exit the cell cycle and enter the G0 phase, and enter the G0 phase, where they remain until where they remain until stimulated to divide, at stimulated to divide, at which point they reenter which point they reenter the cell cycle.the cell cycle.

Cell CycleCell Cycle

Obviously, the initiation of mitosis and normal cell Obviously, the initiation of mitosis and normal cell division depends on the orderly occurrence of division depends on the orderly occurrence of events during what has come to be called the cell events during what has come to be called the cell cycle. A diagram of these events is shown in. cycle. A diagram of these events is shown in.

There is intense interest in the biochemical There is intense interest in the biochemical machinery that produces mitosis, in part because machinery that produces mitosis, in part because of the obvious possibility of its relation to cancer. of the obvious possibility of its relation to cancer. When DNA is damaged, entry into mitosis is When DNA is damaged, entry into mitosis is inhibited, giving the cell time to repair the DNA; inhibited, giving the cell time to repair the DNA; failure to repair damaged DNA leads to cancer. failure to repair damaged DNA leads to cancer.

MitosisMitosis

Interphase. DNA, Interphase. DNA, which is dispersed as which is dispersed as chromatin, replicates. chromatin, replicates. The two strands of The two strands of each DNA molecule each DNA molecule separate, and a copy separate, and a copy of each strand is of each strand is made. Consequently,made. Consequently,

two identical DNA two identical DNA molecules are molecules are produced. The pair of produced. The pair of centrioles replicates to centrioles replicates to produce two pairs of produce two pairs of centriolescentrioles

Prophase. Chromatin Prophase. Chromatin strands condense to strands condense to form chromosomes. form chromosomes. Each chromosome is Each chromosome is composed of two composed of two identical strands of identical strands of chromatin called chromatin called chromatids, which are chromatids, which are joined together at one joined together at one point by a specialized point by a specialized region called the region called the centromere. Each centromere. Each chromatid contains chromatid contains one of the DNA one of the DNA molecules replicated molecules replicated during interphase. during interphase.

MitosisMitosis

Metaphase. Metaphase. The The

chromosomes chromosomes align along the align along the equator with equator with spindle fibers spindle fibers from each pair from each pair of centrioles, of centrioles, located at located at opposite poles opposite poles of the cell, of the cell, attached to their attached to their centromeres.centromeres.

MitosisMitosis

Anaphase. The Anaphase. The centromeres separate, centromeres separate, and each chromatid is and each chromatid is then referred to as a then referred to as a chromosome. Thus, chromosome. Thus, when the centromeres when the centromeres divide, the divide, the chromosome number chromosome number doubles, and there are doubles, and there are two identical sets of two identical sets of chromosomes. chromosomes.

The two sets of The two sets of chromosomes are chromosomes are pulled by the spindle pulled by the spindle fibers toward the poles fibers toward the poles of the cell.of the cell.

MitosisMitosis

Telophase. The Telophase. The migration of each set of migration of each set of chromosomes is chromosomes is complete. A new complete. A new nuclear envelope nuclear envelope develops from the develops from the endoplasmic reticulum, endoplasmic reticulum, and the nucleoli and the nucleoli reappear. reappear.

During the latter During the latter portion of telophase the portion of telophase the spindle fibers spindle fibers disappear, and the disappear, and the chromosomes unravel chromosomes unravel to become less distinct to become less distinct chromatin threads.chromatin threads.

MitosisMitosis

Interphase. Cytokinesis, Interphase. Cytokinesis, which continued from which continued from anaphase through anaphase through telophase, becomes telophase, becomes complete when the complete when the plasma membranes move plasma membranes move close enough together at close enough together at the equator of the cell to the equator of the cell to fuse, completely fuse, completely separating the two new separating the two new daughter cells, each of daughter cells, each of which now has a complete which now has a complete set of chromosomes (a set of chromosomes (a diploid number of diploid number of chromosomes) identical chromosomes) identical to the parent cell.to the parent cell.

MitosisMitosis

MeiosisMeiosis In germ cells, reduction division (meiosis) takes place In germ cells, reduction division (meiosis) takes place

during maturation. The net result is that one of each during maturation. The net result is that one of each pair of chromosomes ends up in each mature germ pair of chromosomes ends up in each mature germ cell; consequently, each mature germ cell contains cell; consequently, each mature germ cell contains half the amount of chromosomal material found in half the amount of chromosomal material found in somatic cells. Therefore, when a sperm unites with somatic cells. Therefore, when a sperm unites with an ovum, the resulting zygote has the full an ovum, the resulting zygote has the full complement of DNA, half of which came from the complement of DNA, half of which came from the father and half from the mother. father and half from the mother.

The term "ploidy" is sometimes used to refer to the The term "ploidy" is sometimes used to refer to the number of chromosomes in cells. Normal resting number of chromosomes in cells. Normal resting diploid cells are euploid and become tetraploid just diploid cells are euploid and become tetraploid just before division. Aneuploidy is the condition in which before division. Aneuploidy is the condition in which a cell contains other than the haploid number of a cell contains other than the haploid number of chromosomes or an exact multiple of it, and this chromosomes or an exact multiple of it, and this condition is common in cancerous cells. condition is common in cancerous cells.