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