Structure of DNA (Deoxyribonucleic acid)
• DNA is made of nucleotides - 3 components: phosphate, sugar called deoxyribose, and 1 of
4 bases – adenine, cytosine, thymine, and guanine
• There are 2 strands of nucleotides held together by hydrogen bonds; two strands twist
around each other to form a double helix
• Complimentary base pairing: A & T always pair together; C & G always pair together
DNA Replication
• Purpose: DNA makes an exact copy of itself prior to cell division
• Importance: Ensures that each new cell gets a complete copy of the DNA
• Steps:
• Helicase (an enzyme) attaches to DNA & breaks H2 bonds between bases – DNA
chain unzips.
• DNA polymerase (an enzyme) attaches to separated strand.
• Complementary nucleotides bind to each side, forming 2 complete strands.
DNA vs. RNA (Ribonucleic acid)
Sugar
Bases
Strands
Location in cell
DNA
Deoxyribose
Adenine, Thymine, Guanine,
and Cytosine
Double-stranded
Nucleus
RNA
Ribose
Adenine, Uracil, Guanine,
Cytosine
Single-stranded
Nucleus, cytoplasm
Types of RNA and roles:
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Messenger RNA - carries genetic information to the ribosomes
• Ribosomes - part of the cell where proteins are made
• Ribosomal RNA - found in the ribosomes
• Transfer RNA - transfers amino acids to the ribosomes
Protein synthesis
• Two processes are involved in the synthesis of proteins in the cell:
• Transcription –DNA is copied into mRNA
• Translation - the process of building proteins
• Codon - each 3-letter unit of an mRNA molecule; each codon represents 1 amino acid
• 1st step in protein synthesis - Transcription
• Purpose – Makes a copy of the DNA code that can leave the nucleus and travel to the
ribosome to direct protein synthesis – makes mRNA
• Occurs in the nucleus; occurs at only 1 gene at a time
• A in DNA pairs with U (Uracil) in RNA, not T; T in DNA pairs with A in RNA
• Steps:
• Segment of DNA unwinds and unzips
• RNA polymerase joins the RNA nucleotides that are complementary to the
code on ½ of the DNA.
• End of gene is reached, process ends, and DNA closes back up
Picture of transcription:
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2nd step of protein synthesis - Translation
• Protein constructed during this process; occurs at the ribosomes
• Each tRNA has a sequence of nucleotides called an anticodon
• Steps:
• Ribosome attaches to the mRNA near the start codon; anticodon of tRNA,
pairs with the start codon on mRNA
• 2nd tRNA attaches to second binding site on ribosome; amino acids are joined
by peptide bond
• Ribosome moves forward; subsequent tRNA molecules bring amino acids to
ribosome and are joined by peptide bonds
• Process continues until stop codon is reached
Picture of translation
Mutations
• Definition - a change in the sequence of bases within a gene
• Passed on only if on gametes (eggs or sperm), not in body cells
• Causes – can be spontaneous or caused by environmental influences called mutagens (such
as X-rays, UV radiation, and organic chemicals (in cigarette smoke and pesticides).
• Effects on organism: may have no effect, be harmful, or result in new beneficial trait
• Mutations that prove beneficial make the organism more likely to survive and pass on the
new trait to the next generation
• If the new trait gives the organism an adaptive advantage, the trait will become more
prevalent in future generations
The Cell Cycle
• An orderly sequence of events that occurs from the time a cell is first formed until it divides
into two new cells.
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Most of the cell cycle is spent in interphase:
• G1 stage – growth, doubling of organelles, prepares for DNA replication
• S stage – DNA replication occurs
• G2 stage – cell makes proteins needed for cell division
• Following interphase is the M stage, including mitosis and cytokinesis
• Mitosis: DNA made during replication splits to become the nuclei of 2 daughter cells
• The cell cycle ends when cytokinesis, the splitting of the cytoplasm, is complete.
Mitosis
• Occurs in body cells; diploid cells (2 sets of chromosomes) divide to produce diploid cells
• Daughter cells are genetically identical to parent cells
• In a non-dividing cell – genetic material is in the form of chromatin (DNA & protein)
• In a dividing cell, chromatin undergoes coiling to form chromosomes
• After replication, there are 2 identical sister chromatids, held together by a centromere
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Body cells contain the diploid (2n) number of chromosomes – 2 chromosomes of each kind
(1 from each parent)
Sex cells (eggs and sperm) contain only 1 chromosome of each kind – haploid (n) number of
chromosomes
Phases:
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A. Late Interphase: centrioles duplicate; chromatin condenses into chromosomes
B. Prophase: Chromosomes become visible, centrioles move to opposite ends of the
cell, nuclear membrane and nucleolus disappear, spindle fibers form, chromosomes
become attached to spindle fibers – centromere attaches to spindle fibers
C. Metaphase: Chromosomes line up at metaphase plate – equidistant from poles
D. Anaphase: Sister chromatids separate, becoming daughter chromosomes, and
move toward opposite ends of cell
E. Telophase: Spindle disappears, nuclear membrane and nucleolus reappear,
chromosomes turn into chromatin
Mitosis in Plant Cells
• Same phases as in animal cells; plants have no centrioles
Cytokinesis, or division of cytoplasm, accompanies mitosis.
• Begins in anaphase, but is not completed until just before the next interphase.
• Newly-formed cells receive a share of organelles made during interphase.
• Cytokinesis in Animal Cells - Cell membrane pinches inward to form a cleavage
furrow beginning at the end of anaphase.
• Cytokinesis in Plant Cells - The rigid cell wall surrounding plant cells cannot pinch
inward; instead, a cell plate forms, which will later become a new cell wall
Cell Division in Prokaryotes - called binary fission.
• Following DNA replication, the two resulting chromosomes separate as the cell
elongates.
• Cell divides without cell structures seen in plants & animals
Meiosis
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Humans have 23 pairs of homologous chromosomes (chromosomes with the same genes), or
46 chromosomes total.
Involves 2 cell divisions & produces 4 haploid cells; produces sex cells (gametes) – eggs &
sperm
Reduces the chromosome number so that egg or sperm cells each have only one of each kind
of chromosome (2n  1n).
Fertilization (fusion of egg and sperm) -restores the diploid number of chromosomes
Phases of Meiosis
• Prior to meiosis I, DNA replication occurs.
• Meiosis I
• Prophase I - Nuclear memebrane & nucleolus disappear, spindle forms,
homologous chromosomes pair during synapsis
• Metaphase I - Homologous chromosomes line up at metaphase plate
• Anaphase I - Homologous chromosomes separate & move to opposite poles
• Telophase I - Nuclear membrane and nucleolus reappear, cytokinesis occurs
• Interkinesis – no replication occurs
• Meiosis II
• Prophase II - Spindle reappears, nucleolus and nuclear membrane disappear,
chromosomes attach to spindle
• Metaphase II - Chromosomes line up at metaphase plate
• Anaphase II - Sister chromatids separate, becoming daughter chromosomes
• Telophase II - Spindle disappears, nuclear membrane and nucleolus reappear,
cytokinesis divides the cells
Genetic Recombination - occurs in several ways:
1) Crossing-over of nonsister chromatids – occurs during prophase I
2) Independent assortment of homologous chromosomes – separate in a random manner
3) Combining of chromosomes of genetically different gametes during fertilization
DNA replication
Number of cell divisions
Number of cells
Daughter cells
Occurs in
Number of parents
Offspring
Mitosis
Occurs during interphase
1
2 diploid cells
Identical to each other & parent
Somatic cells – growth & repair
Sexual reproduction
2
Different from parent
Meiosis
Occurs during interphase
2
Up to 4 haploid cells
Different from each other & parent
Reproductive cells - gametes
Asexual reproduction
1
Identical to parent
Genetics
• Gregor Mendel – Austrian monk, bred pea plants, developed laws of heredity
• Experiments:
• Bred P generation to be pure-breeding (ex: tall x tall = tall offspring)
• Cross-pollinated plants showing opposite forms of same trait (ex: tall x short)
• F1 offspring (1st filial generation) – all tall; allowed F1 to self-pollinate
• F2 generation (2nd filial generation) – ¾ offspring tall; ¼ offspring short
- Conclusions
• Principle of Dominance: Offspring inherits 2 genes, one from each parent. One gene could be
dominant over the other recessive gene, preventing its expression. The only way to show a recessive
trait is to inherit 2 copies (one from each parent)
• Principle of Segregation: Each sex cell only inherits one copy of each gene, so sex cells are haploid;
fertilization (fusion of egg & sperm) restores the diploid number, giving the zygote 2 copies of each
gene
• Principle of Independent Asssortment: Genes on different chromosomes separate independently
during meiosis (ex: so hair color & eye color are independent of each other)
- Alleles – different forms of a trait; ex: red, brown, or blonde hair color
- Genotype – refers to the genes of an individual
- Homozygous dominant – BB (2 copies of dominant gene)
- Homozygous recessive – bb (2 copies of recessive gene)
- Heterozygous – Bb (1 copy of each gene)
- Phenotype – the appearance of an individual (Homozygous dominant and heterozygous show dominant
phenotype; homozygous recessive shows recessive phenotype)
- Punnett square – chart used to show the probability of certain traits
- Polygenic traits – traits are determined by more than one gene; phenotypes distributed along bell-shaped
curve
- Multiple allele traits – More than 2 forms of an allele exist for a trait (ex: blood type – A, B, O alleles)
- Codominance – both alleles are equally expressed in heterozygote (ex AB blood)
- Incomplete dominance – heterozygote shows phenotype that is intermediate to homozygous forms (ex: hair
– curly and straight are homozygous, wavy is appearance of heterozygote)
- Karyotype – visual display of the chromosomes in an organism; shows gender & any abnormalities
- 22 pairs of autosomes (non-sex-determining chromosomes)
- 1 pair of sex chromosomes (determine gender; XX = female; XY = male)
- Chromosomal mutations – organism has extra or missing chromosomes; result of errors during meiosis
(chromosomes fail to separate correctly); may result in syndrome (set of symptoms)
- Sex-linkage – traits are inherited on the X or Y chromosome; ex: color blindness
- Linked genes – traits inherited on the same chromosome; independent assortment does not apply