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
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
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.
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
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.
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
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.
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
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
Campbell & Reece (2002)
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.
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.
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
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.
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
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.
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
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
Campbell & Reece (2002)
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.
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
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.