Presentation on theme: "DNA and Cell Reproduction. Structure of DNA Deoxyribonucleic Acid DNA is one of the two nucleic acids –DNA –RNA It is composed of repeating subunits (monomers)"— Presentation transcript:
DNA and Cell Reproduction
Structure of DNA Deoxyribonucleic Acid DNA is one of the two nucleic acids –DNA –RNA It is composed of repeating subunits (monomers) called NUCLEOTIDES. A nucleotide has three components –– deoxyribose sugar –A phosphate –A nitrogenous base
Nitrogenous Bases Adenine (A) Guanine (G) Cytosine (C) Thymine (T) These are grouped into –Purines – two carbon rings (A and G) –Pyrimidines – one carbon ring (T and C)
The Double Helix A always pairs with T C always pairs with G
DNA Replication 1. Helicase enzymes cause the strands to unwind and separate. (The point where the two chains separate is the replication fork.) 2. DNA polymerases bind to each chain of DNA. 3. The DNA polymerase moves along the chain and aids in the addition of nucleotides for the formation of the new chain.
DNA replication does not begin at one end of the molecule and proceed to the other end. DNA replication occurs in many points simultaneously along the molecule.
Accuracy and Repair DNA replication is very accurate – about 1 error in every 10,000 paired nucleotides. DNA has the ability to proofread and repair mistakes. An error in replication is known as a MUTATION. DNA may also be damaged by a variety of agents which include chemicals and ultraviolet light.
What is an enzyme? An enzyme is a catalyst that is usually an protein. Enzymes are substrate specific. They have a lock and key fit. Allow reaction in living organisms to occur at lower temperatures by reducing the activation energy. Usually end in –ase and identify the substrate the work on. Example: proteases, nucleases
Chromosomes are Composed of DNA DNA in humans is about 6 billion pairs of nucleotides and is located in the nucleus as chromosomes. Each chromosome is a single DNA molecule associated with protein. HISTONESDNA in eukaryotes wraps tightly around proteins called HISTONES. A chromosome consists of two identical halves called chromatids.
The consticted area of each chromosome is called a chromatid (a protein disk), Between cell divisions, the DNA is not so tightly wound and is called chromatin. Centromere chromatid
Each species has a characteristic number of chromosomes – see pate 146. Human and animal chromosomes are categorized as sex chromosomes or autosomes. –Sex chromosomes determine the sex of the organism –Autosomes are all other chromosomes Every cell has two copies of autosomes. These copies are called homologous chromosomes
HOMOLOGOUS CHROMOSOMES SAME SIZE SAME SHAPE SAME GENES FOR THE SAME TRAITS Homologous chromosomes may be arranged in order to form a karyotype.
Diploid – having two sets of chromosomes –Both chromosomes of the homologous pair –Both of the sex chromosomes Haploid – having only one set of chromosomes –Half of the chromosome number –One chromosome of the homologous pair –Only one sex chromosome
Cell Division Prokaryote –No membrane bound organelles –Only one circular chromosome –Reproduce by binary fission Eukaryotes –Membrane bound organelles –DNA is in chromosomes –Two processes for cell division Mitosis Meiosis