Genes, Chromosomes and DNA

The structure of DNA Composed of 4 nucleotide bases, 5 carbon sugar and phosphate. Base pair = rungs of a ladder. Edges = sugar-phosphate backbone. Double Helix Anti-Parallel

The structure of DNA Figure 2.21

DNA Replication Adenine (A) always base pairs with thymine (T) Guanine (G) always base pairs with Cytosine (C) Requires steps: H bonds break as enzymes unwind molecule New nucleotides (always in nucleus) fit into place beside old strand in a process called Complementary Base Pairing.

DNA Replication Figure 2.22a Remember – the two strands run in opposite directions Synthesis of a new (daughter) strand occurs in the opposite direction of the old (parental) strand. Complementary base-pairing occurs A with T and G with C G and C have three hydrogen bonds A and T have two hydrogen bonds

DNA Replication Figure 2.22b

DNA Replication Figure 2.22c New nucleotides joined together by enzyme DNA Polymerase

DNA Replication Each new double helix is composed of an old (parental) strand and a new (daughter) strand. As each strand acts as a template, process is called Semi-conservative Replication. Replication errors can occur. Cell has repair enzymes that usually fix problem. An error that persists is a mutation. This is permanent, and alters the phenotype.

Gregor Mendel observed phenotypes and formed hypotheses How do offspring come to resemble their parents physically? Genetics begins with the unifying assumption that biological inheritance is carried by structures called Genes. The same basic patterns of inheritance apply to most organisms. The inheritance of some human traits can be explained from work on plants Sex-linked traits in humans is more complicated

Terms: Phenotype Genotype An organism’s physical traits An organism’s genetic makeup

Allele Allele: Alternate form of a gene at same position on pair of chromosomes that affect the same trait. Dominant Allele: Capital Letter--O Recessive Allele: lowercase letter--o Homozygous Dominant--OO Homozygous Recessive--oo Heterozygous--Oo

Dominant Recessive Dominant Recessive Pod shape Inflated Constricted Flower color Purple White Pod Color Green Yellow Flower position Axial Terminal Seed color Yellow Green Stem length Tall Dwarf Seed shape Round Wrinkled

Monohybrid Crosses F2 = 3:1 ratio P Generation (true-breeding parents) Purple flowers White flowers All plants have purple flowers F1 Generation Fertilization among F1 plants (F1  F1) F2 Generation F2 = 3:1 ratio 3/4 of plants have purple flowers 1/4 of plants have white flowers

Using a Punnett square to explain the results of a monohybrid cross P plants PP PP Using a Punnett square to explain the results of a monohybrid cross Gametes All P All p F1 plants: (hybrids) All Pp Gametes 1/2 P 1/2 p P P Eggs Sperm F2 plants: p PP p Phenotypic ratio 3 purple : 1 white Pp Pp pp Genotypic ratio 1 PP : 2 Pp : 1 pp Figure 9.8b

from the monohybrid crosses, Mendel derived 4 hypotheses… from the monohybrid crosses, Mendel derived 4 hypotheses….combined, we now refer to these as… = Mendel’s Principle of Segregation There are alternative forms of genes, now called alleles For each characteristic, each organism has two genes Gametes carry only one allele for each inherited characteristic Alleles can be dominant or recessive

Mendel’s Principle of Independent Assortment What happens when you follow the inheritance of more than a single trait at one time? How do two different traits get passed to offspring? A Dihybrid Cross

Dihybrid Cross 9:3:3:1 RrYy RrYy 9/16 3/16 3/16 1/16 RRYY rryy Gametes Eggs Sperm RrYy RrYy rY rY RRYY Ry RrYY RrYY Ry ry ry RRYy rrYY RRYy RrYy RrYy RrYy RrYy Yellow round 9/16 rrYy RRyy rrYy Green round 3/16 Rryy Rryy Yellow wrinkled 3/16 rryy 1/16 Green wrinkled 9:3:3:1

Mendel’s principle of independent assortment Each pair of alleles segregates independently of the other pairs during gamete formation P a B P a b Genotype: PP aa Bb

Incomplete Dominance in Plants and People Red RR White rr In incomplete dominance F1 hybrids have an appearance in between the phenotypes of the two parents Gametes R r Pink Rr 1/2 Gametes 1/2 R r Eggs 1/2 R 1/2 R Sperm Red RR 1/2 r 1/2 r Pink Rr Pink rR White rr Figure 9.18

Types of cells Not all cells of an organisms have the same number of chromosomes. Two types of cells: Somatic Cells Gametes

Somatic Cells Non-sex Cells. Diploid These cells do not carry genetic information for sexual reproduction. Contain a full compliment of chromosomes Characteristic to their species. Referred to as the diploid number of chromosomes. Diploid Means double number. Designated 2n All somatic cells in an organism have the 2n or diploid number of chromosomes.

Gametes Haploid Sex Cells Means single number. Designated n All gametes formed by an organism have the n or haploid number of chromosomes. Sex Cells Cell which carry genetic information for sexual reproduction. Contain one half the compliment of chromosomes characteristic to their species. Referred to as the haploid number of chromosomes.

Human Life Cycle Adults produce gametes--egg and sperm. Gametes fuse to produce zygote. Zygote grows and develops to produce baby. Meiosis--process of division that produces gametes. Mitosis--process of replication and division required for growth. Adults, zygote and baby--2n. 2n=diploid Gametes--n. n=haploid

Mitosis Process of division that produces two daughter cells with identical chromosomal content of parent cell. Mitosis is one stage of the cell cycle. Cell cycle--cycle of stages a cell goes through in order to grow and divide. Stages: I=Interphase, Growth 1=G1, DNA synthesis=S, Growth 2=G2, Mitosis=M

The Human cell cycle Interphase--G1, S, G2 Mitosis--M G1--growth S--DNA Synthesis, replication G2--growth M: mitosis-- nuclear division cytokinesis--cell division pg.339

Stages of Division- Mitosis Prophase--nuclear envelope breakdown, chromosome condensation, spindle formation. Metaphase--chromosomes are lined up precisely on the metaphase plate, or middle of the cell. Anaphase--spindle pulls sister chromatids apart. Telophase--chromatids begin to decondense and become chromatin. Spindle disappears. Cytokinesis--divide cell and organelles. Actin ring, or cleavage furrow splits cell.

Gamete Production -Meiosis In order to reproduce we must produce gametes. Gametes are sperm and egg. Why is that siblings are not identical? Meiosis blends DNA from parental contributions to produce a mixed up “half” or haploid, set of DNA. Crossing over is critical for producing haploid DNA with genetic diversity.

The Process of Meiosis Interphase Haploid gametes are produced in diploid organisms Two consecutive divisions occur, meiosis I and meiosis II, preceded by interphase Centrosomes (with centriole pairs) Nuclear envelope Chromatin Chromosomes duplicate

Prophase -I Replicated pairs of chromosomes line up side by side. These pairs are called Homologous--both have same gene order (gene for eye color, hair color, etc). Sister chromatid from one pair interact with a Sister chromatid from another pair. One sister is from father, one sister from mother, but they have same gene order.

Prophase -I This interaction is called Synapsis. Synapsis results in the formation of a Tetrad (4 sisters together). Crossing over swaps sections of homologous genes.

Meiosis - I Figure 2.9 (1) Prophase I Metaphase I Anaphase I Telophase I

Meiosis - II Figure 2.9 (2) Prophase II Metaphase II Anaphase II Telophase II

Meiosis I: Homologous chromosomes separate Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Sites of crossing over Microtubules attached to Chromosomes Sister chromatids remain attached Cleavage furrow Spindle Sister chromatids Tetrad Centromere Homologous chromosomes pair and exchange segments Tetrads line up Pairs of homologous chromosomes split up Two haploid cells form: chromosomes are still double

Sister chromatids separate Meiosis II Meiosis II: Sister chromatids separate Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

Genes on sex chromosomes determine Sex and sex-linked traits Micrograph of the chromosomes of an organism paired and numbered. Used to check for chromosomal abnormalities in individuals.

Summary Genetics is the study of biological traits. These traits are coded for in genes, which are parts of chromosomes. An Allele is a variant of a gene. These can be dominant or recessive, and these are the basis of inherited traits, both structural and behavioral. Chromosomes exist as homologous pairs.

Summary Somatic Cells - Non-sex Cells. Contain a full compliment of chromosomes. Characteristic to their species. Referred to as the diploid number of chromosomes. Gametes - Sex Cells. Cell which carry genetic information for sexual reproduction. Contain one half the compliment of chromosomes characteristic to their species .Referred to as the haploid number of chromosomes.

The End. Any Questions?