Minggu, 06 Juli 2008

meiosis and sexual lifecycle

Meiosis and Sexual Life Cycles

Key Concepts

1.Keturunan mendapatkan gen dari orang tuanya melalui pewarisan kromosom

2.Fertilisasi dan meiosis terjadi bergantian dalam siklus hidup seksual

3.Meiosis mengurangi jumlah set kromosom dari diploid ke haploid

4.Variasi genetik yang dihasilkan dari siklus hidup seksual memberikan kontribusi terhadap evolusi

Hereditary Similarity and Variation

Ú Living organisms are distinguished by their ability to reproduce their own kind. Only oak trees produce oaks, and only elephants can make more elephants.
Ú The transmission of traits from one generation to the next is called inheritance or heredity
Ú Along with inherited similarity, there is also variation : Offspring differ somewhat in appearance from parents and siblings
Ú Genetics is the scientific study of heredity and hereditary variation

Inheritance of Genes

Ú Parents endow their offspring with coded information in the form of hereditary units called genes . Our genes program the specific traits that emerge as we develop from fertilized eggs into adults.
Ú Genes are segments of DNA. Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action produces an organism′s inherited traits
Ú In animals and plants, reproductive cells called gametes are the vehicles that transmit genes from one generation to the next. During fertilization, male and female gametes (sperm and eggs) unite, thereby passing on the genes of both parents to their offspring

DNA and Chromosome
Ú Except for tiny amounts of DNA in mitochondria and chloroplasts, the DNA of a eukaryotic cell is subdivided into chromosomes within the nucleus. Every living species has a characteristic number of chromosomes
Ú Each chromosome consists of a single long DNA molecule elaborately coiled in association with various proteins. One chromosome includes several hundred to a few thousand genes, each of which is a specific sequence of nucleotides within the DNA molecule
Ú A gene′s specific location along the length of a chromosome is called the gene′s locus (plural, loci ).

Comparison of Asexual and Sexual Reproduction

Ú In asexual reproduction:
– a single individual is the sole parent and passes copies of all its genes to its offspring ----- produce offspring that are exact copies -------gives rise to a clone , a group of genetically identical individuals
– Genetic differences occasionally arise in asexually reproducing organisms as a result of changes in the DNA called mutations
Ú In sexual reproduction:
– two parents give rise to offspring that have unique combinations of genes inherited from the two parents
– offspring of sexual reproduction vary genetically from their siblings and both parents
– What mechanisms generate this genetic variation? The key is the behavior of chromosomes during the sexual life cycle
Sets of Chromosomes in Human Cells

Ú In humans, each somatic cell has 46 chromosomes. During mitosis, the chromosomes become condensed enough to be visible in a light microscope.
Ú Because chromosomes differ in size, in the positions of their centromeres, and in the pattern of colored bands produced by certain stains, they can be distinguished from one another by microscopic examination when sufficiently condensed
Ú Karyotype: A display of the chromosome pairs of a cell arranged by size and shape
Ú homologous chromosomes: Chromosome pairs of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism′s father, the other from the mother.
Ú The two distinct chromosomes referred to as X and Y are an important exception to the general pattern of homologous chromosomes in human somatic cells
Ú Most of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y chromosome has genes lacking on the X
Ú The X and Y chromosomes are called sex chromosomes . The other chromosomes are called autosomes