Chapter 5 DNA and Chromosomes
DNA as the genetic material
Heat-killed bacteria can transform living cells S Smooth R Rough Fred Griffith, 1920
DNA is the genetic material Oswald Avery Colin MacLeod Maclyn McCarty 1935 1. The first evidence that DNA could serve as the genetic material 2. The genetic material was likely to be made of protein
Genes are made of DNA Martha Chase Alfred Hershey 1952 The empty viral coats T2 virus Radioactively label (w/o S) (w/o P)
The Nobel Prize in Physiology or Medicine 1969 The replication mechanism and the genetic structure of viruses
What is gene? Genes the information-containing elements that determine the characteristics of a species as a whole and of the individuals within it. A gene is usually defined as a segment of DNA that contains the instructions for making a particular protein (or, in some cases, a set of closely related proteins).
What is genome? Genome the totality of DNA genetic information in each cell is called its genome. = 3.2x10 9 nucleotides.
What is chromosome? Chromosomes in eucaryotic cells, very long double-strand DNA molecules are packaged into structure called chromosomes.
Homologous chromosomes (homologs) Homologus chromosomes the maternal and paternal chromosomes of a pair.
Chromosomes become visible as cells prepare to divide DAPI DNA staining
Nucleotide
DNA is made of four nucleotide building blocks Sugar-phosphate backbone Polynucleotide chain Antiparallel
The two strands of the DNA double helix are hold together by hydrogen bonds between complementary base pairs Base pair Antiparallel 5 -PO 4 3 -OH 5 -PO 4 3 -OH
A space-filling model shows the conformation of DNA double helix 10 bases/turn P O H N
Genetic code Genetic code the exact correspondence between the 4-letter nucleotide alphabet of DNA and the 20-letter amino acid alphabet of proteins
Transcription and translation
Linear messages come in many forms
Gene contain information to make proteins
Each human chromosome can be painted a different color to allow its unambiguous identification under the light microscope DNA hybridization Karyotype Karyotypes - The homologus chromosomes are numbered and arranged in pairs
Unique banding patterns allow the identification of the human chromosomes Giemsa stain A-T rich centromere rrnas centromere
Abnormal chromosomes are associated with some inherited genetic defects Ataxia / Chromosome 12 Chromosome 4
Genes are arranged along the chromosomes
Junk DNA Gene
Closely related species can have very different chromosome numbers The two species shown have roughly the same number of genes
The cell cycle
The replication and segregation of chromosomes occurs through an ordered cell cycle in proliferating cells microtubules G0/G1 S G2 M
Three DNA sequence elements are needed to produce a eucaryotic chromosome that can be replicated then segregation at mitosis x2 x1
Replication origin
DNA Replication
Centromere
The role of telomere in life span
A typical mitotic chromosome is highly compact Mitotic chromosome SEM
Interphase chromosomes occupy different territories within the nucleus
The nucleolus is the most prominent structure in the interphase nucleus (Chromosome13, 14, 15, 21, 22) TEM
Nucleolus Nucleolus The parts of the different chromosomes carrying genes for ribosomal RNA (rrna) cluster together.
Heterochromatin & Euchromatin Heterochromatin The most highly condensed form of interpahse chromatin is called heterochromatin.
DNA in interphase chromosome is less compact than that in mitotic chromosomes Interphase nuclear DNA Mitotic chromosome TEM
Nucleosomes can be seen in the electron microscope Chromatin Unpacked, decondensed 30-nm thick beads-on-a-string Nucleosome Nucleosome core particle DNA TEM
Chromatin Chromatin The complex of both classes of protein with nuclear DNA. Most of the chromatin is in the form of fibers, each with a diameter of about 30 nm.
Nucleosome Nucleosome The first and most fundamental level of chromatin packing.
Histones
Nucleosomes contain DNA wrapped around a protein core of eight histone molecules High proportion of positively charged amino acids (lysine and arginine) Negatively-charged sugar-phosphate backbone of DNA 80bp
The structure of the nucleosome core particle, as determined by X-ray diffraction analysis, reveals how DNA is tightly wrapped around a disc-shaped histone core Highly conserved 1.7 turns in a left-handed coil N N
A linker histone helps to pull nucleosomes together into the 30-nm fiber
DNA packing occurs on several levels in chromosomes Interphase chromatin + Histone H1 Mitotic chromasome
The mitotic chromosome is formed from tightly packed chromatin SEM
Chromosome & Chromatids
Chromatin-remodeling complexes reposition the DNA wrapped around nucleosomes DNA-binding proteins
The pattern of modification of histone tails can dictate how a stretch of chromatin is treated by the cell To bind specific proteins N Methyl group (+) Acetyl group (-) Phosphate (-)
Interphase chromatin Heterochromatin: 10% Euchromatin: 90%
Expression of a gene can be altered by moving it to another location in the genome (expression) Position effect (non-expression)
An X chromosome can be inactivated by heterochromatin formation Double dose of X-chromosome prodicts would be lethal
The structure of chromatin varies along a single interphase chromosome
How histone modifications may be inherited by daughter chromosomes Restore the parental modification
Epigenetic inheritance Epigenetics The study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence.