Eukaryotic Genetics



The Role of the Cell Cycle

The cell cycle allows multiicellular organisms to grow and divide and single-celled organisms to reproduce.

Learning Objectives

Explain the role of the cell cycle in carrying out the cell’s essential functions

Key Takeaways

Key Points

  • All multicellular organisms use cell division for growth and the maintenance and repair of cells and tissues.
  • Single-celled organisms use cell division as their method of reproduction.
  • Somatic cells divide regularly; all human cells (except for the cells that produce eggs and sperm) are somatic cells.
  • Somatic cells contain two copies of each of their chromosomes (one copy from each parent).
  • The cell cycle has two major phases: interphase and the mitotic phase.
  • During interphase, the cell grows and DNA is replicated; during the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides.

Key Terms

  • somatic cell: any normal body cell of an organism that is not involved in reproduction; a cell that is not on the germline
  • interphase: the stage in the life cycle of a cell where the cell grows and DNA is replicated
  • mitotic phase: replicated DNA and the cytoplasmic material are divided into two identical cells

Introduction: Cell Division and Reproduction

A human, as well as every sexually-reproducing organism, begins life as a fertilized egg or zygote. Trillions of cell divisions subsequently occur in a controlled manner to produce a complex, multicellular human. In other words, that original single cell is the ancestor of every other cell in the body. Once a being is fully grown, cell reproduction is still necessary to repair or regenerate tissues. For example, new blood and skin cells are constantly being produced. All multicellular organisms use cell division for growth and the maintenance and repair of cells and tissues. Cell division is tightly regulated because the occasional failure of regulation can have life-threatening consequences. Single-celled organisms use cell division as their method of reproduction.

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Cell Division and Growth: A sea urchin begins life as a single cell that (a) divides to form two cells, visible by scanning electron microscopy. After four rounds of cell division, (b) there are 16 cells, as seen in this SEM image. After many rounds of cell division, the individual develops into a complex, multicellular organism, as seen in this (c) mature sea urchin.

While there are a few cells in the body that do not undergo cell division, most somatic cells divide regularly. A somatic cell is a general term for a body cell: all human cells, except for the cells that produce eggs and sperm (which are referred to as germ cells), are somatic cells. Somatic cells contain two copies of each of their chromosomes (one copy received from each parent). Cells in the body replace themselves over the lifetime of a person. For example, the cells lining the gastrointestinal tract must be frequently replaced when constantly “worn off” by the movement of food through the gut. But what triggers a cell to divide and how does it prepare for and complete cell division?

The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produces two identical (clone) cells. The cell cycle has two major phases: interphase and the mitotic phase. During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides.

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The Cell Cycle: The cell cycle consists of interphase and the mitotic phase. During interphase, the cell grows and the nuclear DNA is duplicated. Interphase is followed by the mitotic phase. During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei. The cytoplasm is usually divided as well, resulting in two daughter cells

The Relationship Between Genes and Proteins

Proteins, encoded by individual genes, orchestrate nearly every function of the cell.

Learning Objectives

Describe transcription and translation

Key Takeaways

Key Points

  • Genes are composed of DNA arranged on chromosomes.
  • Some genes encode structural or regulatory RNAs. Other genes encode proteins.
  • Replication copies DNA; transcription uses DNA to make complementary RNAs; translation uses mRNAs to make proteins.
  • In eukaryotic cells, replication and transcription take place within the nucleus while translation takes place in the cytoplasm.
  • In prokaryotic cells, replication, transcription, and translation occur in the cytoplasm.

Key Terms

  • DNA: a biopolymer of deoxyribonucleic acids (a type of nucleic acid) that has four different chemical groups, called bases: adenine, guanine, cytosine, and thymine
  • messenger RNA: Messenger RNA (mRNA) is a molecule of RNA that encodes a chemical “blueprint” for a protein product.
  • protein: any of numerous large, complex naturally-produced molecules composed of one or more long chains of amino acids, in which the amino acid groups are held together by peptide bonds

Genes and Proteins

Since the rediscovery of Mendel’s work in 1900, the definition of the gene has progressed from an abstract unit of heredity to a tangible molecular entity capable of replication, transcription, translation, and mutation. Genes are composed of DNA and are linearly arranged on chromosomes. Some genes encode structural and regulatory RNAs. There is increasing evidence from research that profiles the transcriptome of cells (the complete set all RNA transcripts present in a cell) that these may be the largest classes of RNAs produced by eukaryotic cells, far outnumbering the protein-encoding messenger RNAs (mRNAs), but the 20,000 protein-encoding genes typically found in animal cells, and the 30,o00 protein-encoding genes typically found in plant cells, nonetheless have huge impacts on cellular functioning.

Protein-encoding genes specify the sequences of amino acids, which are the building blocks of proteins. In turn, proteins are responsible for orchestrating nearly every function of the cell. Both protein-encoding genes and the proteins that are their gene products are absolutely essential to life as we know it.

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Genes Encode Proteins: Genes, which are carried on (a) chromosomes, are linearly-organized instructions for making the RNA and protein molecules that are necessary for all of processes of life. The (b) interleukin-2 protein and (c) alpha-2u-globulin protein are just two examples of the array of different molecular structures that are encoded by genes.

Replication, Transcription, and Translation are the three main processes used by all cells to maintain their genetic information and to convert the genetic information encoded in DNA into gene products, which are either RNAs or proteins, depending on the gene. In eukaryotic cells, or those cells that have a nucleus, replication and transcription take place within the nucleus while translation takes place outside of the nucleus in cytoplasm. In prokaryotic cells, or those cells that do not have a nucleus, all three processes occur in the cytoplasm.

Replication is the basis for biological inheritance. It copies a cell’s DNA. The enzyme DNA polymerase copies a single parental double-stranded DNA molecule into two daughter double-stranded DNA molecules. Transcription makes RNA from DNA. The enzyme RNA polymerase creates an RNA molecule that is complementary to a gene-encoding stretch of DNA. Translation makes protein from mRNA. The ribosome generates a polypeptide chain of amino acids using mRNA as a template. The polypeptide chain folds up to become a protein.