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The Flow of Genetic Information

•  Cells Inherit Molecules, Energy, and Genetic Information •  Genetic Information Is Encoded in DNA •  DNA Has the Structure of a Double Helix •  DNA Is Replicated Before Cell Division •  Genetic Information Flows from DNA to RNA to Protein •  Cells Express Their Genetic Information Selectively •  Chromosome Structure Changes During the Cell Cycle •  Chromosome Number Changes During the Life Cycle

Cells Are the Smallest Units of Life

As Cells Divide They Pass on Matter, Energy, and … ?

As Cells Divide They Pass on Matter, Energy, and … Genetic Information

Human Sperm

Sperm, carrying a

complete set of genetic

information, are essentially mobile nuclei.

Genetic Traits Are Transfered

with Nuclei

Dolly, the cloned sheep, showed the racial and molecular characteristics of the nuclear donor cell (Finn Dorsett),

not the characteristics of her foster mother (Blackface).

Finn Dorset ewe

G1

S

G2

Cell fusion

Blackface ewe

Mammary glandepithelial cells

Egg in metaphase II

M Cellculture

Starved cells (G0)

Enucleated egg

Gestation in Blackfacefoster ewe

Renucleated egg

Finn Dorset lamb

Chromosomes in Cell Nucleus

The most conspicuous elements in a cell nucleus

are the chromosomes,

which stain deeply (blue in this photo)

for DNA.

Griffith Experiment

Campbell & Reece (2002)

An experiment done in 1928 by Frederick Griffith led to the identification of DNA as the genetic material. He showed that live Pneumococcus bacteria could change both their morphological appearance and their pathogenicity by taking up a “transforming principle” from dead bacteria.

Deoxyribonucleic Acid (DNA)

DNA is a polymer of nucleotides, each consisting of a phosphate group, a sugar (deoxyribose), and one of four nitrogenous bases (A, C, G, or T).

The Four Nitrogenous Bases of DNA

T and C have single-ring structures, A and G have double-ring structures. One of the nitrogen atoms of the ring is bonded to the deoxyribose ring in DNA.

X-Ray Diffraction Pattern of DNA

Rosalind Franklin obtained the critical X-ray diffraction pattern of DNA, from which the double

helix model was deduced.

Double Helix Model of DNA

James Watson (left) and Francis Crick proposed the double helix model of DNA. The model is critical to the ability of DNA to serve as carrier of genetic information.

The Twisted Rope Ladder Model of DNA

The ropes on the sides represent the sugar-phosphate

backbones of two DNA polymer

strands. The rungs of the ladder are

formed by matched pairs of

nitrogenous bases.

DNA Replication Is Semi-Conservative DNA replication generates two daughter molecules, each of which consists of one parental DNA strand and one newly synthesized complementary strand.

Transcription and Translation

1.  Transcription into RNA in the nucleus

2.  Translation into protein in the cytoplasm

Cells express genetic information in two steps:

RNA and DNA Both are long nucleotide polymers. RNA differs from DNA in •  being mostly single-stranded •  having the nitrogenous base uracil instead of thymine •  having ribose as a sugar instead of deoxyribose


Codons Are Informative Units of Three Nucleotides

During translation, mRNA is read in units of 3 nucleotides called codons, and for each codon a specific amino acid is incorporated into the newly synthesized protein.

Genetic Code

Polypeptides Fold into Functional Proteins

The immediate result of translation is a linear polypeptide, which is later cut and folded into a three-dimensional, biologically active protein.

Cells Express Genes Selectively

Controlling the Initiation of Transcription Key elements are regulatory proteins known as transcription factors. Binding to specific DNA sequences called promoters and enhancers, the proteins form a three-dimensional complex that positions RNA polymerase at the transcription initiation site.

Cell Cycle

For clarity, only one of 2n chromosomes is shown

DNA Packaging in Chromosomes

In eukaryotic cells, DNA is packaged with histones and other proteins. The resulting chromatin fibers are coiled and supercoiled. The chromatin exists in segments called chromosomes, which form

a loose meshwork during interphase. During mitosis,

chromosomes become condensed even further and are then visible

as compact units.

Chromosome Duplication and Separation

This illustration focuses on a single

chromosome. During S phase, DNA replication lags behind at a region called the centromere. Each chromosome then

consists of two sister chromatids,

which are separated during mitosis.

Homologous Pairs of Chromosomes

In most somatic cells, chromosomes occur in homologous pairs, one derived from the paternal and the other from the maternal parent. The human chromosome set (karyotype) includes 22 autosomal pairs and one pair of sex chromosomes. Error: Lac/lac refer to genes for lactase, not lactose.

Number of Chromosomes per Nucleus Changes During Life Cycle

The number of chromosomes per cell nucleus is reduced from diploid (2n) to haploid (1n) during gamete formation. The diploid number is restored at fertilization.

Meiosis


This diagram represents only one pair of homologous chromosomes. Meiosis I is unusual because the two homologs from each pair are segregated into different daughter cells. (This is not so in mitosis.)

Trisomy 21 and Down Syndrome

Errors in meiosis may lead to odd sets of chromosomes, such as the human trisomy 21 (three copies of chromosome 21). The presence of the extra

gene copies causes developmental abnormalities known as Down syndrome.

Transfer RNAs have dual specificities

Transfer RNAs (tRNAs) have two specific binding sites:

•  The anticodon binds to one out of 64 codons. •  The amino acid attachment site binds to one out of 20 amino acids.

There are tRNAs with the same amino acid attachment site but different codons, but not vice versa. Thus, the genetic code is redundant but not ambiguous.

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