DNA and Sex Chromosomes

DNA Structure The DNA structure is like a twisted ladder with each step made of nitrogen bases The four nitrogen bases are: Adenine (A) Thymine (T) Guanine (G) Cytosine ( C) Adenine only pairs with Thymine Guanine only pairs with Cytosine Each rung is made up of a pair of molecules called nitrogen bases. Nitrogen bases are molecules that contain the element nitrogen and other elements. DNA has four kinds of nitrogen bases: adenine (ad uh neen), thymine (thy meen), guanine (gwah neen), and cytosine (sy tuh seen). The capital letters A, T, G, and C are used to represent the four bases. The bases on one side of the ladder pair with the bases on the other side. Adenine (A) only pairs with thymine (T), while guanine (G) only pairs with cytosine (C). This pairing pattern is the key to understanding how DNA replication occurs.

DNA Replication DNA replication begins when the two sides of the DNA molecule unwind and separate, somewhat like a zipper unzipping. As you can see in Figure 15, the molecule separates between the paired nitrogen bases. DNA replication begins when the two sides of the DNA molecule begin to “unzip”

Next, nitrogen bases that are floating in the nucleus pair up with the bases on each half of the DNA molecule. Because of the way in which the nitrogen bases pair with one another, the order of the bases in each new DNA molecule exactly matches the order in the original DNA molecule

Mendal Revisited Earlier we learned about Mendal’s work with pea plants Mendal discovered traits but he did not have knowledge of DNA or genes His work proved that genes existed by no one knew what they looked like or where they were located Enter Walter Sutton…….

Walter Sutton (1903) Sutton wanted to now how egg and sperm cells formed While studying grasshoppers he proposed that chromosomes were the key to why offspring look like their parents

Sex Cells The large egg is a female sex cell, and the smaller sperm is a male sex cell

The Chromosome Theory of Inheritance By studying the fretilization of sex cells in grasshoppers Sutton developed the chromosome theory of inheritance According to the chromosome theory of inheritance, genes are carried from parents to their offspring on chromosomes Sutton observed what happened when a sperm cell and an egg cell joined during fertilization. The fertilized egg that formed had 24 chromosomes. As a result, the grasshopper offspring had exactly the same number of chromosomes in its cells as did each of its parents. The 24 chromosomes existed in 12 pairs. One chromosome in each pair came from the male parent, while the other chromosome came from the female parent.

Meiosis Meiosis- the process by which sex cells are formed; the process by which the number of chromosomes is reduced by half to form sex cells—sperm and eggs.

Meiosis I

Meiosis II

Taking a Look Back at Punnett Squares A Punnett square is actually a way to show the events that occur at meiosis. When the chromosome pairs separate and go into two different sex cells, so do the alleles carried on each chromosome. One allele from each pair goes to each sex cell.

Genes Although you have only 23 pairs of chromosomes, your body cells each contain about 35,000 genes. Each gene controls a trait. Genes on Chromosomes Genes are located on chromosomes. The chromosomes in a pair may have different alleles for some genes and the same alleles for others. Classifying For which genes is this organism homozygous? For which genes is it heterozygous?

The Genetic Code

Proteins The main function of genes is to control the production of proteins in an organism’s cells Proteins help to determine the size, shape, color, and many other traits of an organism Recall what DNA is made of …..The four nitrogen bases are…?

Order of the Bases A gene contains the code for the structure of a protein The order of the nitrogen bases along a gene forms a genetic code That code determines what type of protein will be produced A gene contains the code that determines the structure of a protein. The order of the nitrogen bases along a gene forms a genetic code that specifies what type of protein will be produced. Remember that proteins are long-chain molecules made of individual amino acids. In the genetic code, a group of three DNA bases codes for one specific amino acid. For example, the base sequence CGT (cytosine-guanine-thymine) always codes for the amino acid alanine. The order of the three-base code units determines the order in which amino acids are put together to form a protein.

Protein Synthesis The production of proteins is called protein synthesis During protein synthesis, the cell uses information from a gene on a chromosome to produce a specific protein Protein synthesis takes place on the ribosomes in the cytoplasm of a cell. As you know, the cytoplasm is outside the nucleus. The chromosomes, however, are found inside the nucleus. How, then, does the information needed to produce proteins get out of the nucleus and into the cytoplasm?

RNA Transfer RNA Messenger RNA RNA that copies the coded message from DNA in the nucleus and carries the message into the cytoplasm. RNA in the cytoplasm that carries an amino acid to the ribosome and adds it to the growing protein chain.