Chapter 3 The Biological Basis of Life

Chapter Outline The Cell DNA Structure DNA Replication Protein Synthesis Cell Division: Mitosis and Meiosis New Frontiers

The Cell Cells are the basic units of life in all living organisms. Complex life forms, such as plants and animals, are made up of billions of cells. The cells of all living organisms share many similarities as a result of their common evolutionary past.

Cells Prokaryotic cells are single celled organisms, such as bacteria and blue-green algae. Life on earth can be traced back 3.7 billion years in the form of prokaryotic cells. Eukaryotic cells, structurally complex cells, appeared 1.2 billion years ago.

Structure of a Eukaryotic Cell The outer boundary of a cell is the cell membrane. Organelles are structures found in the cytoplasm: Mitochondria produce energy. Ribosomes manufacture protein The nucleus is surrounded by the cytoplasm and contains chromosomes.

Two Types of Cells Somatic cells are the components of body tissues. Gametes are sex cells. Ova are egg cells produced in female ovaries. Sperm are sex cells produced in male testes. A zygote is the union between a sperm and an ovum.

DNA Structure Cellular function and an organism’s inheritance depends on the structure and function of DNA. DNA is composed of two chains of nucleotides. A nucleotide consists of a sugar, a phosphate, and one of four nitrogenous bases.

DNA Structure Nucleotides form long chains. The two chains are held together by bonds formed on their bases with their complement on the other chain. – Adenine (A) is the complement of Thymine(T) – Guanine(G) is the complement of Cytosine(C)

The DNA Replication Process 1. Enzymes break the bonds between the DNA molecule. 2. Two nucleotide chains serve as templates for the formation of a new strand of nucleotides. 3. Unattached nucleotides pair with the appropriate complementary nucleotide

The DNA Replication Process 3. The result is two newly formed strands of DNA. 4. Each new strand is joined to one of the original strands of DNA.

Proteins The major structural components of tissue. Enzymes are proteins that serve as catalysts, initiating chemical reactions in the body. Amino acids are the building blocks of protein. Proteins differ according to number of amino acids and the sequence in which they are arranged.

Protein Synthesis Ribosomes help convert the genetic message from the DNA into proteins. Messenger RNA (mRNA) carries the genetic message from the cell nucleus to the ribosome. Transfer RNA (tRNA),found in the cytoplasm, binds to one specific amino acid.

Protein Synthesis: Transcription The process of coding a genetic message for proteins by formation of mRNA. A portion of the DNA unwinds and serves as a template for the formation of a mRNA strand.

Protein Synthesis: Translation The mRNA travels through the nuclear membrane to the ribosome. tRNAs arrive at the ribosome carrying their specific amino acids. The base triplets on the tRNA match up with the codons on the mRNA. As each tRNA line up in the sequence of mRNA codons their amino acids link to form a protein.

Genes A gene is the entire sequence of DNA bases responsible for the synthesis of a protein. A mutation occurs when the sequence of bases in a gene is altered. Mutations may interfere with an organisms ability to produce vital protein and may lead to a new variety within the species, hence, evolution.

Gene Structure The gene consists of exons and introns. Exons are DNA segments transcribed into mRNA that code for specific amino acids. Introns are DNA sequences not expressed during protein synthesis.

Universal Genetic Code The DNA code of all life on earth is composed of the same molecules and carries on similar functions. The universality of the genetic code implies a common ancestry for all life on the planet. Organisms differ according to the arrangement of the DNA.

Cell Division: Mitosis and Meiosis Cell division results in production of new cells. During cell division: – Cells are involved with normal cellular and metabolic processes. – The cell’s DNA becomes tightly coiled. – DNA is visible under a microscope as chromosomes.

Chromosome Structure A chromosome is composed of a DNA molecule and associated proteins. During normal cell functions, chromosomes exist as single-stranded structures. During cell division, chromosomes consist of two strands of DNA joined at the centromere. Since the DNA molecules have replicated, one strand of a chromosome is an exact copy of the other.

Chromosomes and Genetics Each species is characterized by a specific number of chromosomes. – Humans have 46 chromosomes. Chromosome pairs are called homologus. – Homologous chromosomes carry genetic information influencing the same traits. – Homologous chromosomes are not genetically identical.

Types of Chromosomes Autosomes - govern all physical characteristics except sex determination. Sex chromosomes - X and Y chromosome. – Mammal females have two X chromosomes. – Mammal males have one X and one Y chromosome.

Mitosis Mitosis is cell division in somatic cells. Mitosis occurs during growth and repair/replacement of tissues. The result of mitosis is two identical daughter cells that are genetically identical to the original cell.

Steps in Mitosis 1. The 46 chromosomes line up in the center of the cell. 2. The chromosomes are pulled apart at the centromere. 3. The strands separate and move to opposite ends of the dividing cell. 4. The cell membrane pinches in and two new cells exist.

Meiosis Production of gametes (sex cells). 2 divisions result in 4 daughter cells. Each daughter cell contains 23 chromosomes. Resulting gamete may unite with another gamete to create a zygote. The zygote inherits the DNA, half from each parent, to develop and function normally.

Evolutionary Significance of Meiosis Meiosis and sexual reproduction are highly important evolutionary innovations. Meiosis increases genetic variation at a faster rate than mutation. Offspring in sexually reproducing species represent the combination of genetic information from two parents.

Problems With Meiosis Errors in meiosis may lead to miscarriage. Nondisjunction occurs when chromosomes don’t separate during meiosis. – A gamete containing one less chromosome that fuses with a normal gamete will produce a zygote containing 45 chromosomes. – A gamete containing one extra chromosome that fuses with a normal gamete will produce a zygote containing 47 chromosomes.

Abnormal Numbers of Chromosomes Down's syndrome occurs because of three copies of chromosome #21. – Problems include mental retardation, heart defects and respiratory infections. Nondisjunction may occur in the X and Y chromosomes and result in sterility.