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Lecture 7

BIOL 3113 Lecture 7: 7.DNA Structure

Course Code
BIOL 3113
Barbara S

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7 DNA Structure
DNA - Deoxyribonucleic Acid
*Relatively simple molecule
*Carrier of genetic information
*Genes of every cell on Earth are made of DNA
*Cells maintain, replicate and express genetic information carried in DNA
*Information in genes:
copied and transmitted to daughter cell multiple times
contains primarily instructions for making proteins
Consists of two long polynucleotide chains - DNA strands
Each strand made of 4 types of nucleotide subunits
(linked by phosphodiester bonds sugar-phosphate backbone with N-bases sticking out)
Two strands held together by hydrogen bonds between the bases - purines (adenine,
guanine) and pyrimidines (cytosine, thymine)
The opposing sugar-phosphate backbones have opposite polarity (antiparallel)
Complementary base pairing:
*enables the base pairs to be packed in the energetically most favorable arrangement
(same width, 1-ring base pairs with 2-ring base same distance between sugar-
phosphate backbones along the molecule)
*provides the basis for replication of nucleic acids
2 sugar-phosphate backbones twist around one another form a double helix with 10
bases per helical turn
Base pairs fit together when the two strands are antiparallel (opposite polarity)
Each strand of DNA molecule has a nucleotide sequence exactly complementary to its
partner strand critical for copying DNA
DNA - carrier of genetic information
Genome - a complete set of genetic information in a cell
Genes - fragments of DNA molecule coding for proteins + many non-coding sequences
extremely long sequence of nucleotides (message written in 4 letter code)
Human cell contains ~2 meters of DNA
How this long DNA thread is packed to fit into a cell nucleus?

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Eucaryotic Cell Nucleus
Provides a compartment in which the DNA and DNA- dependent functions are
May contain one or more nucleoli - site of rRNA synthesis and ribosome assembly
Enclosed by the nuclear envelope:
Supported internally by the nuclear lamins (intermediate filaments)
Surrounded externally by network of intermediate filaments
Nuclear Pores
Nuclear envelope is penetrated by numerous nuclear pores
*allow passage of molecules and large particles:
from the nucleus to the cytosol and from the cytosol to the nucleus
*movement through the pores is regulated
Functions of the nucleus
*DNA replication
*DNA packing - chromosomes
*DNA transcription mRNA, rRNA and tRNA
*processing of mRNA
*mRNA transport
*ribosome assembly
*dissolution and reformation of the nuclear envelope during mitosis and meiosis
DNA packing - Chromosomes
Name derived from Greek: (chroma + soma) - colored body
Composed of DNA + proteins = chromatin
Human genome - 3.2 x 109 nucleotides distributed over 24 chromosomes
Each cell contains 2 copies of each chromosome (with a few exceptions) - one from the
mother and one from the father
Ploidy - number of sets of chromosomes per cell:
*haploid - one set of chromosomes per cell
*diploid - two sets of chromosomes per cell
*tetraploid - four sets of chromosome per cell
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