Reproduction and Heredity
Chapters 8-9
Heredity
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Heredity = biological inheritance
– Or why we look like our parents!
Genetics is study of heredity
Sexual reproduction
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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
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In each diploid cell, chromosomes have a partner or homologous chromosome
or homolog
Chromosome structure
Meiosis
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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
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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
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Alleles are alternate forms of the same gene
Alleles occupy the same locus (plural = loci) on homologous chromosomes
Mendel’s crosses
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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
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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
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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
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Phenotype is a product of genes, environment and a gene by environmental
interaction
Asexual reproduction
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Also known as vegetative reproduction
Progeny are identical to parent, or are clones
Strawberry
Kalanchoë
Fern
Advantages of sexual reproduction
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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
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Copying DNA
Catalyzed by DNA polymerase
RNA
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Nucleotide sugar is ribose rather than deoxyribose
Thymine replaced by uracil
Single-stranded
Information transfer
Transcription
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RNA polymerase is used in transcription
Genetic code is universal
RNA
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mRNA is translated into protein
tRNA has two attachment sites, one for mRNA and one for amino acid
Translation occurs at ribosome
Polypeptide sorting
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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
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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
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50-70% of genome is single-copy DNA
– Protein-coding DNA
Introns and extrons
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The protein coding sequences of DNA or structural genes are not continuous
Interrupted by noncoding introns
Exons are coding sequences
Next time….
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Plant biotechnology and genomics