CELL BIOLOGYDaniel Svozil

Molecular biology• Though all aspects of biology can be studied at the

molecular level, molecular biology is usually restricted to the molecules of genes/gene products/heredity – molecular genetics

• Experiments in molecular biology are done using model organisms

• Two classes of organism• Prokaryotes• Eukaryotes

Prokaryotes vs. Eukaryotes

bacteria• 1 bacteria = 1 cell• lower organisms • Escherichia coli (E. coli)

• plasma membrane• nucleus• organelles

Cells in eukaryotes• body (somatic) cells

• differentiated into special cell types (brain cells, liver cells …)• produce by simple cell division – mitosis

• sex cells (gametes)• egg, sperm• used for sexual reproduction (only eukaryotes)• meiosis – reduction of the amount of genetic material

Eukaryotic chromosomes• Threadlike DNA, carries genes• Each organism has specific number of chromosomes• Sex chromosomes (determine gender – XX (female), XY

(male)), autosomal chromosomes• 46 in human, 2 sex, 44 autosomal• Come in pairs (two in pair have the same shape and

same set of genes (but different alleles)), homologs, diploid

Cell cycle• Division of the cell in two exact copies.

Genetics for Dummies, Tara Robinson

homologous chromosomes

homologous chromosomes copied

http://www.bothbrainsandbeauty.com/wp-content/uploads/2009/11/chromosomes.jpg

Karyotype

Genetics for Dummies, Tara Robinson

Mitosis

2c

4c

2c 2c

diploid (2n) mother cell

identical diploid (2n) daughter cells

division

DNA synthesis

Sexual reproduction• Egg gets fertilized by sperm. Zygote is cretaed.• Zygote is diploid (divides by mitosis), thus the gametes

must be haploid!• In organism with diploid

cells, how do you get haploid?

• Meiosis (another type of cell division)

Meiosis• The result of meiosis is haploid cell.• From one parent diploid cell you get four cells haploid. In

addition, homologous chromosomes go through recombination.

http://www.britannica.com

DNA – The Basis of Life

DNA• Every living thing on Earth.• Biomacromolecule

• Consists of repeating units

• DNA in organism does not usually exist in one piece• chromosomes

Deconstructing DNA• http://www.umass.edu/molvis/tutorials/dna/• bases, deoxyribose sugar, phosphate – nucleotide• Bases are flat → stacking• pYrimidines – C, T• puRines – A, G

O3‘

O5‘

C3‘

C5‘

base

sugar

Nucleoside

Nucleotide• nucleosides are interconnected by phospohodiester bond• nucleotide monophosphate

nucleoside

Bases complement each other.

Chargaffs’ rules• amount of G = C• amount of A = T

DNA conformations

B-DNA A-DNA Z-DNA

B

A

Z

Biological role of different DNAs• B-DNA

• canonical DNA• predominant

• A-DNA• Conditions of lower humidity, common in crystallographic

experiments. However, they’re artificial.• In vivo – local conformations induced e.g. by interaction with

proteins.

• Z-DNA• No definite biological significance found up to now.• It is commonly believed to provide torsional strain relief

(supercoiling) while DNA transcription occurs. • The potential to form a Z-DNA structure also correlates with

regions of active transcription.

Different sets of DNA• nuclear DNA

• cell’s nucleus• majority of functions cell carries out• sequencing the genome – scientists mean nuclear DNA

• mitochondrial DNA• mtDNA• circular, in human very short (17 kbp) with 37 genes (controling

cellular metabolism)• all mtDNA comes from mom

• chloroplast DNA• cpDNA• circular and fairly large (120 – 160 kbp), with only 120 genes• inheritance is either maternal, or paternal

Structure of DNA in the eukaryotic cell

• DNA in human chromosomes: 3.2 109 bp. As we’re diploid: 6.4 109 bp.

• 0.33 nm per bp 2.1 m in each nucleus, size of the nucleus: 5-10 m across

• DNA is highly compacted. Combination DNA + proteins.• During interphase, when cells are not dividing, the genetic

material exists as a nucleoprotein complex called chromatin, which is dispersed through much of the nucleus.

• Further folding and compaction of chromatin during mitosis produces the visible metaphase chromosomes.

• euchromatin – extended• heterochromatin – condensed

Chromatin

nucleosome

Nucleosome

Central dogma of molecular biology

Wikipedia

Molecular Cell Biology, Harvey Lodish