Bio D1 - DNA and Genes

How are nucleotide monomers linked together to form a single strand?

Nucleotide monomers are linked together via condensation reactions in which the phosphate grouch of one nucleotide attaches to the sugar of another nucleotide at the 3’ of the 5 carbon sugar, resulting in a phosphodiester bond.

What is RNA and its role in the cell?

RNA is a more versatile single stranded form of nucleic acid that transfers genetic information for decoding

what are sone similarities between DNA and RNA

• both have adenine, guanine, cytosine

• both polymers of nucleotides

• both have five carbon sugars

What are the three components of a nucleotide?

• phosphate group (circle)

• 5 carbon pentose sugar (pentagon)

• nitrogenous base (rectangle)

what are two types of nucleic acids present in cells?

RNA and DNA

What is DNA and what is its role in the cell?

DNA (deoxyribonucleic acid) is a stable double stranded form of nucleic acid and acts as the genetic blueprint of a cell

How does DNA and RNA differ in terms of composition?

DNA

• deoxyribose pentose sugar

• AGCT

  • adenine

  • guanine

  • cytosine

  • thymine

• double stranded

• longer

• antiparallel

• double helix

RNA

• ribose pentose sugar

• AGCU

  • Adenine

  • guanine

  • cytosine

  • uracil

• single stranded

• shorter

How are two polynucleotide chains of DNA held together?

Two polynucleotide chains of DNA are held together via hydrogen bonding between complementary nitrogenous bases.

• Adenine (A) pairs with Thymine (T) via two hydrogen bonds

• Guanine (G) pairs with Cytosine (C) via three hydrogen bonds

Why must the strands of DNA be running in opposite directions for the bases to be able to pair?

The strands of DNA must be running in opposite directions for the bases to be able to pair because the nitrogenous bases are only complementary in a specific orientation.

How are the two strands of DNA described in terms of their directionality and how do they organise themselves?

The two strands of DNA are described as being antiparallel.

As the antiparallel chains lengthen, the atoms will organise themselves into the most stable energy configuration

• This atomic arrangement results in the double-stranded DNA forming a double helix

What is DNA?

DNA is the genetic blueprint which codes for, and determines, the characteristics of an organism, including the physical, behavioural, and physiological features.

How is DNA packaged and organized in a cell?

DNA is packaged and organized into discrete structures called chromosomes.

What is a gene?

A gene is a sequence of DNA that encodes for a specific trait. Traits may also be influenced by multiple genes.

What is the name given to the position of a gene on a particular chromosome?

The position of a gene on a particular chromosome is called the locus (plural = loci).

How do genes determine the characteristics of an organism?

Genes contain the information necessary to produce specific proteins, which in turn determine the traits of an organism.

What are alleles and how do they differ from one another?

Alleles are alternative forms of a gene that code for the different variations of a specific trait.

As alleles are alternative forms of the one gene, they possess very similar gene sequences

• Alleles only differ from each other by one or a few bases

What is an example of a gene that has alleles that encode different variations of a trait?

The gene for eye colour has alleles that encode different shades/pigments.

What is a gene mutation?

A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait.

and also forms new alleles

What is a beneficial mutation?

Missense Mutation

A beneficial mutation changes the gene sequence (missense mutations) to create new variations of a trait that is an advantage to the organism.

What is a detrimental mutation?

Nonsense Mutation

A detrimental mutation truncates the gene sequence (nonsense mutations) to abrogate the normal function of a trait, which can negatively impact the organism's survival or reproductive success.

What is a neutral mutation?

Silent Mutation

A neutral mutation has no effect on the functioning of the specific feature (silent mutations), and does not confer an advantage or disadvantage to the organism.

What is a genome?

The genome is the totality of genetic information of a cell, organism or organelle

What does the human genome consist of?

• 46 chromosomes (barring aneuploidy)

• ~3 billion base pairs

• ~21,000 genes

What is the DNA organisation of prokaryotes?

• Prokaryotes do not possess a nucleus – instead genetic material is found free in the cytoplasm in a region called the nucleoid, bacterial chromosomes have long uncoiled DNA

• Some have shorter DNA strands called plasmids that only contain a few genes and are capable of self-replication

What is the organisation of DNA in a Eukaryotic cell?

• consist of multiple linear molecules of DNA that are associated with histone proteins

  • DNA coiled and wrapped around a set of 8 histones form a nucleosome

  • where genes are found and is a locus for gene position

• has pairs of chromosomes = homologous chromosomes

  • same genes, loci, length, centromere position

What is a homologous pair?

sexually reproducing organisms inherit their genetic sequences from both parents

• organisms possess two copies of each chromosome (1 mom and 1 dad)

• same genes at the same loci but not the always the same allele

what are the difference between diploids and haploids?

as sexually reproducing organisms receive genetic material from both parents, that have two sets of chromosomes (23 pairs of 46 chromosomes), called diploids

however, to reproduce, organisms must create gametes with half the number of chromosomes in order to fuse with another chromosome’s gametes, called haploids

What does a diploid possess?

• Nuclei possessing pairs of homologous chromosomes

• These nuclei will possess two gene copies (alleles) for each trait

• All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis

What does a haploid possess?

• Nuclei possessing only one set of chromosomes

• These nuclei will possess a single gene copy (allele) for each trait

• only gametes

What is a heterosome?

• sex is determined by a pair of chromosomes called the sex chromosomes (or heterosomes) 23rd pair in a karyogram

• Female is XX

• Male is XY (y is short)

• female is default without the y present

what is an autosome?

any other chromosome in the organism and do not determine sex

What is a karyogram?

a process to determine the number and types of chromosomes in a eukaryotic cell

• chromosomes obtained by chorionic villus sampling or amniocentesis (placenta or white blood cells)

• inducing cell division then arresting mitosis when chromosomes are supercoiled in P1

• stained to generate an image

How are karyograms organised and used for?

• chromosomes of an organism are arranged into homologous pairs according to size, length

• Determine the gender of the unborn child (via identification of the sex chromosomes)

• Test for chromosomal abnormalities

What is down syndrome?

• disjunction of chromosome 21, where there is an extra chromosome

• The extra genetic material causes mental and physical delays in the way the child develops

What is autoradiography?

• process to measure the length of DNA

  1. grow prokaryotic cells in radioactive thymine and there is no T in RNA

  2. DNA extracted from cells and fixed onto photographic paper

  3. radiation released and after a period of exposure an image appears on the page and can be visualised

What is random assortment?

the order line up of chromosomes in the equator of the cell in metaphase with mom left and dad right or dad left and mom right

• order is completely random

What is non-disjuction and where are the two places they can occur in meiosis?

• chromosomes failing to spearate correctly, resulting in gametes with one extra or one less chromosome

occurs via:

• Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)

• Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected)

What is the Law of Dominance?

• alleles can be dominant or recessive

• the dominant phenotype is expressed with one or two dominant alleles: homozygous dominant or heterozygous genotypes. PP, Pp

• the recessive phenotype is expressed with two recessive alleles: homozygous recessive genotype. pp

What is the law of Segregation?

• each phenotype is controlled by two alleles

• only one allele for a trait is passed on through gametes to offspring from each parent

• offspring receive one allele for a trait from each parent

What is the Law of Independent Assortment?

• the inheritance of one gene does not affect the inheritance of another gene

• homologous chromosomes undergo random orientation in Metaphase I

• genes on non-homologous chromosomes are randomly assorted in Anaphase I

What is a haploid gamete?

only carry one allele of each gene

the two alleles of each gene is separated into different haploids during meiosis

what are Punnett grids?

A monohybrid cross determines the allele combinations for potential offspring for one gene only

What is cystic fibrosis?

• autosomal recessive disease

• extra info: thick and sticky mucus clogs bronchioles and secretory ducts; leads to respiratory failure and pancreatic cysts

What is sickle cell anaemia?

• autosomal codominant disease

• causes fibrous hemoglobin rather than globular; cells become sickle shaped; leads to poor oxygen transport, clogged vessels, and malaria resistance

What is Hemophilia and Red-green colorblindness?

• sex-linked recessive disorders

• extra info: hemophilia is a blood clotting disorder; colorblindness is the inability to perceive certain colors

What are the Human ABO blood types?

• multiple alleles (≥2): IA IB i

• autosomal dominant-recessive and codominant

• controls type of glycoprotein used for cell recognition

What are the inheritance patterns?

What is a pedigree chart and what does is do?

A pedigree is a chart of the genetic history of a family over several generations

• Males are represented as squares, while females are represented as circles

• Shaded symbols mean an individual is affected by a condition, while an unshaded symbol means they are unaffected

• A horizontal line between man and woman represents mating and resulting children are shown as offshoots to this line

• Generations are labeled with roman numerals and individuals are numbered according to age (oldest on the left)

Nitrogenous Bases

- Adenine | A
- Thymine | T
- Cytosine | C
- Guanine | G
- Uracil | U

DNA

Deoxyribonucleic Acid

DNA Function

Holds the code for protein synthesis within a cell, called genes. Processes of protein synthesis is called “transcription” and “translation”.

It’s able to copy itself for cell division called replication within the S-phase of the cell cylce.

DNA Structure

It’s a double helix, it’s double strands are attached to each other by hydrogen bonds which are twisted into a spiral. The backbones are made of phosphate and sugars while the rungs are composed of nitrogenous base pairs.

These nucleotides are composed of phosphate, 5 C-sugar/deoxyribose and a nitrogenous base.

Nitrogenous Bases: Adenine, Thymine, Guanine & Cytosine.

RNA

Ribonucleic Acid

RNA Function

There are several functions:
mRNA - Copy of a gene that leaves the nucleus and carries the message into the cytoplasm and to a ribosome for protein synthesis.

tRNA - Reads the mRNA and carries amino acids to the correct ribosome.

RNA Structure

It only has a single strand of nucleotides which are composed of 5-C sugar ribose.

Nitrogenous Bases: Adenine, Guanine, Cytosine & Uracil.

Nucleotides

In a chain connected to each other by covalent bonds, meaning electrons are shared between atoms.

Complementary nucleotides are held together by hydrogen bonds which are weak bonds where there’s a proton in one atom while the other one is an electron.

Protein Synthesis

DNA is the holder of codes responsible for protein synthesis.

Transcription Summary

Makes a copy of a gene on DNA to turn into a molecule of RNA.

Transcription Steps

1. Happens in the nucleus.

2. Enzyme RNA polymerase is needed to open the hydrogen bonds between the double-stranded DNA which makes a strand of mRNA next to one of the DNA strands.

3. RNA nucleotides are already present in the nucleus and places the nitrogenous bases into complementary pairs.

   C —> G | A —> U

4. The mRNA leaves the nucleus and the DNA goes back together.

Translation Summary

Translates the code on mRNA into amino acids in correct order.

Translation Steps

1. Happens on the ribosomes in the cytoplasm.

2. mRNA has the code copy of DNA and goes to the ribosome.

3. The code is read in groups of three nitrogenous bases called codons.

4. tRNA reads the code and brings the correct amino acid to the ribosome for each codon. tRNA has a complementary anticodon which will bind with the mRNA codon.

5. When two tRNAs are in the ribosome a peptide bond will form between the amino acids and the chain will further longate.

6. The ribosome will move further along the mRNA until it hits a certain codon. All parts will separate and the protein can leave the ribosome.

What is mitosis?

the division of a cell into two identical daughter cells

What are sister chromatids?

replication to form chromosome in S phase, genetically identical DNA molecules

What processes occur in Meiosis 1?

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated

• P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs

• M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell

• A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell

• T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

What occurs in Meiosis 2?

The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)

• P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)

• M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator

• A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles

• T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells

What is crossing over?

• when two homologous chromosomes pair up in prophase 1 and one of each sister chromatids cross over and trade genetic material, then when separated in metaphase 1, each chromatid is genetically unique. and sisters are no longer identical