You-Nique

Cell Structure and Function

What is a cell?

• A cell is the building block of all living things

• There are many different types of cells.
  Each is specialised because it has a different function.

Cell Membrane:

• All cells have a cell membrane

• Its function is to control the movement of substances in and out of the cell

• These substances include water, waste and food

Cytoplasm:

• All cells have cytoplasm

• It is the site of chemical reactions and cellular processes

• Jelly-like fluid contained within the cell membrane 

Nucleus and Components:

• The nucleus controls all cell activities and contains genetic material

• The nucleolus is found within the nucleus and produces ribosomes

• Around the nucleus is the nuclear membrane (its gaps are called nuclear pores). Its job is to control the movement of substances in and out of the nucleus.

Chloroplasts:

• Found only in plant cells (and some protists)

• The site of photosynthesis

• Green in colour due to pigment called chlorophyll

Cell Wall:

• Found in plant cells, as well as algae, fungi and most bacteria (NOT animal cells)

• Protects the cell and gives it shape and support

Mitochondria:

• The “powerhouse” of the cell

• Its function is to produce energy through cellular respiration

• No energy = no cell. The mitochondria is therefore very important!

Ribosomes:

• Found in all cells

• Can be found free-floating in the cytoplasm or attached to the rough endoplasmic reticulum

• Produce proteins (very, very important!)

Endoplasmic Reticulum:

• Folded membranes that synthesise and transport materials around the cell

• Can be smooth (no ribosomes attached) or rough (ribosomes attached)

• Rough ER makes and transports proteins; Smooth ER makes and transports lipids (fats)

Golgi Body:

• Modifies and packages proteins (usually for release out of the cell)

• Usually found closer to the cell membrane than endoplasmic reticulum

• Also known as the golgi apparatus or golgi complex

Vacuole:

• Temporary storage space in a cell for water, food and waste

• One large vacuole in plant cells; many, small vacuoles in animal cells

Plant vs Animal Cells:

Not all cells have the same organelles! 

Plant cells contain the following organelles, which are absent in animal cells:

• Cell wall

• Chloroplasts

Plant cells also have a large, central vacuole whereas animal cells have multiple, small vacuoles. 

DNA: The Universal Code of Life

What is DNA?

• DNA stands for deoxyribonucleic acid

• It is a chemical substance found in all living cells (usually in the nucleus)

• DNA is a molecule

• It contains instructions that control the development and function of all living things

• While your DNA is unique to you, everyone’s DNA is made of the same building blocks, so it is called universal

The Structure of DNA

DNA Molecule:

• DNA is a very large molecule made up of a long chain of subunits.

• Each subunit is called a nucleotide.

Nucleotide:

• In every nucleotide there is always phosphate, a nitrogenous base and a sugar.

• The sugar in DNA is always deoxyribose

• The nitrogenous base in a DNA nucleotide will be either:

Adenine 

Cytosine 

Thymine 

Guanine

Joining Nucleotides:

• To join together, the phosphate from one nucleotide forms a bond with the sugar on the next.

• This forms the sugar-phosphate backbone

• The diagram shows one strand of DNA, and millions of nucleotides join together in this one strand.

DNA is Double Stranded:

• DNA is double stranded, which means it is two chains of nucleotides joined together.

• The sugar-phosphate backbones are on the outside

• The strands are held together by hydrogen bonds between the bases. 

Complementary Bases:

• Bases can only join to their complementary base.

• Adenine always bonds with Thymine (A-T)

• Cytosine always bonds with Guanine (C-G)

• These bonds hold the two DNA strands together

Double Helix:

• The 3D shape of DNA is called the double helix

• As the strands join together they twist into a spiral staircase shape

Genes, Chromosomes and DNA

Genes:

• A typical strand of DNA contains millions of bases.

• A gene is one section of DNA and codes for a specific protein

• These proteins determine your traits (such as hair colour, eye colour, skin colour etc)

Chromosomes:

• There is about six feet (1.8 metres) of DNA in each of your cells. 

It must be condensed!

• The DNA is very tightly wound up into structures called chromosomes

• A average human body cell has 46 chromosomes.

• 23 chromosomes come from your mother’s egg cell

• 23 chromosomes come from your father’s sperm cell

• This means that you get two copies of every chromosome

• These two copies are called homologous pairs

• A cell or organism that contains two copies of every chromosome is described as diploid.

• A cell or organism that contains one copy of every chromosome is described as haploid

• In humans, the only haploid cells (cells with one set of chromosomes) are the sex cells. 

  Female = egg (23 chromosomes)

  Male = sperm (23 chromosomes)

• In humans, every other cell in the body is a diploid cell (cell with two sets of chromosomes).

Secret of Photo 51 The Contribution of Rosalind Franklin

Chromosomes and Karyotypes

• Sometimes there can be abnormalities in an organism’s chromosomes. 

• To determine any abnormalities, a karyotype can be made. 

• A karyotype is a picture of all the chromosomes in a cell, arranged in pairs.

• Karyotypes are arranged in order of size. 

• The first 22 pairs are autosomes (non-sex chromosomes), and pair 23 are the sex

Homologous Chromosomes:

• Each homologous pair of chromosomes has three unique features:

  • Size

  • Banding Pattern

  • Centromere Position

This helps scientists identify chromosomes

Common Chromosomal Abnormalities:

Trisomy 21:

• Extra chromosome on #21

• 47 chromosomes

• Down Syndrome

• Learning disabilities in varying degrees

• Heart and circulatory problems

• Almond shaped eyes, short limbs

Klinefelter’s Syndrome:

• XXY

• 47 chromosomes

• Only in males

• Tall, sterile, small testicles, appear normal

Turners Syndrome:

• Single X in female

• 45 chromosomes

• Short, webbed neck

• Will never have menstrual cycle (sterile)

Other Abnormalities:

• Edward’s Syndrome/Trisomy 18 (extra chromosome 18)

• Patau Syndrome/Trisomy 13 (extra chromosome 13)

• XYY Syndrome (extra Y chromosome in males)

• XXX Syndrome (extra X chromosome in females)

https://www.youtubeeducation.com/watch?v=3IaYhG11ckA

Genes and Inheritance

Each gene is responsible for making a protein. These proteins can cause skin colour, eye colour, rolling of your tongue etc. A single chromosome may contain hundreds of genes. Because we inherit one chromosome from our mother and one from our father, we usually end up with two copies of each gene.

• Alleles are different versions of the same gene

• Genotypes are the particular combination of alleles of a gene.

• Genotypes can be:

  HOMOZYGOUS (two of the same allele eg: BB or bb) or
  HETEROZYGOUS (one of each allele eg: Bb)

• Phenotypes are visible expression of a genotype. (How this gene affects the way you look or your body changes.)

• Your phenotype can be influenced by the environment. (eg. your genotype may cause you to have fair skin, but in summer your skin may darken)

• What is meant by genetic difference?

• Traits can be dominant or recessive

• The dominant trait requires only one allele for the trait to show in the phenotype

• Dominant traits requires only one allele for the trait to show in the phenotype. (brown eyes, widows peak, pigmented skin, rolled tongue, cleft chin)

• Recessive traits need two copies of an allele before it shows in the phenotype. (red hair, blue eyes, attached earlobes)

Punnett Squares

Homozygous and Heterozygous

• Homozygous – having two identical alleles for a particular trait. 

This can be further classified as homozygous dominant (AA) or homozygous recessive (aa)

• Heterozygous – having two different alleles for a particular trait (Aa)

For example, two parents with brown hair end up with a child who has red hair. how did this happen? How many of their kids will end up with red hair?

• Carriers have an allele for the recessive trait in their genotype, but do not show the trait in their phenotype.

• Homozygous dominant only carry the dominant trait

• Homozygous recessive only carry the recessive trait

A punnett square is a tool used in Biology to predict inheritance.

Pedigree Analysis

Does it run in the family?

• When more than one individual in a family is afflicted with a disease, it suggests that the disease might be inherited. 

• A basic method for determining the pattern of inheritance of any trait, including a disease, is to look at its occurrence in several individuals within a family, spanning as many generations as possible.

Pedigree

A pedigree is a family tree diagram that shows the phenotypes of individuals in a family across multiple generations.

• Males are represented by squares

• Females are represented by circles

• Non-specified gender is represented by diamonds

• Filled symbols show individuals who exhibit the trait in question

• Mating, offspring and twins can also be represented by symbols

• When the symbols are connected, they form a family tree

• Roman numerals are used to label generations

• Arabic numerals are used to label individuals in each generation

• Therefore, we refer to individuals as II-3 or IV-4 etc

• When interpreting pedigrees, you must determine whether the trait is dominant or recessive

• Pedigrees show phenotypes but often genotypes can be determined from them.

Dominant trait (autosomal)

• Every affected child has at least one affected parent

• Once the trait disappears from the family pedigree it doesn’t reappear

Recessive trait (autosomal)

• Two unaffected parents can have an affected child

• The trait may skip a generation

When you see a pedigree, see if you can spot the following:

1. Does it skip a generation?

→ recessive

2. Does an affected child have unaffected parents?

→ recessive

3. Does every affected child have affected parents?

→ dominant

Once you think you know the mode of inheritance (dominant/recessive), check by working out the genotypes for all individuals.

Revision Notes

CELL STRUCTURE AND FUNCTION

What is a Cell?

• The basic unit of life.

• Many types, each specialized for different functions.

Cell Organelles & Their Functions

Differences Between Plant and Animal Cells

DNA: THE UNIVERSAL CODE OF LIFE

What is DNA?

• Stands for Deoxyribonucleic Acid.

• Found in all living cells (usually in the nucleus).

• Holds genetic instructions for life.

Structure of DNA

• Double-stranded molecule.

• Made of nucleotides, each containing:

  • Phosphate

  • Deoxyribose sugar

  • Nitrogenous base (A, T, C, G)

• Base Pairing:

  • Adenine (A) pairs with Thymine (T)

  • Cytosine (C) pairs with Guanine (G)

• Twists into a double helix shape.

Genes & Chromosomes

• Gene: A section of DNA that codes for a protein.

• Chromosome: Tightly wound DNA.

  • Humans have 46 chromosomes (23 pairs).

  • Diploid Cells: Two sets of chromosomes (body cells).

  • Haploid Cells: One set (sperm & egg).

CHROMOSOMES AND KARYOTYPES

• Karyotype: Picture of chromosomes arranged in pairs.

• Abnormalities:

  • Trisomy 21 (Down Syndrome): Extra chromosome 21.

  • Klinefelter’s Syndrome (XXY): Affects males, sterile.

  • Turner’s Syndrome (X0): Affects females, sterile.

GENES AND INHERITANCE

Key Terms

Punnett Squares

Used to predict inheritance of traits.

• Example: Two heterozygous (Bb) parents have children.

  • 75% chance of brown eyes (BB or Bb).

  • 25% chance of blue eyes (bb).

PEDIGREES

What is a Pedigree?

• A family tree that tracks how traits are passed down.

• Used to study genetic inheritance (e.g. diseases, eye color).

Symbols in a Pedigree

Types of Inheritance

Example: Hemophilia (X-Linked Recessive)

• More common in males because they only have one X chromosome.

• If a mother is a carrier, she has a 50% chance of passing it to her sons.