Chapter 3 The Biological Basis of Life

Chapter Outline  The Cell  DNA Structure  DNA Replication  Protein Synthesis  What is a Gene?  Cell Division: Mitosis and Meiosis  New Frontiers

The Cell  Cells are the basic units of life in all living organisms.  In some forms, such as bacteria, a single cell constitutes the entire organism.  Complex life forms, such as plants and animals, are made up of billions of cells.  An adult human is made up of perhaps as many as 1,000 billion cells, all functioning in complex ways that promote the survival of the individual.

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

Structure of a Eukaryotic Cell

Types of Cells  Somatic cells are components of body tissue.  Gametes are sex cells.  Ova are produced in female ovaries.  Sperm are 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.

Part of a DNA Molecule

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 4. The result is two newly formed strands of DNA. 5. Each new strand is joined to one of the original strands of DNA.

DNA Replication

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.

RNA and DNA RNA differs from DNA in three important ways: 1. It’s usually single-stranded. (This is true of the forms we discuss, but it’s not true for all.) 2. It contains a different type of sugar. 3. It contains the base uracil as a substitute for the DNA base thymine. (Uracil is attracted to adenine, just as thymine is.)

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.

Transcription

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.

Assembly of an Amino Acid Chain in Protein Synthesis  As the ribosome binds to the mRNA, tRNA brings a particular amino acid, specified by the mRNA codon, to the ribosome.

Assembly of an Amino Acid Chain in Protein Synthesis  The tRNA binds to the first codon while a second tRNA–amino acid complex arrives at the ribosome.

Assembly of an Amino Acid Chain in Protein Synthesis  The ribosome moves down the mRNA, allowing a third amino acid to be brought into position by another tRNA molecule. Note that the first two amino acids are now joined together.

Diagram of a DNA Sequence Being Transcribed

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:  These carry genetic information that influences the same traits.  They 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.

Standard Chromosomal Complement in Various Organisms Organism Chromosome Number in Somatic Cells Chromosome Number in Gametes Human4623 Chimpanzee4824 Gorilla4824 Dog7839 Chicken7839

Standard Chromosomal Complement in Various Organisms Organism Chromosome Number in Somatic Cells Chromosome Number in Gametes Frog2613 Housefly126 Onion168 Corn2010 Tobacco4824

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.

Mitosis

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.

Meiosis

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.

Mitosis and Meiosis Compared

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.

Polymerase Chain Reaction (PCR)  Enables scientists to make of copies of DNA samples which can then be analyzed.  Scientists use PCR to:  Examine nucleotide sequences in Neandertal fossils and Egyptian mummies  Identify DNA sequences unique to an individual.  Identify remains of victims of 911.  Exonerate people wrongly convicted of crimes and imprisoned for years.

Human Genome Project  Effort begun in 1990 to sequence the entire human genome, which consists of some 3 billion bases comprising approximately 25,000 to 30,000 genes.  The goal was achieved in  Scientists are still several years away from identifying the functions of many of the proteins produced by these genes.

Quick Quiz

1. The entire sequence of DNA bases on the chromosome that code for a particular polypeptide chain is a(n) a) ribosome. b) amino acid. c) gene. d) polypeptide chain.

Answer: c  The entire sequence of DNA bases on the chromosome that code for a particular polypeptide chain is a gene.

2. Which of the following is NOT true about RNA? a) It is single stranded. b) Some forms of RNA are involved with protein synthesis. c) It has a different type of sugar than DNA has. d) It contains the base thymine.

Answer : d  The following is NOT true about RNA.  It contains the base thymine.

3. Which of the following is true for meiosis? a) It has only one division that duplicates the parent cell exactly. b) It produces gametes. c) When a mutation occurs it affects only the individual. d) It has no effect on evolution.

Answer: b  The following is true for meiosis:  It produces gametes.

4. Chromosomes that carry genes for the same traits are _________________ pairs.

Answer: homologous Chromosomes that carry genes for the same traits are homologous pairs.

5. Somatic cell division is termed _________________.

Answer: mitosis  Somatic cell division is termed mitosis.