Genetics and DNA

Punnett Squares

Biologists can predict the probable outcomes of a cross by using a diagram called a Punnett square. In the Punnett square illustrated at the right, the yellow pea pods are dominant, as designated by a capital Y, and the green pea pods are recessive, as designated with a lowercase y. In a cross between one homozygous recessive (yy) parent and a heterozygous dominant parent (Yy), the outcome is two heterozygous dominant offspring (Yy) and two homozygous recessive offspring (yy), which gives a ratio of 2:2.

Chromosomes

A gene is a segment of DNA, deoxyribonucleic acid, which transmits information from parent to offspring. A single molecule of DNA has thousands of genes. A chromosome is a rod-shaped structure that forms when a single DNA molecule and its associated proteins coil tightly before cell division.

Chromosomes have two components:

• Chromatids: two copies of each chromosome
• Centromeres: protein discs that attach the chromatids together

Human cells have 23 sets of different chromosomes. The two copies of each chromosome are called homologous chromosomes, or homologues. An offspring receives one homologue from each parent. When a cell contains two homologues of each chromosome, it is termed diploid (2n). A haploid (n) cell contains only one homologue of each chromosome. The only haploid cells humans have are the sperm and eggs cells known as gametes.

Deoxyribonucleic Acid

The DNA molecule is a long, thin molecule made of subunits called nucleotides that are linked together in a nucleic acid chain. Each nucleotide is constructed of three parts: a phosphate group, five-carbon sugar, and nitrogen base.

The four nitrogenous bases are:

• adenine (A);
• guanine (G);
• thymine (T); and
• cytosine (C).

Adenine and guanine belong to a class of large, organic molecules called purines. Thymine and cytosine are pyrimidines, which have a single ring of carbon and nitrogen atoms. Base pairs are formed as adenine pairs with thymine and guanine pairs with cytosine. These are the only possible combinations.

Cells maintain genetic information through the Central Dogma (of biology). The Central Dogma includes three DNA processes:

1. Replication
2. Transcription
3. Translation

Replication

The process of synthesizing a new strand of DNA is called replication. A DNA molecule replicates by separating into two strands, building a complementary strand, and twisting to form a double helix.

Transcription

The first step in using DNA to direct the making of a protein is transcription, the process that “rewrites” the information in a gene in DNA into a molecule of messenger RNA (ribonucleic acid). Transcription manufactures three types of RNA:

• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
• Ribosomal RNA (rRNA)

Messenger RNA is an RNA copy of a gene used as a blueprint for a protein. In eukaryotes, transcription does not produce mRNA directly; it produces a pre-mRNA molecule. Transfer RNA translates mRNA sequences into amino acid sequences. Ribosomal RNA plays a structural role in ribosomes.

Transcription proceeds at a rate of about 60 nucleotides per second until the RNA polymerase (an enzyme) reaches a stop codon on the DNA called a terminator and releases the RNA molecule.

Translation

The components necessary for translation are located in the cytoplasm. Translation is the making of proteins by mRNA binding to a ribosome with the start codon that initiates the production of amino acids. A peptide bond forms and connects the amino acids together. The sequence of amino acids determines the protein’s structure, which determines its function.

Central Dogma Summary

Let’s Review

Genetics

• Gregor Mendel is considered the father of genetics because of his Theory of Heredity. His theory revolves around the idea that parents do not transmit traits directly to their offspring. Rather, they pass on units of information called genes.
• A gene is a segment of DNA that transmits information from parent to offspring, also known as an allele.
• For each trait, an individual has two alleles: one from each parent. If the two factors have the same information, the individual is homozygous for that trait. If the two factors are different, the individual is heterozygous for that trait.
• The alleles determine the physical appearance, or phenotype. The set of alleles an individual has is its genotype.
• An allele may be dominant or recessive. A dominant allele guarantees a trait’s expression in the phenotype. The presence of a recessive allele does not guarantee that the trait will be expressed.
• Biologists can predict the probable outcomes of a genetic cross by using a diagram called a Punnett square.

DNA (Deoxyribonucleic Acid)

• Deoxyribonucleic acid is a long, thin molecule made of subunits called nucleotides that are linked together in a nucleic acid chain.
• Nucleotides consist of three parts: a phosphate group, a five-carbon sugar, and nitrogen base.
• There are four nitrogen bases: adenine, guanine, thymine, and cytosine. These bases occur in pairs on a DNA strand.
• Adenine pairs with thymine and guanine pairs with cytosine. These are the only possible pairings.
• Replication is the process of synthesizing a new strand of DNA. DNA will separate into two strands and build a complementary strand.
• Transcription is the first step in using DNA to direct the making of a protein. It is the process that “rewrites” the information in a gene in DNA into a molecule of RNA which will be used to code for proteins.
• Translation is the process of making proteins.

Genetics and DNA Flashcards