HSC Biology Module 5: Heredity Reproduction Cell Replication

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Cell Replication

DNA replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules. Replication is an essential process because, whenever a cell divides, the two new daughter cells must contain the same genetic information, or DNA, as the parent cell.

The replication process relies on the fact that each strand of DNA can serve as a template for duplication. DNA replication initiates at specific points, called origins, where the DNA double helix is unwound. A short segment of RNA, called a primer, is then synthesized and acts as a starting point for new DNA synthesis. An enzyme called DNA polymerase next begins replicating the DNA by matching bases to the original strand. Once synthesis is complete, the RNA primers are replaced with DNA, and any gaps between newly synthesized DNA segments are sealed together with enzymes.

In sexually reproducing organisms, the genomes of two parents are combined to create offspring with unique genetic profiles. This is beneficial to the population because genetically diverse populations have a higher chance of withstanding survival challenges such as disease and environmental changes.

Asexually reproducing organisms can suffer a dangerous lack of diversity – but they can also reproduce faster than sexually reproducing organisms, and a single individual can create a new population without the need for a mate.

Some organisms that practice asexual reproduction can exchange genetic information to promote diversity using forms of horizontal gene transfer such as bacteria who use plasmids to pass around small bits of DNA. However, this method results in fewer unique genotypes than sexual reproduction.

Some species of plants, animals, and fungi are capable of both sexual and asexual reproduction, depending on the demands of the environment.

Asexual reproduction is practiced by most single-celled organisms including bacteria, archaebacteria, and protists. It is also practiced by some plants, animals, and fungi.

Cell replication

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Mitosis and Meiosis

There are two types of cell division: mitosis and meiosis. Most of the time when people refer to “cell division,” they mean mitosis, the process of making new body cells. Meiosis is the type of cell division that creates egg and sperm cells.

Mitosis is a fundamental process for life. During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. Because this process is so critical, the steps of mitosis are carefully controlled by a number of genes. When mitosis is not regulated correctly, health problems such as cancer can result.

The other type of cell division, meiosis, ensures that humans have the same number of chromosomes in each generation. It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells. When the sperm and egg cells unite at conception, each contributes 23 chromosomes so the resulting embryo will have the usual 46. Meiosis also allows genetic variation through a process of DNA shuffling while the cells are dividing.

Mitosis and Meiosis

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DNA structure

DNA is made up of molecules called nucleotides. Each nucleotide contains a phosphate group, a sugar group and a nitrogen base. The four types of nitrogen bases are adenine (A), thymine (T), guanine (G) and cytosine (C). The order of these bases is what determines DNA's instructions, or genetic code. Human DNA has around 3 billion bases, and more than 99 percent of those bases are the same in all people.

Similar to the way the order of letters in the alphabet can be used to form a word, the order of nitrogen bases in a DNA sequence forms genes which in the language of the cell, tells cells how to make proteins. Another type of nucleic acid, ribonucleic acid, or RNA, translates genetic information from DNA into proteins.

Nucleotides are attached together to form two long strands that spiral to create a structure called a double helix. If you think of the double helix structure as a ladder, the phosphate and sugar molecules would be the sides, while the bases would be the rungs. The bases on one strand pair with the bases on another strand: adenine pairs with thymine, and guanine pairs with cytosine.

DNA molecules are long — so long, in fact, that they can't fit into cells without the right packaging. To fit inside cells, DNA is coiled tightly to form structures we call chromosomes. Each chromosome contains a single DNA molecule. Humans have 23 pairs of chromosomes, which are found inside the cell's nucleus. 

DNA Structure

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Polypeptide or Protein Synthesis

The Art of Protein SynthesisThis amazing artwork shows a process that takes place in the cells of all living things: the production of proteins. This process is called protein synthesis, and it actually consists of two processes — transcription and translation. In eukaryotic cells, transcription takes place in the nucleus. During transcription, DNA is used as a template to make a molecule of messenger RNA (mRNA). The molecule of mRNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs. During translation, the genetic code in mRNA is read and used to make a protein. These two processes are summed up by the basic equation of molecular biology: DNA → RNA → Protein.mRNA

transcription translation eukaryotic ribosomecytoplasm

DNA → RNA → Protein

Polypeptide Synthesis

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Eukaryotic transcriptionEukaryotic transcription is the process that eukaryotic cells use to copy genetic information stored in DNA into units of transportable RNA.  Gene

transcription occurs in both eukaryotic and prokaryotic cells., RNA polymerase in eukaryotes (including humans) comes in three variations, each translating a different type of gene. A eukaryotic cell has a nucleus that separates the processes of transcription and translation. Eukaryotic transcription occurs within the nucleus where DNA is packaged into nucleosomes and higher order chromatin structures. Eukaryotic transcription proceeds in three sequential stages: initiation, elongation, and termination. The RNAs transcribed serve functions. For example, structural components of the ribosome are transcribed by RNA polymerase I. Protein coding genes are transcribed by RNA polymerase II into messenger RNAs (mRNAs) that carry the information from DNA to the site of protein synthesis.

eukaryotic cells prokaryotic cells nucleosomes

 chromatin structures.

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Effects of Environment on PhenotypeGenes play an important part in influencing phenotype, but genes are not the only influence. Environmental conditions, such as temperature and availability of nutrients can affect phenotypes. For example, temperature affects coat colour in Siamese cats.

The dark “points” on this Siamese cat are caused by a gene that codes for a temperature-sensitive enzyme. The enzyme, which causes a darkening of the cat’s fur, is active only in the cooler parts of the body such as the tail, feet, ears, and area around the nose.The pointed pattern is a form of partial albinism, which results from a mutation in an enzyme that is involved in melanin production. The mutated enzyme is heat-sensitive; it fails to work at normal body temperatures. However, it is active in cooler areas of the skin. This results in dark coloration in the coolest parts of the cat's body, such as the lower limbs and the face.

The cat’s face is cooled by the passage of air through the nose. Generally adult Siamese cats living in warm climates have lighter coats than those in cooler climates.

Height in humans is a complex phenotype influenced by many genes, but it is also influenced by nutrition. A person who eats a diet poor in nutrients will not grow as tall as they would have had they eaten a more nutritious diet.

phenotype enzyme mutated

Effect of environment on phenotype

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What are proteins and what do they do?

Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs.

Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains. There are 20 different types of amino acids that

can be combined to make a protein. The sequence of amino acids determines each protein’s unique 3-dimensional structure and its specific function.

Proteins can be described according to their large range of functions in the body, listed in alphabetical order:

Examples of protein functions

Function Description Example

Antibody Antibodies bind to specific foreign particles, such as viruses and bacteria, to help protect the body.

Immunoglobulin G (IgG)

Enzyme Enzymes carry out almost all of the thousands of chemical reactions that take place in cells. They also assist with the formation of new molecules by reading the genetic information stored in DNA.

Phenylalanine hydroxylase

Messenger Messenger proteins, such as some types of hormones, transmit signals to coordinate biological processes between different cells, tissues, and organs.

Growth hormone

Structural component

These proteins provide structure and support for cells. On a larger scale, they also allow the body to move.

Actin

Transport/storage These proteins bind and carry atoms and small molecules within cells and throughout the body.

Ferritin

Proteins

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https://ghr.nlm.nih.gov/primer/howgeneswork/protein

https://www.khanacademy.org/science/biology/gene-expression-central-dogma/transcription-of-dna-into-rna/v/transcription-and-mrna-processing

https://www.nature.com/scitable/definition/phenotype-phenotypes-35/

https://en.wikipedia.org/wiki/Eukaryotic_transcription#/media/File:Eukaryotic_Transcription.png

https://en.wikipedia.org/wiki/Eukaryotic_transcription

https://opentextbc.ca/anatomyandphysiology/chapter/3-4-protein-synthesis/

https://courses.lumenlearning.com/boundless-biology/chapter/rna-processing-in-eukaryotes/

https://www.ck12.org/book/CK-12-College-Human-Biology/section/5.6/

https://en.wikipedia.org/wiki/Protein_biosynthesis

https://www.sciencedirect.com/topics/medicine-and-dentistry/protein-synthesis

http://faculty.ccbcmd.edu/~gkaiser/biotutorials/protsyn/protsyn.html

https://www.thoughtco.com/differences-between-mitosis-and-meiosis-373390

https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Biofundamentals_(Klymkowsky_and_Cooper)/08%3A_Peptide_bonds%2C_polypeptides_and_proteins/8.07%3A_The_translation_(polypeptide_synthesis)_cycle

https://www.nature.com/scitable/definition/replication-33/

https://www.biology-online.org/dictionary/Protein_synthesis

https://en.wikipedia.org/wiki/Molecular_Structure_of_Nucleic_Acids:_A_Structure_for_Deoxyribose_Nucleic_Acid

https://www.livescience.com/37247-dna.html

https://www.mun.ca/biology/scarr/Watson-Crick_Model.html

https://www.thoughtco.com/differences-between-mitosis-and-meiosis-373390