Genetics and Cellular Reproduction

1. Genetics and Cellular Reproduction

2. Cell Reproduction • All living things must reproduce to keep species alive. • This is a major driving force for zoology -Evolution • These two facts, reproduction and passing traits necessary for success on to offspring, are essentially interelated.

3. Cell Reproduction • Cell reproduction serves different functions for different organisms. • Unicellular organisms - producing a copy of itself. • Multicellular organisms - growth, repair of damaged structures, development of new structures, and most importantly… Formation of gametes!

4. Cell Reproduction • For reproduction to occur, there must be a set of instructions. • These instructions are carried in the code of DNA. • DNA codes for specific proteins • Proteins make EVERYTHING!

5. Eukaryotic Chromosomes • DNA that is responsible for the code in organisms, is contained in the form of chromosomes. • When in the chromatin state, the units of inheritance called genes can actively participate in protein production. • Chromatin consists of DNA and histone proteins. • The proteins serve as a coiling point for the DNA and when combined becomes the nucleosome.

7. Types of Chromosomes • All of the cells in the body contain chromosomes and the same genetic information. • Somatic cells (body cells) have the code for male and female. • However, these cells are not responsible for the reproduction of the organism. • Sex cells are the cells that actually determine sex in the organism.

8. Chromosome Numbers • The number of chromosomes in a species is constant. • However, the number of chromosomes between species varies greatly. • As seen in the picture, the chromosomes are present in pairs. • If N represents the number of different chromosomes, most animals have two sets and are 2N.

9. Chromosome Numbers • This 2N condition is what we call diploid in animals. • Some animals may only have one set of chromosomes, which we refer to as haploid. • Even more bizarre and rare, some animals have more than two sets of chromosomes, which we refer to as polyploidy.

10. Mitosis • There are two types of cellular division: • Mitosis • Meiosis • Mitosis - the type of division necessary for growth and repair processes. • Involves two stages, mitosis (dividing of the nucleus) and cytokinesis (dividing of the cytoplasm).

11. Mitosis • The normal cell cycle consists of distinct phases. • We are mainly concerned with everything but interphase. • Mitosisis divided into four specific stages. • Prophase • Metaphase • Anaphase • Telophase

12. Mitosis • Prophase - first stage of mitosis; begins when the cells become visible. • Nuclear envelope begins to dissolve and the centrioles begin to move apart. • The centrioles, asters, and microtubules once formed make up the mitotic spindle.

13. Mitosis • The next phase for the dividing cell is metaphase. • Chromatids begin to move towards the center of the cell. • Near the end of this phase the centromeres will detach the two chromatids.

14. Mitosis • Once the cell has formed chromosomes, the cell moves into anaphase. • The microtubules of the mitotic spindle begin to shorten. • This phase ends when each chromosome reaches its respective pole.

15. Mitosis • Once the chromosomes are at opposite poles, the cell begins telophase. • The mitotic spindle begins to disassembles, and the nuclear envelope reforms. • The chromosomes begin to uncoil which allows for gene expression and the nucleolus is reformed.

16. Mitosis • The final step for this cell to complete its division is cytokinesis. • The cytoplasm will begin to divide either late in anaphase or early telophase. • The contractile ring will pinch the cell in half eventually forming two new cells.

17. Meiosis • We have already said that reproduction involves the combination of two different cells called gametes. • Uniting of these gametes forms a zygote - the first new cell of the animal. • To have the number of chromosomes remain constant in future reproductions, the reproducing animals must produce gametes with half the number of chromosomes as somatic cells.

18. Meiosis • Meiosis involves two nuclear divisions - Meiosis I and II. • These divisions result in four daughter cells, each with half the number of chromosomes as parent cell. • It is important to note that these daughter cells are NOT genetically identical!

19. Meiosis • Just as prophase was the first step in Mitosis, it is called prophase I in Meiosis. • The chromosomes become visible. • The cells contain a diploid number of chromosome at this point.

20. Meiosis • Crossing over - a series of events where non-sister chromatids of two homologous chromosomes exchange DNA. • This allows for the exchange of genetic information which is a major source of genetic variation in populations.

21. Meiosis • Again, the second phase of this division is referred to as Metaphase I. • The microtubules form spindles just as in mitosis. • The difference in this division is that the chromosomes do not pair up like in mitosis. • The chromosomes remain in the center of the cell unpaired.

22. Meiosis • Anaphase is when the homologous chromosomes move toward each pole. • Each pole randomly receives a pair of homologues. • This results in independent assortment of the homologues.

23. Meiosis • Just as in mitosis, there is a disassembly of the spindle. • This is called Telophase I in meiosis. • The transition to the second nuclear division is called interkinesis.

24. Meiosis • Meiosis differs from mitosis in that there is a second nuclear division. • This division resembles the first division, except the number of chromosomes will be reduced by half. • Repectively, the phases are Prophase II, Metaphase II, Anaphase II, and Telophase II. • The final step is cytokinesis as in mitosis.

25. Meiosis

26. Meiosis • The end result of this 2 phase nuclear division is the formation of two gametes (sperm and egg cells). • Spermatogenesis results in the production of the sperm cell. • Oogenesis produces a mature ovum or egg. • Oogenesis differs slightly in that only one of the four cells produced in meiosis is functional.

27. Modes of Reproduction • There are many modes of reproduction in the animal kingdom • Viviparous – giving birth to live young (ex. Mammals) • Oviparous – animals lay their eggs (ex. Butterflies) • Ovoviviparous – eggs are held within the mother and hatched giving birth to young (ex. Sharks)

28. Genetic Material • For cells to divide, there has to be a molecule that can perform certain functions: • The substance must serve as a code for the amino acids and proteins. • It must be able to copy itself. • It must be able to be contained in the nucleus. • It must be able to undergo changes over time to account for adaptations.

29. Genetic Material • This substance can be none other than DNA. • DNA (deoxyribonucleic acid). • Another genetic molecule just as important is RNA and it is responsible for protein synthesis.

30. Genetic Material • DNA and RNA are made up of molecules called nucleotides. • A nucleotide consists of: • A nitrogen base (either a purine or pyrimidine). • A pentose sugar • A phosphate group • There are some differences between DNA and RNA in terms of the nitrogen base and the sugar group.

31. Genetic Material • The DNA structure is ladder like with the rails consisting of alternating sugar and phosphate groups. • The pairing of the nitrogenous bases is complimentary and is held by a hydrogen bond. • Each strand of DNA is a template for a new strand. The complimentary structure of DNA allows for the coding of a new strand.

32. Genetic Material • For a trait to be expressed, there has to be a gene to code for that trait. • A gene is a sequence in the bases of DNA that codes for the synthesis of a polypeptide. • When RNA is synthesized from DNA it is called transciption. • When RNA then forms a protein in the ribosome, the process is called translation.

33. Genetic Material • There are three types of RNA, each with a specific role in protein synthesis. • mRNA is the strand that carries the instructions for proteins to the cytoplasm. • tRNA pick up the amino acid in the cytoplasm and carries them to ribosome for formation of the polypeptide. • rRNA and proteins make up the ribosome.

34. Genetic Material • There are 20 different amino acids found in living organisms. • However, there are only 4 different nitrogen bases. • How can we possibly code for these different amino acids?

35. Genetic Information • The genetic code is a triplet code or a three base code called a codon. • There are a total of 64 possible codons coding for 20 amino acids. • Some of the codons do not code for amino acids, but rather for stop and start signals.

36. Transcription • Information from DNA is transcribed into mRNA. • This involves the unwinding of a portion of DNA and copying the code to mRNA. • Unlike direct DNA transcription where an exact copy is made, with RNA only one or a few genes are exposed and only one of the two DNA strands is copied.

37. Translation • Translation is the process of protein synthesis in the ribosome in the cytoplasm. • This occurs because of the information transcribed to mRNA. • mRNA codes for different amino acids which are then aligned by tRNA so that a polypeptide can be made.

38. Mutations • DNA, while it has the ability to undergo replication, also is at risk for “mistakes” to be made during replication. • These mistakes are another means of driving the evolutionary process. • Mutations are changes in the sequencing of bases in DNA.

39. Mutations • Point mutations change result from changes in the base sequence of DNA. • This may be due to replacement, addition, or deletion of a nucleotide base.

40. Changes in Chromosome Number • Sometimes the number of chromosomes can be changed. • This may happen to an entire set of chromosomes or may happen to one particular set. • Aneuploidy is the addition or deletion of one or more chromosomes. • Another problem is when chromatids fail to separate during meiosis. • This condition is referred to as nondisjunction.

41. Changes in Chromosome Structure • A final problem with chromosomes can occur when the chromosome breaks. • This can result in pieces of the chromosome being lost or the piece may re-attach, but in the wrong direction. • Either of these situations can result in the wrong protein being synthesized.

42. Inheritance Patterns • One of the main principles in genetics is segregation. • Segregation is the idea that pairs of genes are distributed between gametes during gamete formation. • This ultimately means that genes from each parent are mixed and passed onto offspring.

43. Inheritance Patterns • The genes that exhibit specific traits exist in alternative forms called alleles. • Alleles can be present in dominant and recessive forms • Dominant always shows up when present AA vs Aa