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Reproduction and Heredity Chapters 8-9 Heredity • • Heredity = biological inheritance – Or why we look like our parents! Genetics is study of heredity Sexual reproduction • Alternation between meiosis and fertilization – Meiosis is the process of nuclear division in which the chromosome number is halved • Usually 2n to n – Fertilization is the process where two haploid cells (gametes) fuse Homologs • In each diploid cell, chromosomes have a partner or homologous chromosome or homolog Chromosome structure Meiosis • Through meiosis and cytokinesis, a single diploid cell becomes four haploid cells – Haploid cells can be gametes • Unite to form zygote then divides – Haploid cells can be meiospores • Divides without uniting with another cell Meiosis • Meiosis produces genetic variability through genetic recombination – Crossing over, as well as possible haploid chromosome combinations Meiosis vs. mitosis • • • Two nuclear divisions in meiosis, only one in mitosis Four haploid cells result from meiosis, Two diploid cells result from mitosis Nuclei produced by mitosis have identical gene combinations Genetics • • Alleles are alternate forms of the same gene Alleles occupy the same locus (plural = loci) on homologous chromosomes Mendel’s crosses • Complete dominant versus recessive alleles Mendel’s two principles • Principle of segregation – Alleles (of one gene) separate during meiosis • Principle of independent assortment – Alleles separate independently of alleles of other genes Linkage • Genes found in close proximity do not always segregate independently • Linkage maps are created Mutations • Change in heredity state of an organism is a mutation – Point mutation – Deletion – Duplication – Transposons Chromosomal mutations • Pieces of chromosomes – Inversion • Piece of chromosome inserted backwards – Translocation • Exchange of parts between two non-homologous chromsomes • Entire chromosomes – Aneuploidy is any number different from normal – Polyploidy is duplication of entire set of chromosomes Allele interactions • • • • • Incomplete dominance can occur Multiple alleles can occur Epistasis can occur Some traits are controlled by more than one gene – polygenic traits Pleiotropy can occur Genes in plastids • Plastids and mitochondria contain their own DNA • Cytoplasmic inheritance is usually determined by female parent, and is therefore known as maternal inheritance • Leaf mottling is a character controlled by maternal inheritance in chloroplasts Role of the environment • Phenotype is a product of genes, environment and a gene by environmental interaction Asexual reproduction • • Also known as vegetative reproduction Progeny are identical to parent, or are clones Strawberry Kalanchoë Fern Advantages of sexual reproduction • • • Produces genetic diversity, which allows the population to adapt through the process of evolution by natural selection Requires a lot of energy (so it must be advantageous!) Widely used Chemistry of heredity DNA • Double helix with sugar-phosphate backbone and nitrogenous base pairs • Strands are antiparallel and complementary DNA Replication • • Copying DNA Catalyzed by DNA polymerase RNA • • • Nucleotide sugar is ribose rather than deoxyribose Thymine replaced by uracil Single-stranded Information transfer Transcription • RNA polymerase is used in transcription Genetic code is universal RNA • • • mRNA is translated into protein tRNA has two attachment sites, one for mRNA and one for amino acid Translation occurs at ribosome Polypeptide sorting • Polypeptide targeting assures that each peptide moves to the correct location within a cell – Cotranslational transport • Ribosomes on endoplasmic reticulum – Posttranslational transport • Free ribosomes Gene regulation • Only certain genes are expressed • Totipotency – an entire plant can be regenerated from a differentiated cell – Points towards regulation of gene expression • Transcription factors directly or indirectly affect the initiation of transcription Genome size varies Repetitive sequences • • Tandemly repeated DNA – Repeated DNA are arranged in tandem series – Simple-sequence repeated DNA (satellite DNA) Interspersed repeated DNA units – Dispersed throughout the DNA – 20-40% of most multicellular organisms is this type Unique DNA • 50-70% of genome is single-copy DNA – Protein-coding DNA Introns and extrons • • • The protein coding sequences of DNA or structural genes are not continuous Interrupted by noncoding introns Exons are coding sequences Next time…. • Plant biotechnology and genomics