DNA and Chromosomes

DNA and Chromosomes

• DNA, or deoxyribonucleic acid, is a chemical substance found in the nucleus of all cells in living organisms.
• DNA is consistent across all cells in an individual.
• DNA codes for the production of proteins, like enzymes.
• DNA determines unique characteristics.
• Chromatin refers to DNA in long, thread-like fibers.
• Chromosomes are formed when chromatin condenses as the cell prepares to divide.
• Human body cells contain 46 chromosomes, arranged in 23 pairs.
• 44 chromosomes are autosomes, while 2 are sex chromosomes.
• Chromosomes contain genes.
• A karyotype is a picture of all the chromosomes in a cell.
• Genes are sections of DNA that code for specific traits.
• Alleles are variations of genes.

DNA Structure

• Nucleotides are the subunits of DNA.
• A nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
• DNA exists as a double helix, with two complementary strands of nucleotide polymers.
• The DNA backbone comprises sugar and phosphate units linked by phosphodiester bonds.
• Nitrogenous bases form base pairs through hydrogen bonding, connecting the two DNA strands.
• Nitrogenous bases attach to the DNA backbone via a glycosyl bond.
• A polynucleotide chain consists of nucleotides linked by phosphodiester bonds, forming the DNA backbone.
• Pyrimidine bases, which have a single ring, include thymine (T) and cytosine (C).
• Purine bases, which have a double ring, include adenine (A) and guanine (G).
• DNA contains deoxyribose, a phosphate group, and the nitrogenous bases A, T, C, or G, but not uracil.
• Two polynucleotide chains are connected via nitrogenous bases.
• Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C).
• The two DNA strands are antiparallel, meaning they run in opposite directions.

Cell Cycle Stages

• Humans develop from a single fertilized egg into bodies with trillions of cells through cell division.
• Cell division allows for organism reproduction, growth, and damage repair.

Interphase

• Involves cell growth, normal functions, and DNA replication.
• It constitutes the majority of a cell's life.

Mitosis

• Mitosis is the division of the nucleus.
• It is the dividing phase of a eukaryotic cell, excluding interphase.
• The genetic material in the nucleus divides into two identical nuclei.

Cytokinesis

• The process of a eukaryotic cell dividing its cytoplasm into two daughter cells.

Mitosis Phases

• Interphase sees chromosome replication within the nucleus, stored as chromatin, resulting in pairs of sister chromatids.
• Prophase involves chromosomes shortening and thickening, the nuclear membrane dissolving, centrioles moving to opposite poles, and spindle fibers attaching to centromeres.
• Metaphase is when chromosomes align in the middle of the cell.
• Anaphase involves sister chromatids being pulled apart by spindle fibers to opposite ends of the cell.
• Telophase sees nuclear membranes form around the chromosomes at each end of the cell.
• Cytokinesis: The cell pinches in the middle, forming two identical daughter cells.

Other Definitions

• Telomeres are protective caps on chromosomes that shorten with each DNA replication.
• After numerous divisions, shortened telomeres prevent the cell from dividing further.
• Cloning involves replacing an egg cell's nucleus with a nucleus from the organism being cloned.
• The altered cell replicates via mitosis in a surrogate mother, producing a clone.

GMOs

• Genetically modified organisms are created by inserting a desired gene from one species into the DNA of another.
• Examples include frost/pest-resistant crops, insulin in safflowers, hepatitis vaccines in bananas, and healthy fats in pork.

Meiosis

• Homologous chromosomes are pairs consisting of one chromosome from each parent, coding for the same features.
• Homologous chromosomes carry the same genes at the same loci but may have different alleles and are not identical.
• Diploid cells (2n) have two sets of each chromosome, one from each parent.
• Somatic cells are diploid (2n=46).
• Haploid cells (n) have one of each chromosome.
• Gametes are haploid (n=23).

Sexual Reproduction

• Meiosis occurs during gamete formation.
• Male gametes are sperm, produced in the testes.
• Female gametes are eggs, produced in the ovaries.
• Fertilization is the fusion of haploid sperm and egg to form a diploid zygote.
• The diploid zygote divides by mitosis to develop into a new, unique organism.
• The purpose of meiosis: To divide one parent cell into four non-identical haploid daughter cells, which are gametes.

Meiosis Stages

• Interphase duplicates each chromosome into two genetically identical sister chromatids attached at the centromere (46 chromosomes to 92 chromatids).
• Non-sister chromatids include one paternal and one maternal chromosome, while sister chromatids are duplicates of either the maternal or paternal chromosome.
• Prophase I involves homologous chromosomes lining up to form a tetrad, spindle fibers and centrioles forming, and crossing over occurs, exchanging genetic material between homologous chromosomes.
• Metaphase I has homologous chromosomes (tetrads) aligned in the middle of the cell, with crossing over having occurred. They are in a double file.
• Anaphase I sees homologous chromosomes separate and move to opposite poles, with independent assortment occurring where maternal and paternal chromosomes go to either side randomly.
• Orientation of each pair happens at random which promotes uniqueness, with sister chromatids remaining together and centromeres.
• Telophase I two nuclei form, spindle fibres break down, 2n becomes n.
• Prophase II has spindle fibres and centrioles form.
• Metaphase II aligns sister chromatids along the middle of the cell, with spindle fibers attaching to the centromere of sister chromatids; there are no homologous chromosomes lining.
• Anaphase II separates sister chromatids, halving the chromosome number in each daughter cell.
• Telophase II forms nuclear membranes, producing four daughter haploid cells (sperm or egg cells).
• Uniqueness of these cells results from independent assortment and crossing over.
• Meiosis I separates homologous chromosomes, reducing diploid cells (2n) to haploid cells (n).
• Meiosis II separates sister chromatids, producing four haploid daughter cells (n).

Gametogenesis

• Gametogenesis is the process of creating gametes through meiosis.
• Spermatogenesis creates four non-identical haploid sperm cells.
• Oogenesis creates four non-identical haploid cells, one ovum and three polar bodies.

Sex Chromosomes

• The 23rd pair of chromosomes determines sex.
• XX signifies female.
• XY signifies male.

Chromosomal Abnormalities

• Nondisjunction is the failure of homologous chromosomes to separate during meiosis, leading to an abnormal chromosome number in daughter cells.
• Trisomy is having three copies of a chromosome instead of two.
• Monosomy is having one copy of a chromosome instead of two.
• Down's Syndrome: Trisomy 21, results in developmental challenges, and is identified with karyotype analysis.
• Karyotype analysis helps identify chromosomal abnormalities such as Patau syndrome, Edwards syndrome, Klinefelter syndrome and Turner syndrome.
• Genetic variations, like point mutations, deletions, insertions, and base-pair substitutions, can cause genetic disorders.
• Spontaneous mutations occur naturally, while induced mutations are caused by external factors.
• Understanding these syndromes can aid in early diagnosis and intervention.