Genetic Recombination

-Each chromosome has hundreds of thousands of genes (map of the human genome)
-Genes code for proteins, which ribosomes synthesize
-Linked genes are those that are located on the same chromosome, and are frequently inherited together
Crossing Over
Crossing Over
-However, during meiosis, the chromosomes trade genes with the corresponding (homologous) chromosomesof the other parent. This process, "crossing over," allows genes to be inherited independently of each other, though genes located close together are usually inherited together
-Scientists are able to map the chromosome based on the frequency of genes occurring together.
-Genetic recombination is the production of offspring with new combinations of traits inherited from both parents



Genetic Disorders
Chromosome
Chromosome

-Nondisjunction occurs when tetrad chromosomes do not separate during Anaphase I, or sister chromatids do not separate during Anaphase II. This causes the gamete to receive either two of a certain chromosome, or none
-Aneuploidy means abnormal chromosome number. Trisomic cells have three copies of a chromosome, and monosomic cells have only one. In the case of survival, this leads to a distinct phenotype,
-Sometimes, an organism experiences nondisjunction of all of its chromosomes. This leads to polyploidy, and the organism will have three sets of chromosomes
Chromosome breakage problems:
-Deletion- chromosome fragment breaks off of the chromosome
-Duplication- a fragment attaches to a sister chromatid
-Inversion- a fragment joins a nonhomologous chromosome

-Down syndrome- 3 copies of chromosome 21. Causes mental and physical problems in development
-Klinefelter's syndrome- XXY. Have male sex organs and normal intelligence, but are sterile, and may have some feminine characteristics.
-XYY- tall, healthy male
-Trisomy X- XXX. Healthy female
-Turner's syndrome- X. Immature female


DNA Replication


-DNA strands serve as templates for new complementary strands, because of base pairing
-There are many enzymes involved in DNA replication, and many that proofread and repair damage in the DNA
-The phosphate backbones of DNA are arranged in opposite directions; one side goes 3'-5', while the other goes 5'-3'

First, the enzyme helicase unwinds and separates the two complementary strands of DNA. Then:
Leading Strand
Lagging Strand
Primase primes the parent strand for replication by creating a short chain of RNA
Primase primes for Okazaki fragments (it must do this in several different places)
DNA polymerase adds a chain of DNA to the RNA in a process called elongation
DNA polymerase elongates the individual Okazaki fragments (this is necessary because DNA can only by synthesized in a 3'-5' direction
DNA polymerase replaces the RNA primer with DNA
DNA polymerase replaces the RNA primer with DNA

Ligase joins the Okazaki fragments



external image ReplicationFork.gif