MITOSIS AND MEIOSIS

Objectives 2. Discuss the relationships among chromosomes, genes, and DNA. 2.1 Describe how the genetic code is carried on the DNA. 2.2 Outline the process of replication. 2.3 Compare mitosis and meiosis. 2.4 Describe the process of transcription.

DNA, genes and chromosomes DNA (deoxyribonucleic acid) carries the genetic information in the body’s cells. DNA is made up of four similar chemicals (called bases and abbreviated A, T, C, and G) that are repeated over and over in pairs A gene is a distinct portion of a cell’s DNA. Genes are coded instructions for making everything the body needs, especially proteins. Human beings have about 25,000 genes. Researchers have discovered what some of our genes do, and have found some that are associated with disorders (such as cystic fibrosis or Huntington’s disease). There are, though, many genes whose functions are still unknown Genes are packaged in bundles called chromosomes. Humans have 23 pairs of chromosomes (for a total of 46). Of those, 1 pair is the sex chromosomes (determines whether you are male or female, plus some other body characteristics), and the other 22 pairs are autosomal chromosomes (determine the rest of the body’s makeup).

DNA Replication An enzyme called DNA gyrase makes a nick in the double helix and each side separates An enzyme called helicase unwinds a portion of the double stranded DNA, in an area called the replication fork Several small proteins called single strand binding proteins (SSB) temporarily bind to each side and keep them separated An enzyme called DNA polymerase “walks” down the DNA strands and adds new nucleotides to each strand. The nucleotides pair with the complementary nucleotides on the existing strand A sub unit of the DNA polymerase proofreads the new DNA An enzyme called DNA ligase seal up the fragments into one long continuous strand The new copies automatically wind up again.

Building Proteins Building proteins is much like building a house: The master blueprint is DNA which contains all of the information to build a new protein (house) The working copy of the master blueprint is called messenger RNA (mRNA), which is copied from DNA The construction site is either the cytoplasm in prokaryotic cells (bacteria) or the endoplasmic reticulum (ER) in eukaryotic cells (plants and animals) The building materials are amino acids The construction workers are ribosomes and transfer RNA molecules This is done in two steps – RNA transcription and translation

RNA Transcription This is the copying of the master blueprint (DNA) to the working blueprint (mRNA) The transcription is preformed by an enzyme called RNA polymerase goes through the following process: First the RNA polymerase binds to the DNA strand at a specific sequence of the gene called a promoter Unwinds and unlinks the two strands of DNA Uses one of the DNA strands as a guide or template Matches new nucleotides with their complements on the DNA strand ( G with C, and A with U) Binds these new RNA nucleotides together to form a complementary copy of the DNA strands (mRNA) Stops when it encounters a termination sequence of bases (stop codon)

Translation Translation is where the construction workers take the working blueprint (mRNA) and use its instructions to build the house (protein) mRNA strand moves out of the nucleus into the cytoplasm and attaches to a ribosome tRNA anticodons (series of 3 nucleotides) match or pair with the first codon on the mRNA Another tRNA anticodon matches or pairs with the second codon on the mRNA The amino acids on the other side of the tRNA bond First tRNA leaves to go pick up another amino acid Process continues down the length of the mRNA strand

Mitosis Mitosis: division of somatic (body) cells. The process by which a eukaryotic cell divides the genetic material in its nucleus into two new identical nuclei

Interphase During interphase, the cell grows and carries out its normal functions. The chromosomes in the nucleus form a mass of thread-like structures called chromatin, which are composed of DNA and proteins The replication of chromosomes during interphase results in pairs of sister chromatids, which containing exactly the same genes at the same loci (location)

Prophase Chromosome pair up! Chromosomes thicken and shorten become visible under a microscope 2 chromatids joined by a centromere Centrioles move to the opposite poles of the nucleus Nucleolus disappears Nuclear membrane disintegrate

Metaphase Chromosomes meet in the middle! Chromosomes arrange at equator of cell Become attached to spindle fibres by centromeres Homologous chromosomes do not associate

Anaphase Chromosomes get pulled apart Spindle fibres contract pulling chromatids, now referred to chromosomes, apart to the opposite poles of the cell Chromosomes are being pulled by their centromeres

Telophase Now there are two! Chromosomes reach the opposite poles of the cell and begin to unwind Spindle fibres disintegrate Centrioles replicate Nucleur membrane forms around the chromosomes resulting in two daughter nuclei Cell divides

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Meiosis 4 daughter cells produced Each daughter cell has half the chromosomes of the parent 2 sets of cell division involved Creates sperm and egg cells

Meiosis I Purpose: Meiosis I has two main purposes: It is the reduction division, so it reduces the number of chromosomes in half, making the daughter cells haploid (when the parent cell was diploid). It is during meiosis I that most of the genetic recombination occurs. Phases: Keep in mind that before meiosis begins at all, the DNA undergoes replication, just like it did before mitosis started. So, when you first see chromosomes in meiosis I, they have sister chromatids, just like in mitosis. It is just that in meiosis I, we will be talking about tetrads becoming visible, lining up, separating, and decondensing (rather than chromosomes, like in mitosis). Finally, cytokinesis occurs, too, any time after the tetrads have moved out of the equator (just like in mitosis)..

Prophase I Just like in mitosis, during prophase, DNA condensation occurs, the nuclear envelope and nucleoli disappear, and the spindle starts to form. The big difference is what is going on with the chromosomes themselves. As DNA condensation proceeds and the chromosomes first become visible, they are visible as tetrads. So, tetrads become visible during prophase.

Metaphase 1 Tetrads line up at the equator. The spindle has completely formed. It is during prophase I and metaphase I that genetic recombination is occurring. Take a look at the genetic recombination page to find out about how that happens here. Keep in mind that it only happens when there are tetrads, so as soon as anaphase I gets going, genetic recombination is over.

Anaphase I Tetrads pull apart and chromosomes with two chromatids move toward the poles.

Telophase I Chromosomes with two chromatids decondense and a nuclear envelope reforms around them. Each nucleus is now haploid. Keep in mind that it is not the number of chromatids per chromosome that determine whether a cell is diploid or haploid, but, it is the number of chromosomes and whether they are paired that determines this.

Meiosis II Purpose: At the end of meiosis I, each chromosome still had two chromatids. That is double the amount of DNA that a cell should have. So, the entire reason to go through meiosis II is to reduce the amount of DNA back to normal-- basically, to split the chromosomes so that each daughter cell has only one chromatid per chromosome (the normal genetic content). Phases: As you read through the phases of meiosis II, you will see that it looks just like mitosis. It is really similar to mitosis-- so keep that in mind. The only difference is that the two chromatids per chromosome are not necessarily identical due to genetic recombination occurring in meiosis I.

Prophase II Chromosomes with two chromatids become visible as they condense (and the nuclear envelope and nucleoli disappear, and the spindle is forming).

Metaphase II Chromosomes with two chromatids line up at the equator. The spindle is fully formed. Although genetic recombination primarily occurs during meiosis I, the way the chromosomes line up during metaphase II can also help to make unique daughter cells.

Anaphase II Chromosomes split, so that a chromosome with only one chromatid heads toward each pole.

Telophase II Chromosomes with only one chromatid decondense and get surrounded by new nuclear envelopes. The four daughter cells are now all haploid and have the right amount of DNA. They are ready to develop into sperm or eggs now.

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