8.2 Structure of DNA

KEY CONCEPT DNA structure is the same in all organisms.

8.2 Structure of DNA

• DNA is short for deoxyribonucleic acid

8.2 Structure of DNA

DNA is composed of four types of nucleotides.

• DNA is made up of a long chain of nucleotides.• Each nucleotide has three parts.

– a phosphate group– a deoxyribose sugar– a nitrogen-containing base

phosphate group

deoxyribose (sugar)

nitrogen-containingbase

8.2 Structure of DNA

• The nitrogen containing bases are the only difference in the four nucleotides. • 2 categories of nitrogenous bases:

• Pyrimidines: single ring, ex. thymine and cytosine• Purines: double ring, ex. adenine and guanine

8.2 Structure of DNA

•Chargaff’s rule states that the amount of thymine = adenine and that the amount of cytosine = guanine in DNA.

8.2 Structure of DNA

Franklin’s x-ray images suggested that DNA was a double helix of even width.

• Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff.

8.2 Structure of DNA

Watson and Crick determined the three-dimensional structure of DNA by building models.

• They realized that DNA is a double helix that is made up of a sugar-phosphate backbone on the outside with bases on the inside.

8.2 Structure of DNA

TA CG

Nucleotides always pair in the same way.

• The base-pairing rules show how nucleotides always pair up in DNA.

• Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width.

– A pairs with T– C pairs with G

8.2 Structure of DNA

• The DNA backbone: alternating deoxyribose (sugar) and phosphate group is connected by strong covalent bonds.

hydrogen bond covalent bond

• The nitrogen bases are connected by hydrogen bonds.

Bonding in DNA Phosphate group ↓ Deoxyribose

Sugar →

8.2 Structure of DNA

hydrogen bond covalent bond

T G A C T A | | | | | |A C T G A T

8.2 Structure of DNA

8.2 Structure of DNA

All the DNA in your body is identical….

Two haploid cells meet (fertilization) then mitosis takes over

8.2 Structure of DNA

You have body cells and gametes. • Body cells are also called somatic cells.• Germ cells develop into gametes.

– Germ cells are located in the ovaries and testes.– Gametes are sex cells: egg and sperm.

body cells sex cells (sperm) sex cells (egg)

8.2 Structure of DNA

• Your body cells have 23 pairs of chromosomes.– Homologous pairs of

chromosomes have the same structure.

– For each homologous pair, one chromosome comes from each parent.

• Chromosome pairs 1-22 are autosomes.

• Sex chromosomes, X and Y, determine gender in mammals.

Your cells have autosomes and sex chromosomes.

8.2 Structure of DNA

Body cells are diploid; gametes are haploid.

• Fertilization between egg and sperm occurs in sexual reproduction.

• Diploid (2n) cells have two copies of every chromosome. (you have 2 sets of chromosomes)– Body cells are diploid.– One set comes from each parent.

8.2 Structure of DNA

• Haploid (n) cells have one copy of every chromosome.

– Gametes are haploid.– Gametes have 22 autosomes and 1 sex chromosome.

8.2 Structure of DNA

• Chromosome number must be maintained in animals.• Mitosis and meiosis are types of nuclear division that

make different types of cells.• Mitosis makes

more diploid cells.

8.2 Structure of DNA

• Meiosis makes haploid cells from diploid cells.

– Meiosis occurs in sex cells. – Meiosis produces gametes.

8.2 Structure of DNA

• Meiosis I and meiosis II each have four phases, similar to those in mitosis.

homologous chromosomes

sisterchromatids

sisterchromatids

– Homologous chromosomes: Pairs of chromosomes that separate in meiosis I; Similar but not identical genes

– Sister chromatids:divide in meiosis II; copies of the same chromosome.

Tetrad: 4 chromatidsCrossing over: exchange of similar DNA

8.2 Structure of DNA

• Meiosis I occurs after DNA has been replicated.

• Meiosis I divides homologous chromosomes in four phases.

8.2 Structure of DNA

Prophase I

•Homologous chromosomes form tetrads

•Crossing over (genetic recombination)

8.2 Structure of DNACrossing over during meiosis increases genetic diversity. • Crossing over is the exchange of chromosome

segments between homologous chromosomes.– occurs during prophase I of meiosis I when

homologous chromosomes form tetrads.– results in new combinations of genes

8.2 Structure of DNA

Metaphase I

Homologous chromosomes line up in middle

8.2 Structure of DNA

Anaphase I

• chromosome pairs separate

8.2 Structure of DNA

Telophase I and Cytokinesis

– separation into two haploid cells

8.2 Structure of DNA

Prophase II

• Each cell prepares for division

8.2 Structure of DNA

Metaphase II

• chromosomes line up in the middle

8.2 Structure of DNA

Anaphase II

– sister chromatids separate

8.2 Structure of DNA

Telophase II and Cytokinesis

– produces 4 unique haploid cells

8.2 Structure of DNA

Haploid cells develop into mature gametes. • Gametogenesis differs between

females and males.

Spermatogenesis– Sperm become motile.– Sperm primarily contribute DNA

to an embryo.

Oogenesis– Eggs contribute DNA, cytoplasm,

and organelles to an embryo.– During meiosis, the egg gets

most of the contents; the other 3 cells form polar bodies.

8.2 Structure of DNA

• Meiosis differs from mitosis in significant ways.

– Meiosis has two cell divisions while mitosis has one.– In mitosis, homologous chromosomes never pair up.– Meiosis results in haploid cells; mitosis results in diploid

cells.