DNA and Chromosomes BIO 224 Intro to Molecular and Cell Biology.

DNA and Chromosomes

BIO 224

Intro to Molecular and Cell Biology

Heredity

• All organisms get genetic information from parents

• All cells come from pre-existing cells• All genetic material needs to be replicated

and passed on from parent to progeny at each cell division

• Molecular biology seeks to understand heredity at the molecular level

Genes and Chromosomes

• Mendel described classical genetics principles with pea plant experiments

• Characteristics carried on inherited factors called genes

• Sexually reproducing organisms get one copy from each parent, called alleles

• Dominant alleles mask the recessive allele• Genotype is the genetic makeup,

phenotype is the physical expression

Chromosomes• Late 1900s role of chromosomes as carriers of genes

proposed• Most higher plant and animal somatic cells diploid • Gametes (germ cells) haploid

– Union of haploid gametes produces diploid zygote• Relationships of mutation, genetic linkage, and chromosomes

worked out in Drosophila in 1900s by T.H. Morgan– Breeding experiments with various mutant strains revealed traits that

were linked and those on different chromosomes• 1909 first understood how genes led to phenotypes

– Beadle and Tatum’s experiments with enzyme deficient mutants led to one gene, one enzyme hypothesis now one gene, one polypeptide

• 1915 chromosome basis of heredity widely accepted• Originally thought proteins were carriers of genetic material

because of presence in chromosomes• DNA was not “complex” enough to carry genetic information

Chromosomes

• Prokaryote and eukaryote chromosomes differ– Prokaryote DNA in single circular chromosome– Eukaryote genomes made of multiple chromosomes

with linear DNA molecules

• All eukaryotes have same basic chromosome structure but can vary in size and number– DNA bound to small basic histone proteins that help

package DNA into cells’ nuclei– DNA and associated proteins make up chromatin

Chromatin• Contains 2X protein to DNA• Histones are major proteins

– 5 types, similar in different eukaryotes• Over 1000 kinds of nonhistone proteins, involved in DNA replication

and gene expression• Nucleosome is basic structural unit

– DNA wrapped around histone core– Contributes to DNA coiling and chromatin condensation

• Chromatosome is subunit with nucleosome core locked in place by histone protein

• Highly condensed chromatin in interphase is heterochromatin, not transcriptionally active, making up centromere and telomeres

• Euchromatin is decondensed in interphase for replication and transcription prior to cell division

5.13 Chromatin fibers

Chromosomes• Condense when entering mitosis for distribution of material

to daughter cells• Transcription ends when condensation occurs• Centromeres ensure correct distribution of duplicated

chromosomes to daughter cells during mitosis– Hold sister chromatids together during mitosis and allow for spindle

tubule attachment– Specific DNA sequences found there for binding of associated

proteins forming kinetochore• Telomeres are sequences at ends of eukaryotic

chromosomes– Sequences have clusters of Gs, allows loop formation– Important in chromosome replication and maintenance– Seem to have role in lifespan and reproductive capability of cells– Damage or loss of portions of telomeres associated with human

disease

DNA as Genetic Material

• Experiments in 1928 with Pneumococcus bacteria showed a “transforming principle” was responsible for conversion of R strains to S strains

• 1944 experiments by Avery, MacLeod, and McCarty showed transforming principle to be DNA

• Established DNA as the genetic material

DNA Structure

• 1953 Watson and Crick discovered 3-D structure of DNA• Known to be 4 base polymer of 2 purines (A & G), 2

pyrimidines (C & T) plus attached phosphorylated sugars

• X-ray crystallography studies, H-bonding evidence, base pair ratios led to DNA model

• Double helix with phosphate-sugar backbone, turns every 3.4 nm

• Bases internally H-bonded in complementary fashion• Purines pair with pyrimidines (A-T and G-C)• One strand carries the information to dictate sequence of

the other strand

DNA

• Provides blueprint to direct cellular activities and specify development in multicellular organisms

• Recombinant DNA techniques have allowed sequencing of whole genomes

• Complete sequences available for several organisms

Eukaryote Genomes

• Larger and more complex than prokaryotes

• Complexity not necessarily related to size• Most eukaryotic cells have functional and

nonfunctional DNA sequences• Most higher eukaryotes have large amount

of noncoding sequences

Genome Organization• A gene is a sequence of DNA expressed to yield a final product like

RNA or protein• Some noncoding sequences in eukaryotes are spacers between

genes• Eukaryote DNA contains introns and exons

– Exons are coding regions of genes– Introns are sequences between exons

• Entire gene is transcribed to mRNA and introns are spliced out• Introns have no known function• May contain more DNA than exons• Allow for different combinations of exons, giving multiple products

from one gene by alternative splicing• Allow exon shuffling- recombination of exons from different genes• Lead to new genes or chimeras

Repetitive Sequences

• Large portions of complex eukaryote genomes have repeated noncoding DNA sequences

• Can be hundreds to thousands, even millions, of copies

• Up to 50% of mammalian DNA made of highly repetitive sequences

• Several types of repeats were identified by sequence analysis

Repetitive Sequences• Repetitive sequences and introns contribute to most of

the genome size of higher eukaryotes• Simple sequence repeats (SSR) are tandem arrays of

short sequences, up to thousands of copies– Will form separate bands from main DNA called satellite DNAs– Not transcribed, have no functional information, make up part of

chromosome structure• SINEs and LINEs found throughout genome, not in

tandem– Both are examples of transposable elements– SINEs are 100-300 bp, 1.5 million copies, transcribed but not

translated, function unknown – LINEs are 4-6 kb, 8.5X105 copies, some transcribed and

translated, function unknown

Transposable Elements

• Capable of moving to different sites in genomic DNA

• Retrotransposon transposition mediated by reverse transcriptase– SINEs and LINES are retrotransposons

• Retrovirus-like elements resemble retroviruses and also move by reverse transcriptase activity with 45,000 in human genome, from 2-10 kb

• DNA transposons are copied and re-inserted as DNA sequences with up to 300,000 copies in genome, from 80 to 3000 bp

Gene Duplication and Pseudogenes

• Genes present in multiple copies, many nonfunctional– Need multiple genes for RNA or proteins needed in large quantities

• Gene families are members of related groups of genes transcribed in different tissues or different developmental stages

• Thought to have originated due to one of two mechanisms– Duplication of block DNA sequence that moved to new location

• Range from 1kb to 50 kb• Copies of genes mutated until they are nonfunctional become

pseudogenes– Duplicate obtained by reverse transcription, copies lack introns and

sequences to direct transcription, are nonfunctional• Becomes a processed pseudogene

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